DEFINITION OF ASTRONOMY:
The scientific study of the universe, especially of the motions, positions, sizes, composition, and behavior of astronomical objects.Astronomy is a natural science that is the study of celestial objects (such as stars, galaxies, planets, moons, and nebulae), the physics, chemistry, and evolution of many others.
FEW PICTURES ABOUT ASTRONOMY:
ABERRATION OF LIGHT
The aberration of light is a phenomenon in which light appears to be slanted (the angle at which the light appears to be coming is different than the angle at which it's actually coming) if the observer is in motion. (An analogous situation is watching rain fall from a moving train - although a raindrop is falling straight to the ground, it will appear to fall at a slant, and the slant increases as the train's speed increases.) The aberration of light is due to the fact that light travels at a finite speed (as do the raindrops). The aberration of light was discovered and demonstrated by James Bradley (an English amateur astronomer) in 1725. The discovery of the aberration of light was one of the first proofs that the speed of light was finite; it also provided an early estimate of the speed of light (Bradley calculated that the time it took for light to travel from the Sun to the Earth was 8 minutes
ABLATION
Ablation is a cooling process in which heat is carried away from an object (like a falling meteorite) as the flow of air blows away the hot, melted or vaporized outer layers of the object. Some spacecraft use ablative heat shields; their outer surface is coated with heat-dissipating material (like a phenolic epoxy resin, a reinforced plastic) that burns rapidly, and then falls away during re-entry.
ABSOLUTE MAGNITUDE
Absolute magnitude is a measure of the inherent brightness of a celestial object. This scale is defined as the apparent magnitude a star would have if it were seen from a standard distance of 32.6 light-years (10 parsecs). The lower the number, the brighter the object. Negative numbers indicate extreme brightness.
ABSOLUTE ZERO
At absolute zero, a hypothetical temperature, all molecular movement stops. All actual temperatures are above absolute zero. Absolute zero would occur at -273.16°C, -459.69°F, or 0 K.
ABSORPTION LINES
Absorption Lines are dark lines superimposed over a bright continuous absorption spectrum. Each dark line is formed as a cooler gas absorbs photons emitted by a particular element from a hotter source.
ABSORPTION SPECTRUM
An absorption spectrum (also called a dark-line spectrum) consists of dark absorption lines superimposed on a bright continuous spectrum. An absorption spectrum is created when light from an incandescent source passes through a cooler gas that absorbs photons. Each different element and molecule absorbs light at a unique set of frequencies. Astronomers can determine the composition of gases in stars by looking for their characteristic.
ACCELERATION
Acceleration is the rate of change of velocity. When an object is moving at a constant velocity (or if it is standing still) it has zero acceleration. An object whose velocity is increasing has a positive acceleration.
ACCRETION DISK
An accretion disk is a flat disk of gas and dust in space that surrounds a newborn star, black hole, or other enormous object that is growing by attracting matter to it with its gravitational field.
ACHONDRITIC METEORITE
An achondritic meteorite is a stony meteorite, coarsely crystallized, with large bits of various minerals visible to the naked eye.
ACID RAIN
Acid rain is polluted and harmful to the environment. Acid rain may have been a component of the K-T
ACRUX
Acrux (also known as alpha Crux) is a first magnitude star in the Southern Hemisphere constellation Crux .
ACTIVE GALACTIC NUCLEI
Active galactic nuclei (AGN) are galaxies that have a massive black hole at the galactic center (nucleus). These galaxies produce huge amounts of energy (at all wavelengths of the electromagnetic spectrum).
ACTIVE GALAXY
An active galaxy is a galaxy that produces huge amounts of energy; they produce more energy than the total energy emitted from all of the stars in the nebula. There may be at least three types of active galaxies, including Seyfert galaxies, quasars, and blazars (although these three may be the same type of galaxy viewed from different distances and perspectives).
nglish astronomer and mathematician who, at 24 years old, predicted the existence of the planet Neptune (Le Verrier also predicted its existence, independently).
ADAPTIVE OPTICS
Adaptive optics (AO) is a process in which distortions (like those from the Earth's atmosphere) are removed from a telescope's image in real time. First, a wavefront sensor uses a reference star to measure the distortions that are occurring. The distortions are then removed with a phase corrector. AO was recently installed on the Keck II telescope in Hawaii,
ADRASTEA
Adrastea is one of Jupiter's 16 moons, and the second-closest to Jupiter. Adrastea is 12 miles (20 km) in diameter and orbits 80,000 miles (129,000 km) from Jupiter, within its main ring. Adrastea and the first moon, Metis, are probably the source of the dust in this ring. Adrastea has a mass of 1.91 x 1016kg. It orbits Jupiter in 0.29826 (Earth) days; this is faster than Jupiter rotates on its axis. Adrastea was discovered by D. Jewitt and E. Danielson (Voyager 2) in 1979.
AEROLITE
An aerolite is a meteorite that is consists mostly of stony matter.
AIRY, GEORGE
Sir George Bidell Airy (1801-1892) was the director of Greenwich Observatory/Astronomer Royal of England from 1835 to 1881. Airy installed a transit (a precise surveying device) at Greenwich, England, which was used to define the zero degree meridian of the Earth (zero-degrees longitude). A crater on Mars about 5 degrees south of the equator and on what is defined as Mars' prime meridian (zero-degrees longitude) is call Airy. A small crater within this crater (which is called Airy-0) is where the meridian line (zero-degrees longitude) crosses. A crater on the moon is also named for him (latitude 18.1 degrees, longitude 354.3 degrees, diameter 36 km). Airy is supposed to have stated incorrectly that Charles Babbage's new "analytical engine" (the predecessor of the computer) was "worthless," effectively ending Babbage's government funding.
Aphelion: see Orbit.
Apogee: see Orbit.
Black hole: the theoretical end-product of the total gravitational collapse of a massive star or group of stars. Crushed even smaller than the incredibly dense neutron star, the black hole may become so dense that not even light can escape its gravitational field. In 1996, astronomers found strong evidence for a massive black hole at the center of the Milky Way. Recent evidence suggests that black holes are so common that they probably exist at the core of nearly all galaxies.
Conjunction: the alignment of two celestial objects at the same celestial longitude. Conjunction of the Moon and planets is often determined with reference to the Sun. For example, Saturn is said to be in conjunction with the Sun when Saturn and Earth are aligned on opposite sides of the Sun.
Mercury and Venus, the two planets with orbits within Earth's orbit, have two positions of conjunction. Mercury, for example, is said to be in inferior conjunction when the Sun and Earth are aligned on opposite sides of Mercury. Mercury is in superior conjunction when Mercury and Earth are aligned on opposite sides of the Sun.
Dwarf planet: see Planet.
Elongation: the angular distance between two points in the sky as measured from a third point. The elongation of Mercury, for example, is the angular distance between Mercury and the Sun as measured from Earth. Planets whose orbits are outside Earth's can have elongations between 0° and 180°. (When a planet's elongation is 0°, it is at conjunction; when it is 180°, it is at opposition.) Because Mercury and Venus are within Earth's orbit, their greatest elongations measured from Earth are 28° and 47°, respectively.
Galaxy: gas and millions of stars held together by gravity. All that you can see in the sky (with a very few exceptions) belongs to our galaxy—a system of roughly 200 billion stars. The exceptions you can see are other galaxies. Our own galaxy, the rim of which we see as the “Milky Way,” is about 100,000 light-years in diameter and about 10,000 light-years in thickness. Its shape is roughly that of a thick lens; more precisely, it is a spiral nebula, a term first used for other galaxies when they were discovered and before it was realized that these were separate and distinct galaxies. Astronomers have estimated that the universe could contain 40 to 50 billion galaxies. In 2004, the Hubble Space Telescope and observers at the Keck Observatory in Hawaii discovered a new galaxy 13 billion light-years from Earth.
Neutron star: an extremely dense star with a powerful gravitational pull. Some neutron stars pulse radio waves into space as they spin; these are known as pulsars.
Occultation: the eclipse of one celestial object by another. For example, a star is occulted when the Moon passes between it and Earth.
Opposition: the alignment of two celestial objects when their longitude differs by 180°. Opposition of the Moon and planets is often determined with reference to the Sun. For example, Saturn is said to be at opposition when Saturn and the Sun are aligned on opposite sides of Earth. Only the planets whose orbits lie outside Earth's can be in opposition to the Sun.
Orbit: the path traveled by an object in space. The term comes from the Latin orbis, which means “circle” or “disk,” and orbita, “orbit.” Theoretically, there are four mathematical figures, or models, of possible orbits: two are open (hyperbola and parabola) and two are closed (ellipse and circle), but in reality all closed orbits are ellipses. Ellipses can be nearly circular, as are the orbits of most planets, or very elongated, as are the orbits of most comets, but the orbit revolves around a fixed, or focal, point. In our solar system, the Sun's gravitational pull keeps the planets in their elliptical orbits; the planets hold their moons in place similarly. For planets, the point of the orbit closest to the Sun is the perihelion, and the point farthest from the Sun is the aphelion. For orbits around Earth, the point of closest proximity is the perigee; the farthest point is the apogee. See also Retrograde.
Perigee: see Orbit.
Perihelion: see Orbit.
Planet: the International Astronomical Union (IAU) issued the definition for planet (from the Greek planetes, “wanderers”) at their General Assembly in August 2006. A planet is a body that (a) is in orbit around the Sun, (b) is massive enough that its self-gravity gives it a nearly-spherical shape, and (c) has cleared the neighborhood around its orbit. A body that fulfills the first two criteria but not the third is a dwarf planet, provided that it (d) is not a satellite.
While the exact definition of “clearing the neighborhood” was not established at press time, the eight planets from Mercury through Neptune have either assimilated or repulsed most other objects in their orbits, and each has more mass than the combined total of everything else in its area. The same cannot be said for Pluto, which has now been reclassified as a dwarf planet. There are currently eight planets and three dwarf planets recognized in the solar system, and more dwarf planets are expected to be admitted.
In 1994, Dr. Alexander Wolszcan, an astronomer at Pennsylvania State University, presented convincing evidence of the first known planets to exist outside our solar system. These particular extrasolar planets circle a pulsar, or exploded star, in the constellation Virgo.
In 1995, several of these extrasolar planets were discovered orbiting stars similar to our Sun. Swiss astronomers found the first extrasolar planet (HD 209458b, nicknamed “Osiris”) to circle a normal Sun-like star. As of May 2006, 170 such planets have been discovered.
In Feb. 2004, using the Hubble Space Telescope, a team of scientists at the Institut d'Astrophysique de Paris announced that they had discovered oxygen and carbon in the atmosphere of “Osiris.”
In Aug. 2004, NASA and the National Science Foundation announced the discovery of two new planets, the smallest yet found, about the size of Neptune. The discovery opens up the possibility of smaller, Earth-sized extrasolar planets.
In April 2005, a team of American and European astronomers reported that the first image of an extrasolar planet had been made. The planet is orbiting a brown dwarf near the constellation Hydra, 230 million light-years from Earth.
Pulsar: a celestial object, believed to be a rapidly spinning neutron star, that emits intense bursts of radio waves at regular intervals.
Quasar: “quasi-stellar” object. Originally thought to be peculiar stars in our own galaxy, quasars are now believed to be the most remote objects in the universe.
Quasars emit tremendous amounts of light and microwave radiation. Although they are not much bigger than Earth's solar system, quasars pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars. It is believed that quasars are powered by massive black holes that suck up billions of stars.
Retrograde: describes the clockwise orbit or rotation of a planet or other celestial object, which is in the direction opposite to Earth and most celestial bodies. As viewed from a position in space north of the solar system (from some great distance above Earth's North Pole), all the planets revolve counterclockwise around the Sun, and all but Venus, Uranus, and Pluto rotate counterclockwise on their own axes. These three planets have retrograde motion.
Sometimes retrograde is also used to describe apparent backward motion as viewed from Earth. This motion happens when two objects rotate at different speeds around another fixed object. For example, the planet Mars appears to be retrograde when Earth overtakes and passes by it as they both move around the Sun.
Satellite (or moon): an object in orbit around a planet. Until the discovery of Jupiter's four main moons by Galileo Galilei, celestial objects in orbit around a planet were called moons. However, upon Galilei's discovery, Johannes Kepler (in a letter to Galileo) suggested satellite (from the Latin satelles, which means “attendant”) as a general term for such objects. The word satellite is used interchangeably with moon, and astronomers speak and write about the moons of Neptune, Saturn, etc. The term satellite is also used to describe man-made devices of any size that are launched into orbit.
Small Solar System Objects: at the 2006 IAU General Assembly, solar system bodies not defined as planets, dwarf planets, or satellites were placed in this category. These include most asteroids, most Trans-Neptunian Objects, comets, and other small bodies.
Star: a celestial object consisting of intensely hot gases held together by gravity. Stars derive their energy from nuclear reactions going on in their interiors, generating heat and light. Stars are very large. Our Sun has a diameter of 865,400 mi—a comparatively small star.
A dwarf star is a small star that is of relatively low mass and average or below-average luminosity. The Sun is a yellow dwarf, which is in its main sequence, or prime of life. This means that nuclear reactions of hydrogen maintain its size and temperature. By contrast, a white dwarf is a star at the end of its life, with low luminosity, small size, and very high density.
A red giant is a star nearing the end of its life. When a star begins to lose hydrogen and burn helium instead, it gradually collapses, and its outer region begins to expand and cool. The light we see from these stars is red because of their cooler temperature. There are also red super giants, which are even more massive.
A brown dwarf lacks the mass to generate nuclear fusion like a true star, but it is also too massive and hot to be a planet. A brown dwarf usually cools into a dark, practically invisible object. The existence of brown dwarfs, also called failed stars, was confirmed in Nov. 1995 when astronomers at Palomar Observatory in California took the first photograph of this mysterious object.
Supernova: a celestial phenomenon in which a star explodes, releasing a great burst of light. There are two basic types of supernova. Type Ia happens when a white dwarf star draws large amounts of matter from a nearby star until it can no longer support itself and collapses. The second more well-known kind of supernova, type IIa, is the result of the collapse of a massive star. (Massive is a classification for a star that is at least eight times the size of our Sun.) Once the star's nuclear fuel is exhausted, if its core is heavy enough, the star will collapse in on itself, releasing a huge amount of energy (the supernova), which may be brighter than the star's host galaxy.
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°.
Aperture
The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
Asterism
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.
Asterism
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°.
Aperture
The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
Asterism
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.
Asteroid (Minor Planet)
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.
Astronomical Unit
The average distance from Earth to the Sun, slightly less than 93 million miles.
Averted Vision
Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.
- See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°.
Aperture
The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
Asterism
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.
Asteroid (Minor Planet)
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.
Astronomical Unit
The average distance from Earth to the Sun, slightly less than 93 million miles.
Averted Vision
Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.
- See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Asteroid (Minor Planet)
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.
Astronomical Unit
The average distance from Earth to the Sun, slightly less than 93 million miles.
Averted Vision
Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.
Barlow Lens
A lens that’s placed into the focusing tube to effectively double or triple a telescope’s focal length and, in turn, the magnification of any eyepiece used with it.
Black Hole
A concentration of mass so dense that nothing — not even light — can escape its gravitational pull once swallowed up. Many galaxies (including ours) have supermassive black holes at their centers.
Black Hole
A concentration of mass so dense that nothing — not even light — can escape its gravitational pull once swallowed up. Many galaxies (including ours) have supermassive black holes at their centers.
Blue Moon
Traditionally, something that happens rarely or never. More recently, this has come to mean the second full Moon in a single calendar month.
Celestial Coordinates
A grid system for locating things in the sky. It’s anchored to the celestial poles (directly above Earth’s north and south poles) and the celestial equator (directly above Earth’s equator). Declination and right ascension are the celestial equivalents of latitude and longitude.
Circumpolar
Denotes an object near a celestial pole that never dips below the horizon as Earth rotates and thus does not rise or set.
Collimation
Aligning the optical elements of a telescope so that they all point in the proper direction. Most reflectors and compound telescopes require occasional collimation in order to produce the best possible images.
Comet
A “dirty snowball” of ice and rocky debris, typically a few miles across, that orbits the Sun in a long ellipse. When close to the Sun, the warmth evaporates the ice in the nucleus to form a coma (cloud of gas) and a tail. Named for their discoverers, comets sometimes make return visits after as little as a few years or as long as tens of thousands of years.
Compound Telescope
A telescope with a mirror in the back and a lens in the front. The most popular designs are the Schmidt-Cassegrain telescope (SCT) and the Maksutov-Cassegrain telescope (commonly called a “Mak”).
Conjunction
When the Moon or a planet appears especially close either to another planet or to a bright star.
Constellation
A distinctive pattern of stars used informally to organize a part of the sky. There are 88 official constellations, which technically define sections of the sky rather than collections of specific stars.
Culmination
The moment when a celestial object crosses the meridian and is thus at its highest above the horizon.
Dark Adaptation
The eyes’ transition to night vision, in order to see faint objects. Dark adaptation is rapid during the first 5 or 10 minutes after you leave a well-lit room, but full adaptation
requires at least a half hour — and it can be ruined by a momentary glance at a bright light.
Declination (Dec.)
The celestial equivalent of latitude, denoting how far (in degrees) an object in the sky lies north or south of the celestial equator.
Dobsonian (“Dob”)
A type of Newtonian reflector, made popular by amateur astronomer John Dobson, that uses a simple but highly effective wooden mount. Dobs provide more aperture per dollar than any other telescope design.
Double Star (Binary Star)
Two stars that lie very close to, and are often orbiting, each other. Line-of-sight doubles are a consequence of perspective and aren’t physically related. Many stars are multiples (doubles, triples, or more) gravitationally bound together. Usually such stars orbit so closely that they appear as a single point of light even when viewed through professional telescopes.
Earthshine
Sunlight reflected by Earth that makes the otherwise dark part of the Moon glow faintly. It’s especially obvious during the Moon’s thin crescent phases.
Eccentricity
The measure of how much an orbit deviates from being circular.
Eclipse
An event that occurs when the shadow of a planet or moon falls upon a second body. A solar eclipse occurs when the Moon’s shadow falls upon Earth, which we see as the Moon blocking the Sun. When Earth’s shadow falls upon the Moon, it causes a lunar eclipse.
Ecliptic
The path among the stars traced by the Sun throughout the year. The Moon and planets never stray far from the ecliptic.
Elongation
The angular distance the Moon or a planet is from the Sun. The inner planets of Mercury and Venus are best seen when at maximum elongation, and thus are highest above the horizon before sunrise or after sunset.
Ephemeris
A timetable with celestial coordinates that indicates where a planet, comet, or other body moving in relation to background stars will be in the sky. Its plural is ephemerides (pronounced eff-uh-MEHR-ih-deez).
Equinox
The two times each year, near March 20th and September 22nd, when the Sun is directly overhead at noon as seen from Earth’s equator. On an equinox date, day and night are of equal length.
Eyepiece
The part of a telescope that you look into. A telescope’s magnification can be changed by using eyepieces with different focal lengths; shorter focal lengths yield higher magnifications. Most eyepieces have metal barrels that are 1¼ inches in diameter; other standard sizes are 0.965 and 2 inches across.
The circle of sky that you see when you look through a telescope or binoculars. Generally, the lower the magnification, the wider the field of view.
Finderscope
A small telescope used to aim your main scope at an object in the sky. Finderscopes have low magnifications, wide fields of view, and (usually) crosshairs marking the center of the field.
Focal Length
The distance (usually expressed in millimeters) from a mirror or lens to the image that it forms. In most telescopes the focal length is roughly equal to the length of the tube. Some telescopes use extra lenses and/or mirrors to create a long effective focal length in a short tube.
Focal Ratio (f/number)
A lens or mirror’s focal length divided by its aperture. For instance, a telescope with an 80-mm-wide lens and a 400-mm focal length has a focal ratio of f/5.
A vast collection of stars, gas, and dust, typically 10,000 to 100,000 light-years in diameter and containing billions of stars (from galaxias kuklos, Greek for “circle of milk,” originally used to describe our own Milky Way).
Gibbous
When the Moon or other body appears more than half, but not fully, illuminated (from gibbus, Latin for “hump”).
A plot of the number of pixels in an image at each brightness level. It’s a useful tool for determining the optimum exposure time; the histogram of a properly exposed image generally peaks near the middle of the available brightness range and falls to zero before reaching either end.
The angle between the plane of an orbit and a reference plane. For example, NASA satellites typically have orbits inclined 28° to Earth’s equator.
Libration
A slight tipping and tilting of the Moon from week to week that brings various features along the limb into better view. The main causes are two aspects of the Moon’s orbit: its elliptical shape and inclination to the ecliptic.
Light Pollution
A glow in the night sky or around your observing site caused by artificial light. It greatly reduces how many stars you can see. Special light-pollution filters can be used with your telescope to improve the visibility of celestial objects.
Light-year
The distance that light (moving at about 186,000 miles per second) travels in one year, or about 6 trillion miles.
Limb
The edge of a celestial object’s visible disk.
The amount that a telescope enlarges its subject. It’s equal to the telescope’s focal length divided by the eyepiece’s focal length.
Magnitude
A number denoting the brightness of a star or other celestial object. The higher the magnitude, the fainter the object. For example, a 1st-magnitude star is 100 times brighter than a 6th-magnitude star.
Meridian
The imaginary north-south line that passes directly overhead (through the zenith).
Messier object
An entry in a catalog of 103 star clusters, nebulas, and galaxies compiled by French comet hunter Charles Messier (mess-YAY) between 1758 and 1782. The modern-day Messier catalog contains 109 objects.
Meteor
A brief streak of light caused by a small piece of solid matter entering Earth’s atmosphere at tremendous speed (typically 20 to 40 miles per second). Also called a “shooting star.” If material survives the trip through the atmosphere, it’s called a meteorite after landing on Earth’s surface.
Meteor Shower
An increase in meteor activity at certain times of the year due to Earth passing through a stream of particles along a comet’s orbit around the Sun.
Milky Way
A broad, faintly glowing band stretching across the night sky, composed of billions of stars in our galaxy too faint to be seen individually. It’s invisible when the sky is lit up by artificial light or bright moonlight.
Mount
The device that supports your telescope, allows it to point to different parts of the sky, and lets you track objects as Earth rotates. A sturdy, vibration-free mount is every bit as important as the telescope’s optics. A mount’s top, or head, can be either alt-azimuth (turning side to side, up and down) or equatorial (turning parallel to the celestial coordinate system). “Go To” mounts contain computers that can find and track celestial objects automatically once the mounts have been aligned properly.
Latin for “cloud.” Bright nebulas are great clouds of glowing gas, lit up by stars inside or nearby. Dark nebulas are not lit up and are visible only because they block the light of stars behind them.
A telescope’s main light-gathering lens or mirror.
Occultation
When the Moon or a planet passes directly in front of a more distant planet or star. A grazing occultation occurs if the background body is never completely hidden from the observer.
Opposition
When a planet or asteroid is opposite the Sun in the sky. At such times the object is visible all night — rising at sunset and setting at sunrise.
The apparent offset of a foreground object against the background when your perspective changes. At a given instant, the Moon appears among different stars for observers at widely separated locations on Earth. Astronomers directly calculate the distance to a nearby star by measuring its incredibly small positional changes (its parallax) as Earth orbits the Sun.
Phase
The fraction of the Moon or other body that we see illuminated by sunlight.
Planisphere (Star Wheel)
A device that can be adjusted to show the appearance of the night sky for any time and date on a round star map. Planispheres can be used to identify stars and constellations but not the planets, whose positions are always changing.
Reflector
A telescope that gathers light with a mirror. The Newtonian reflector, designed by Isaac Newton, has a small second mirror mounted diagonally near the front of the tube to divert the light sideways and out to your eye.
Refractor
A telescope that gathers light with a lens. The original design showed dramatic rainbows, or “false color,” around stars and planets. Most modern refractors are achromatic, meaning “free of false color,” but this design still shows thin violet fringes around the brightest objects. The finest refractors produced today are apochromatic, meaning “beyond achromatic.” They use expensive, exotic kinds of glass to reduce false color to nearly undetectable levels.
Retrograde
When an object moves in the reverse sense of “normal” motion. For example, most bodies in the solar system revolve around the Sun and rotate counterclockwise as seen from above (north of) Earth’s orbit; those that orbit or spin clockwise have retrograde motion. This term also describes the period when a planet or asteroid appears to backtrack in the sky because of the changing viewing perspective caused by Earth’s orbital motion.
Right Ascension (R.A.)
The celestial equivalent of longitude, denoting how far (in 15°-wide “hours”) an object lies east of the Sun’s location during the March equinox.
A measure of the atmosphere’s stability. Poor seeing makes objects waver or blur when viewed in a telescope at high magnification. The best seeing often occurs on hazy nights, when the sky’s transparency is poor.
Solar Filter
Material that allows safe viewing of the Sun by blocking nearly all of its light. Proper filters should completely cover the front aperture of a telescope and should never be attached to the eyepiece; they range from glass used by welders to special plastic film. White-light filters will show sunspots, while hydrogen-alpha (Hα) filters let certain red light through that reveals the Sun’s streaming hot gases.
Solstice
The two times each year, around June 20th and December 21st, when the Sun is farthest north or south in the sky. At the summer solstice, the day is longest and the night is shortest, and vice versa at the winter solstice.
Star
A massive ball of gas that generates prodigious amounts of energy (including light) from nuclear fusion in its hot, dense core. The Sun is a star.
Star Cluster
A collection of stars orbiting a common center of mass. Open clusters typically contain a few hundred stars and may be only 100 million years old or even less. Globular clusters may contain up to a million stars, and most are at least 10 billion years old (almost as old as the universe itself).
Star Diagonal
A mirror or prism in an elbow-shaped housing that attaches to the focuser of a refractor or compound telescope. It lets you look horizontally into the eyepiece when the telescope is pointed directly overhead.
Star Party
A group of people who get together to view the night sky. Astronomy clubs often hold star parties to introduce stargazing to the public.
Sunspot
A temporary dark blemish on the surface of the Sun that is a planet-size region of gas cooler than its surroundings. Sunspots can be viewed safely using a solar filter.
Supernova
A star ending its life in a huge explosion. In comparison, a nova is a star that explosively sheds its outer layers without destroying itself.
The line on the Moon or a planet that divides the bright, sunlit part from the part in shadow. It’s usually the most exciting and detailed region of the Moon to view through a telescope.
Transit
When Mercury or Venus crosses the disk of the Sun, making the planet visible as a black dot in silhouette, or when a moon passes across the face of its parent planet. Transit also refers to the instant when a celestial object crosses the meridian and thus is highest in the sky.
Transparency
A measure of the atmosphere’s clarity — how dark the sky is at night and how blue it is during the day. When transparency is high, you see the most stars. Yet crystal-clear nights with superb transparency often have poor seeing.
Twilight
The time after sunset or before sunrise when the sky is not fully dark. Astronomical twilight ends after sunset (and begins before sunrise) when the Sun is 18° below the horizon.
A device for aiming your telescope that shows the sky as it appears to your unaided eye, without magnification. The simplest type is a pair of notches or circles that you line up with your target. Other versions use an LED to project a red dot or circle onto a viewing window.
Universal Time (UT)
Greenwich Mean Time, expressed in the 24-hour system. For example, 23:00 UT is 7:00 p.m. Eastern Daylight Time (or 6:00 p.m. Eastern Standard Time). Astronomers use Universal Time to describe when celestial events happen in a way that is independent of an observer’s time zone.
A star whose brightness changes over the course of days, weeks, months, or years.
The changing illumination of the Moon (or other body) over time. The Moon waxes, growing more illuminated, between its new and full phases, and wanes, becoming less illuminated, between its full and new phases.
Waxing
The changing illumination of the Moon (or other body) over time. The Moon waxes, growing more illuminated, between its new and full phases, and wanes, becoming less illuminated, between its full and new phases.
The point in the sky that’s directly overhead.
Zodiac
Greek for “circle of animals.” It’s the set of constellations situated along the ecliptic in the sky, through which the Sun, Moon, and planets move.
Libration
A slight tipping and tilting of the Moon from week to week that brings various features along the limb into better view. The main causes are two aspects of the Moon’s orbit: its elliptical shape and inclination to the ecliptic.
Light Pollution
A glow in the night sky or around your observing site caused by artificial light. It greatly reduces how many stars you can see. Special light-pollution filters can be used with your telescope to improve the visibility of celestial objects.
Light-year
The distance that light (moving at about 186,000 miles per second) travels in one year, or about 6 trillion miles.
Limb
The edge of a celestial object’s visible disk.
The amount that a telescope enlarges its subject. It’s equal to the telescope’s focal length divided by the eyepiece’s focal length.
Magnitude
A number denoting the brightness of a star or other celestial object. The higher the magnitude, the fainter the object. For example, a 1st-magnitude star is 100 times brighter than a 6th-magnitude star.
Meridian
The imaginary north-south line that passes directly overhead (through the zenith).
Messier object
An entry in a catalog of 103 star clusters, nebulas, and galaxies compiled by French comet hunter Charles Messier (mess-YAY) between 1758 and 1782. The modern-day Messier catalog contains 109 objects.
Meteor
A brief streak of light caused by a small piece of solid matter entering Earth’s atmosphere at tremendous speed (typically 20 to 40 miles per second). Also called a “shooting star.” If material survives the trip through the atmosphere, it’s called a meteorite after landing on Earth’s surface.
Meteor Shower
An increase in meteor activity at certain times of the year due to Earth passing through a stream of particles along a comet’s orbit around the Sun.
Milky Way
A broad, faintly glowing band stretching across the night sky, composed of billions of stars in our galaxy too faint to be seen individually. It’s invisible when the sky is lit up by artificial light or bright moonlight.
Mount
The device that supports your telescope, allows it to point to different parts of the sky, and lets you track objects as Earth rotates. A sturdy, vibration-free mount is every bit as important as the telescope’s optics. A mount’s top, or head, can be either alt-azimuth (turning side to side, up and down) or equatorial (turning parallel to the celestial coordinate system). “Go To” mounts contain computers that can find and track celestial objects automatically once the mounts have been aligned properly.
Latin for “cloud.” Bright nebulas are great clouds of glowing gas, lit up by stars inside or nearby. Dark nebulas are not lit up and are visible only because they block the light of stars behind them.
A telescope’s main light-gathering lens or mirror.
Occultation
When the Moon or a planet passes directly in front of a more distant planet or star. A grazing occultation occurs if the background body is never completely hidden from the observer.
Opposition
When a planet or asteroid is opposite the Sun in the sky. At such times the object is visible all night — rising at sunset and setting at sunrise.
The apparent offset of a foreground object against the background when your perspective changes. At a given instant, the Moon appears among different stars for observers at widely separated locations on Earth. Astronomers directly calculate the distance to a nearby star by measuring its incredibly small positional changes (its parallax) as Earth orbits the Sun.
Phase
The fraction of the Moon or other body that we see illuminated by sunlight.
Planisphere (Star Wheel)
A device that can be adjusted to show the appearance of the night sky for any time and date on a round star map. Planispheres can be used to identify stars and constellations but not the planets, whose positions are always changing.
Reflector
A telescope that gathers light with a mirror. The Newtonian reflector, designed by Isaac Newton, has a small second mirror mounted diagonally near the front of the tube to divert the light sideways and out to your eye.
Refractor
A telescope that gathers light with a lens. The original design showed dramatic rainbows, or “false color,” around stars and planets. Most modern refractors are achromatic, meaning “free of false color,” but this design still shows thin violet fringes around the brightest objects. The finest refractors produced today are apochromatic, meaning “beyond achromatic.” They use expensive, exotic kinds of glass to reduce false color to nearly undetectable levels.
Retrograde
When an object moves in the reverse sense of “normal” motion. For example, most bodies in the solar system revolve around the Sun and rotate counterclockwise as seen from above (north of) Earth’s orbit; those that orbit or spin clockwise have retrograde motion. This term also describes the period when a planet or asteroid appears to backtrack in the sky because of the changing viewing perspective caused by Earth’s orbital motion.
Right Ascension (R.A.)
The celestial equivalent of longitude, denoting how far (in 15°-wide “hours”) an object lies east of the Sun’s location during the March equinox.
A measure of the atmosphere’s stability. Poor seeing makes objects waver or blur when viewed in a telescope at high magnification. The best seeing often occurs on hazy nights, when the sky’s transparency is poor.
Solar Filter
Material that allows safe viewing of the Sun by blocking nearly all of its light. Proper filters should completely cover the front aperture of a telescope and should never be attached to the eyepiece; they range from glass used by welders to special plastic film. White-light filters will show sunspots, while hydrogen-alpha (Hα) filters let certain red light through that reveals the Sun’s streaming hot gases.
Solstice
The two times each year, around June 20th and December 21st, when the Sun is farthest north or south in the sky. At the summer solstice, the day is longest and the night is shortest, and vice versa at the winter solstice.
Star
A massive ball of gas that generates prodigious amounts of energy (including light) from nuclear fusion in its hot, dense core. The Sun is a star.
Star Cluster
A collection of stars orbiting a common center of mass. Open clusters typically contain a few hundred stars and may be only 100 million years old or even less. Globular clusters may contain up to a million stars, and most are at least 10 billion years old (almost as old as the universe itself).
Star Diagonal
A mirror or prism in an elbow-shaped housing that attaches to the focuser of a refractor or compound telescope. It lets you look horizontally into the eyepiece when the telescope is pointed directly overhead.
Star Party
A group of people who get together to view the night sky. Astronomy clubs often hold star parties to introduce stargazing to the public.
Sunspot
A temporary dark blemish on the surface of the Sun that is a planet-size region of gas cooler than its surroundings. Sunspots can be viewed safely using a solar filter.
Supernova
A star ending its life in a huge explosion. In comparison, a nova is a star that explosively sheds its outer layers without destroying itself.
The line on the Moon or a planet that divides the bright, sunlit part from the part in shadow. It’s usually the most exciting and detailed region of the Moon to view through a telescope.
Transit
When Mercury or Venus crosses the disk of the Sun, making the planet visible as a black dot in silhouette, or when a moon passes across the face of its parent planet. Transit also refers to the instant when a celestial object crosses the meridian and thus is highest in the sky.
Transparency
A measure of the atmosphere’s clarity — how dark the sky is at night and how blue it is during the day. When transparency is high, you see the most stars. Yet crystal-clear nights with superb transparency often have poor seeing.
Twilight
The time after sunset or before sunrise when the sky is not fully dark. Astronomical twilight ends after sunset (and begins before sunrise) when the Sun is 18° below the horizon.
A device for aiming your telescope that shows the sky as it appears to your unaided eye, without magnification. The simplest type is a pair of notches or circles that you line up with your target. Other versions use an LED to project a red dot or circle onto a viewing window.
Universal Time (UT)
Greenwich Mean Time, expressed in the 24-hour system. For example, 23:00 UT is 7:00 p.m. Eastern Daylight Time (or 6:00 p.m. Eastern Standard Time). Astronomers use Universal Time to describe when celestial events happen in a way that is independent of an observer’s time zone.
A star whose brightness changes over the course of days, weeks, months, or years.
The changing illumination of the Moon (or other body) over time. The Moon waxes, growing more illuminated, between its new and full phases, and wanes, becoming less illuminated, between its full and new phases.
Waxing
The changing illumination of the Moon (or other body) over time. The Moon waxes, growing more illuminated, between its new and full phases, and wanes, becoming less illuminated, between its full and new phases.
The point in the sky that’s directly overhead.
Zodiac
Greek for “circle of animals.” It’s the set of constellations situated along the ecliptic in the sky, through which the Sun, Moon, and planets move.
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. - See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
Angular Size and Distance
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°.
Aperture
The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
Asterism
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.
Asteroid (Minor Planet)
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.
Astronomical Unit
The average distance from Earth to the Sun, slightly less than 93 million miles.
Averted Vision
Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.
- See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°.
Aperture
The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).
Asterism
Any prominent star pattern that isn’t a whole constellation, such as the Northern Cross or the Big Dipper.
Asteroid (Minor Planet)
A solid body orbiting the Sun that consists of metal and rock. Most are only a few miles in diameter and are found between the orbits of Mars and Jupiter, too small and far away to be seen easily in a small telescope. A few venture closer to the Sun and cross Earth’s orbit.
Astronomical Unit
The average distance from Earth to the Sun, slightly less than 93 million miles.
Averted Vision
Viewing an object by looking slightly to its side. This technique can help you detect faint objects that are invisible when you stare directly at them.
- See more at: http://www.skyandtelescope.com/astronomy-terms/#sthash.VVaRXPyc.dpuf
| Planet | Albedo | |
|---|---|---|
| Mercury | 0.12 | |
| Venus | 0.59 | |
| Earth | 0.39 | |
| Mars | 0.15 | |
| Jupiter | 0.44 | |
| Saturn | 0.46 | |
| Uranus | 0.56 | |
| Neptune | 0.51 | |
| Pluto | 0.5 | |
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