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Glossary of Astronomy Terms

Here is a small glossary of terms that have been used on this webpage, or are used commonly in the field of astronomy. Knowing there definitions will help you grasp the different concepts of astronomy much more easily. I hope you find these useful.


ACCRETION- the accumulation of dust and gas into larger bodies.

ALBEDO- the reflectivity of an object, or the ratio of reflected light to incident light.

ALLOCTHONOUS- Fragmented rock thrown out of a crater during impact that either falls back into the crater or blankets its outer banks after the impact.

ALPHA CENTAURI- The closest bright star to our Solar System other than the Sun.

APHELION- The point in its orbit where a planet is farthest from the Sun.

APOAPSIS- The point in orbit farthest from the planet.

APOGEE- The point in orbit farthest from the Earth.

ASTEROID NUMBER- Asteroids are assigned a serial number when they are discovered.

ASTRONOMICAL UNIT- (AU)- The average distance from the Earth to the Sun; 1 AU is 149,597,870 kilometers.

AURORA- A glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun.


BASALT- A general term for dark colored igneos rock composed of minerals that are relatively rich in iron and magnesium.

BLACK HOLE- An object whose gravity is so strong that the escape velocity exceeds the speed of light.

BOLIDE- An exploding meteorite.


CALDERA- A large, basin-shaped depression that is more or less circular in form. Most volcanic calderas are produced by the collapse of the room of a magma chamber, or by explosive removal of the upper part of the volcano.

CATENA- A chain of craters.

CAVUS- Hollows or irregular depressions

CENTRAL PEAK- The exposed core of uplifted rocks in complex craters.

CHASMA- A canyon.

CHROMOSPHERE- The lower level of the solar atmosphere between the photosphere and the corona.

COMA- The dust and gas surrounding an active comet's nucleus.

CORONA- The upper level of the solar atmosphere, chracterized by low densities and high temperatures. It is not visable from Earth except during total eclipses or by the use of special telescopes.

CRATER- A depression formed by the impact of a meteorite.


DENSITY- Measured in grams per cubic centimeter or kilograms per liter. The density of water is 1.0, iron is 7.9, and lead is 11.3.

DISK- The visable surface of a heavenly body projected against the sky.

DORSUM- A ridge.


E=MC^2- Einstein's famous theory of realativity formula known as the energy-mass relation. The energy e is equal to the mass m multiplied by the speed of light squared. A small mass produces a huge amount of energy.

ECCENTRIC- Noncircularor elliptical (applied to an orbit).

ECLIPSE- The cutting off of light from one celestial body from another.

ECLIPTIC- The plane of the Earth's orbit around the Sun.

EJECTA- Material thrown out of an impact crater during its formation.

ELLIPSE- A closed curve that is from two foci or points in which the sum of the distance from any point on the curve to the two foci are constant. Johannes Kepler first discovered that the planets orbit the Sun in ellipses, and not in circles.


FAULT- A crack or break in the crust of a planet along which slippage or movement can take place.

FISSURE- A narrow opening or crack of considerable length and depth.

FLARE- A sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.


GALILEAN MOONS- Jupiter's four largest moons- Europa, Io, Callisto, and Ganymede- discovered by Galileo.

GRANULATION- A pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas.

GREENHOUSE EFFECT- An increase in temperature caused when the atmosphere absorbs incoming solar radiation but blocks outgoing thermal radiation. Carbon dioxide is the major factor.

GRAVITY- A mutual physical force attracting two bodies.


H-ALPHA- A narrow wavelength of red light which is emitted and absorbed by the element hydrogen. This wavelength is often used to study the Sun.

HELIOCENTRIC- Sun centered, such as in the Copernican theory.

HELIOPAUSE- The point where the solar wind meets the instellar medium or solar wind from other stars.

HEMISPHERE- A half of the celestial sphere that is divided into two halves by either the horizon, the celestial sphere, or the ecliptic.


ICE- This word is used to refer to water, methane, and ammonia, which occur as solids in the outer Solar System.

INFERIOR PLANETS- The planets Mercury and Venus are inferior planets because their orbits are closer to the Sun than is the Earth's orbit.


JOVIAN PLANET- Any of the four outer gaseous planets: Jupiter, Saturn, Uranus, or Neptune.


KILOGRAM- One kilogram is equilvalent to 1,000 grams or 2.2 pounds, which is the mass of a liter of water.

KILOMETER- One kilometer is equal to 1,000 meters or 0.62 miles.


LAVA- A general term for molton rock that is extruded onto the surface.

LEADING HEMISPHERE- The hemisphere that faces foward into the direction of a motion of a satellite that keeps the same face toward the planet.

LIGHT- Electromagnetic radiation that is visable to the eye.

LIGHT YEAR- The distance light travels in a year, at the rate of 300,000 kilometers per second (671 million kilometers per hour). 1 light year is equal to 9.46053e12 kilometers, 5,880,000,000,000 miles, or 63,240 AU.

LIMB- The outer edge if the apparent disk of a celestial body.


MAGNETIC FIELD- A region of space near a magnetized body where magnetic forces can be detected.

MAGNITUDE- The degree of brightness of a celestial body designated on a numerical scale, on which the brighest star has a magnitude of -1.4 and the faintest with a magnitude of 6.

MARE- Latin word for sea. The term is applied to the basalt-filled impact basins common on the face of the Moon and visable from Earth.

METEOR- The luminous phenomenon seen when a meteoroid enters the atmosphere.

METEORITE- A part of a meteorid that survives through the Earth's atmosphere.

MINOR PLANETS- Another term used for asteroids.

MONS- A mountain.


NEBULA- A diffuse mass of instellar dust and gas.


OCCULTATION- The blockage of light by the intervention of another object. A planet can occult (block) the light from a distant star.

OLD- A planetary surface that has been modified little since its formation, and usually feature large numbers of craters

ORBIT- The path of an object that is moving around a second object or point.


PENUMBRA- The outer filamentary region of a sunspot.

PERIAPSIS- The point in the orbit closest to a planet.

PERIGEE- The point in its orbit closest to the Earth.

PERHELION- The point in its orbit where a planet is closest to the Sun

PERTRUB- To cause a planet or satellite to deviate from a theortically regular orbital motion.

PHOTOSPHERE- The visable surface of the Sun

PROMINENCE- An eruption of hot gases above the photosphere of the Sun.

RADIATION- Energy radiated in the form of waves or photon particles.

RED GIANT- A star that has low surface temperature and a diameter that is large relative to the Sun.

RETROGRADE- The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic. This simply means moving in the opposite direction from the majority of Solar System bodies.

SATELLITE- A body that revolves around a larger body.

SINUS- A bay

SOLAR CYCLE- The approximately 11 year variation in the number of solar active events.

SOLAR WIND- A tenious flow of energetic charged particles, mostly protons and electrons, which stream from the Sun. Typical solar wind velocities are almost 350 kilometers per second.

SPECTRUM- The distribution of wavelengths and frequencies.

SULFURIC ACID- A corrosive acid H2SO4 that is colorless when pure

SUNSPOT- An area seen as a dark spot on the photosphere of the Sun. Sunspots are concentrations of magnetic flux, occuring in bipolar clusters or groups. They appear dark because they are cooler than the surrounding photosphere.

SUPERIOR PLANETS- The planets Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto are superior planets because they orbit the Sun further than the Earth.


TERMINATOR- The dividing line between the illuminated and unilluminated part of the Moon's or planet's disk.

TRAILING HEMISPHERE- The hemisphere that faces backwards, away from the direction of motion of a satellite that keeps the same face towards the planet.


ULTRAVIOLET- Electromagnetic radiation at wavelengths shorter than the violet end of visable light. The Earth's atmosphere effectivly blocks the transmission of most ultraviolet light.

UMBRA- The dark central region of a sunspot.


VALLIS- A sinuous valley

VOLCANO- A vent in the planetary surface through which magma and associated gases and ash erupt.


WHITE DWARF- A whitish star of high surface temperature and low intrinsic brightness with a mass appoximately equal to that of the Sun but with a density many times larger.


X-RAY- Electromagnetic radiation of very short wavelength and very high energy. X-rays have shorter wavelengths than ultraviolet light but are longer than those of cosmic rays.

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