Glossary

Astronomy and Science Terms

Comet Hale-Bopp Science has its own language and definitions. Here are some common terms and other places where you can find information. When you are ready, click back to return to the Student Center or click below to explore again.

Scientific Notation

Universal Time

A
Amors: Mars-crossing but not Earth-crossing asteroids;
with perihelion distance 1.017 < Q < 1.3 AU.
Anorthosite: Rock of aluminum and silicates found on the moon.
Aphelion: The farthest point from the Sun of any object orbiting the Sun. Sun Facts
Apogee: The farthest point from Earth of any object orbiting Earth. The word apoapsis is used for objects orbiting other planets.
Apollos: Includes most Earth-crossing asteroids (except for the Atens);
with semimajor axis > 1.0 AU, and perihelion distance Q < 1.107 AU.
Asterism: A named group of stars that is part of a constellation, the Big Dipper is one.
Asteroid: Small, rocky world. Most asteroids are between the orbits of Mars and Jupiter. See Asteroid, Comet and Meteor Facts page.
Astronomical Unit: The average distance between Earth and the Sun, 1.5 x 10⁸ km.
Atens: Asteroids with orbits largely inside the orbit of the Earth;
with semimajor axis < 1.0 AU and aphelion distance Q > 0.983 AU.
Aurora: The glowing light from solar particles interact with Earth's magnetic field.
B
Breccia: Rock made from pieces of rocks formed earlier.
C
Carbonaceous Chondrite: Stony meteorite containing chondrules and volatiles.
Celestial Equator: The imaginary line around the sky directly above the Earth's equator.
Celestial Pole: Points above the Earth's north and south poles.
Celestial Sphere: An imaginary sphere surrounding the Earth where the sun, planets and stars are, a 3 dimensional map of the universe. See Celestial Coordinate System page.
Chondrite: A stony meteorite containing chondrules.
Chondrule: Round, glassy part of meteorite made from silicates.
Comet: One of the small, icy bodies that orbit the sun that make tails of gas and dust when they get close to the sun. See Asteroid, Comet and Meteor Facts page.
Conjunction: The position of two celestial objects when they are 0° apart as viewed from east to west on Earth. When the Sun is one of the objects the other is between it and Earth so is usually not visible.
Constellation: A pattern of stars usually named after animals or people in stories. Now used to designate an area in the celestial sphere. See Celestial Coordinate System page.
E
Ecliptic: The path the Sun seems to follow in the sky. Also, Plane of the Ecliptic
Ejecta: Pulverized rock scattered by impacts on an object's surface.
Emission Nebula: A cloud of glowing gas.
G
Globular Star Cluster: A group of stars that look like a shape of a ball.
L
Light Pollution: Wasted light from city and outdoor lights that makes it hard to see the stars at night.
Light-year: The distance light travels in a year.
Lunar Eclipse: The name of the event when the Earth comes exactly between the Sun and the moon.
M
Mare: Lava flows on the surface of the moon. Plural, maria.
Meteor: Small rocks or sand making a bright trail through the sky as it burns in the atmosphere. See Asteroid, Comet and Meteor Facts page.
Meteorite: A meteor that has landed on the Earth.
N
Nebulae: A glowing clouds of gas or dust reflecting the light of nearby stars.
O
Occultation: A partial or complete temporary hiding of one celestial object by another such as a planet moving in front of a star as seen from Earth.
Open Star Cluster: A group of stars that look close together in the sky.
Opposition: The position of two celestial objects when they are 180° apart as viewed from east to west on Earth. When the Sun is one of the objects the other is directly opposite it in the sky and can be seen all night. See alsoMars Oppositions page.
Orbit: The path an object takes as it moves around another object.
P
Perigee: The closest point to Earth of any object orbiting Earth. The word periapsis is used for objects orbiting other planets.
Perihelion: The closest point to the Sun of any object orbiting the Sun. Sun Facts
Planetary Nebula: An expanding ring of gas around a star.
R
Rays: Ejecta from impacts that spreads out away from a crater.
Reflecting Telescope: A telescope that uses a mirror to focus light.
Reflection Nebula: Dust and gas reflecting light from stars close by.
Refracting Telescope: A telescope that uses a lens to focus light by bending it.
Rilles: Channels in the lunar surface.
Rotation: An object spinning about its center.
S
SETI: The Search for Extra-Terrestrial Intelligence.
Solar Eclipse: The name of the event when the moon comes exactly between the Sun and Earth.
V
Volatiles: Carbon compounds, frozen gases and other materials that when heated vaporize.
Z
Zenith: The point in the sky directly overhead.

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