| A | B |
|---|
| Solstice | the two times of the year when the Earth is tilted farthest off its axis; the longest and shortest days of the year |
| Equinox | 'equal night' - the two days of the year when the sun is directly over the equator and the length of the day and the night are equal |
| Geocentric | Earth centred view of the solar system (planets and sun orbit around us) |
| Heliocentric | sun centered view of the solar system (all planets, including Earth, orbit the sun) |
| Ellipse | the slightly stretched out circular path followed by the planets as they orbit the sun |
| Astronomical Unit | the distance between the center of the Earth and the center of the sun (149.5 million km); used to measure small distances in space |
| Light Year | the distance that light travels in one year (9.4 trillion km); used to measure large distances in space |
| Nebulae | a cloud of interstellar dust and gas; can eventually condense into stars or planets |
| Interstellar Matter | extremely spread out gas and dust located between stars and planets; approximately 1 atom per cubic cm |
| Protostar | a contracting mass of gas in the first stage of a star's formation |
| Massive Star | one of the two main types of stars that can form, larger and heavier than sun-like stars |
| Main Sequence | on the Hertzsprung-Russell diagram, the stage in the life cycle of stars in which they produce energy by the nuclear fusion of hydrogen into helium; stable state |
| Red Giant | the stage in the life cycle of a sun-like star in which it increases in size and becomes extremely bright |
| Red Supergiant | the stage in the life cycle of a massive star in which it increases in size and becomes extremely bright |
| White Dwarf | latter stage in the life of a sun-like star in which it collapses; hot, but very faint |
| Black Dwarf | final phase in the life cycle of a sun-like star |
| Supernova | an enormous explosion that marks the death of a massive star |
| Neutron Star | a small, super dense remnant of a supernova |
| Black Hole | a super denses remnant of a supernova; an object around which the gravity is so intense that even light can not escape |
| Constellation | a grouping of stars that forms a pattern in the night sky (e.g. Ursa Major); officially there are 88 of them |
| Asterism | a distinctive star grouping that is not one of the 88 recognized constellations |
| Galaxy | a grouping of millions or billions of stars, gas, and dust, held together by gravity |
| Solar Wind | stream of electrically charged particles discharged by the sun in every direction; passes the earth at nearly 400 km/s |
| Asteroid | a small, rocky body orbiting the sun; lie mainly in a narrow belt between Mars and Jupiter |
| Comet | a celestial body composed of dust and ice that orbits the sun; it has a bright center and a long, faint tail that always points away from the sun |
| Meteoroid | a solid body, usually a fragment of rock or metal, travelling in space with no particular path |
| Meteor | a meteoroid that enters Earth's atmosphere where the heat of friction causes it to glow brightly |
| Meteorite | the remains of a meteor that do not burn up completely and last long enough to hit Earth's surface |
| Azimuth | the direction of an object in the sky in degrees - north is 0 then go clockwise; bearing |
| Altitude | the angle of elevation (in degrees) of an object in the sky above the horizon; horizon is 0, zenith is 90 |
| Zenith | the highest point in the night sky, directly overhead; altitude of 90 |
| Ecliptic | the apparent path of the sun due to the tilt of the Earth's axis |
| Microgravity | the condition in which the gravitational forces that act on an object are greatly reduced |
| Gravity | the force of attraction between masses |
| Satellite | a small body that orbits a larger one; can be natural (e.g. the Moon) or artificial (e.g. Hubble space telescope orbiting Earth) |
| Refracting Telescope | telescope that uses lenses to bend light and magnify distant objects |
| Reflecting Telescope | telescope that uses mirrors to reflect light and magnify distant objects |
| Interferometry | technique of combining the observations of two or more telescopes to produce images that have better resolution that what one telescope alone could produce |
| Hubble Space Telescope | one of the largest, most complex artificial satellites ever built; uses a series of mirrors to focus light from extremely distant objects |
| Electromagnetic Energy | forms of radiated energy that travel at the speed of light, although different forms have different wavelengths and frequencies (e.g. light, radio waves, x-rays) |
| Electromagnetic Spectrum | the complete range of wavelengths over which electromagnetic energy extends; includes gamma rays, x-rays, ultraviolet rays, visible light, infrared radiation, microwaves, and radio and television signals |
| Radio Telescope | a telescope that collects and analyzes radiation in the radio frequency range from stars and other bodies in space |
| Space Probes | unmanned satellites or remote controlled 'landers' used to explore areas or objects in space that are too difficult or dangerous to send humans to |
| Triangulation | a method of indirectly measuring distance between an observer and an object whose distance away is to be estimated |
| Parallax | the apparent shift in position of a nearby object against a distant background when the object is viewed from 2 different positions |
| Doppler Effect | the apparent change in frequency of sound, light, and other waves as the observer and the wave move towards or away from each other |
