Glossary of Astronomical Terms

As with many science-related hobbies, there are a few new words you'll have to add to your vocabulary. This glossary is not meant to be an exhaustive list of techno-definitions. Rather, it is designed as an easy-to-access aid for novices to help them grapple with the terminology of astronomy.

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Do you know how to pronounce the CONSTELLATIONS, STARS, or the PLANETS and their MOONS?

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A - C
D - F
G - I
J - M
N - R

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S TUVW XYZ

S

SATELLITE: An object that revolves around a planet.

SCHMIDT-CASSEGRAIN: See catadioptric.

SIDEREAL DAY: The length of a planet's rotation period relative to the stars.

SOLAR SYSTEM: The sun and all the objects that are held in its gravitational field and move around it.

SOLAR WIND: Charged particles from the sun moving out in every direction at high velocity.

SPHERICAL ABERRATION: A problem where a lens or mirror in a telescope is not shaped correctly, so the light from the center is focused at a different location than the light from the edges. You should never have to worry about this. This usually shows up only in really cheap telescopes.

SPOTTING SCOPE: A small telescope, always a refractor or catadioptric, generally used for terrestrial viewing. Of limited utility for astronomy, though many are marketed as such. Probably the wrong choice unless you want to use it also for bird watching, or as a powerful telephoto lens on a SLR camera.

SUPERIOR CONJUNCTION: The position of a planet when it is on the opposite side of the Sun from Earth (thus making in invisible).

SUPERNOVAE: A rare celestial phenomenon involving the explosion of most of the material in a star, resulting in an extremely bright, short-lived object that emits vast amounts of energy.

SURFACE:The outside of any solid object.

SURFACE GRAVITY: The strength of the gravitational pull on the surface of an object.

SYNODIC PERIOD: The time from one conjunction of a planet to the next conjunction.

T U V W

TERRESTRIAL PLANETS: The smaller planets: Mercury, Venus, Mars, Earth. Pluto is also considered a terrestrial planet by some.

TIDAL EFFECT: The pull of a body on various parts of another body that might slow down its rotation, round the orbit, alter the distance of that body, or any combination of the three.

TRANSIT: The movement of an object between the Earth and the Sun and showing on the surface of the sun as a black dot.

TROJAN ASTEROIDS: Asteroids traveling in Jupiter's orbit, some in front, some behind the planet.

TRUNNION: A pin or grudgeon, especially either of two small cylindrical projections forming an axis on which a Dobsonian-mounted telescope (or a canon) pivots. If you've ever seen a Dob, then you know how much it looks like a canon!

WATER (H₂O): A substance made up of two hydrogen atoms and one oxygen atom.

WEDGE: This is the thing that a fork-mounted Schmidt-Cassegrain telescope will attach to, to connect it to the tripod. You want it to be sturdy.

WHITE DWARF: A tiny, collapsed star, with the size of a planet but the mass of a regular star.

WORM DRIVE: This is the sort of drive most telescopes come with, if they come with a drive. It is a very accurate and smooth drive. However, due to imperfections in the manufacturing process, there will be periodic errors that occur at the same point in every worm cycle (usually about 8 minutes). To deal with this, higher end telescopes come with drives which compensate for the mechanical defects.

X Y Z

UMBRA: The shadow of the primary body.

VERNAL EQUINOX: The first day of spring. The day in which the sun appears to move from the southern hemisphere to the northern hemisphere. The apparent ascending node of the sun as viewed from earth. This occurs around 21 March. On this day the earth is aligned with

ZENITH: The sky directly overhead. An object "transits" when its line of right ascension crosses the zenith. The opposite of zenith is nadir.

ZENITHAL HOURLY RATE or ZHR: The zenithal hourly rate; used to measure the intensity of a meteor shower. Defined as the number of meteors seen per hour with the naked eye under ideal conditions, assuming the shower is radiating from the zenith . For showers that don't radiate from the zenith, a correction factor can be used to adjust one's meteor count to a true ZHR.

ZODIAC: Comprised of the twelve constellations that lie closest to the ecliptic line. The planets, sun, and moon stay within the zodiac band as they move in the sky.