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?

A - C
D - F
J - M
N - R
S - Z

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G H I

G

GALAXY: A galaxy is made up of billions or trillions of stars bound together by their own gravity. All of the stars we see in our night sky belong to our own Milky Way galaxy. Most galaxies external to our own require a telescope or photography to see clearly. There are some exceptions; most notable are the Large and Small Magellanic Clouds (visible from the southern hemisphere) and the Andromeda Galaxy -- all of which are visible to the naked eye.

GALILEAN SATELLITES: The four large satellites of Jupiter: Io, Europa, Callisto, and Ganymede, so named after Galileo Galilee who discovered them.

GEOSTATIONARY: An orbit whose orbit lies in the equatorial plane (inclination = 0) and whose period is equal to the rotational rate of the earth. Geostationary satellites' groundtracks are points.

GEOSYNCHRONOUS: An orbit whose period is equal to the rotational rate of the earth. Geosynchronous satellites' groundtracks are figure eight shaped.

GERMAN EQUATORIAL MOUNT: The first equatorial mount devised and still the most common for small to moderate sized reflectors and refractors. Unlike the equatorial fork, the German equatorial is suitable for telescopes with either short or long tubes (although, if poorly designed, a long tube may strike the tripod, preventing viewing at the zenith). They usually are designed with movable counterweights, which make them easy to balance, but heavy and bulky.

The tube of the telescope is joined to a shaft (the Declination shaft or axis) which rotates in a housing that in turn is joined at right angles to another shaft (The polar axis). The polar axis is pointed at the celestial pole (just like any other equatorial mount). A counterweight, which is required for balance, is placed on the other end of the declination shaft.

Tracking an object past the zenith requires that the telescope be turned (both Right Ascension and Declination rotated through 180 degrees ), which reverses the field of view. Not so much a problem for visual astronomy, but a limitation on astrophotography.

GIBBOUS: A shape similar to the Moon when it is more than half lit but less than a full.

GRAVITATION: Traditionally, an attraction exerted by one object on the other objects in the universe. Albert Einstein, however believed that it is caused by the curvature of space time. For every day purposes, the first definition will do, though.

GREATEST ELONGATION: When a planet is the furthest from the sun in the sky, usually making it the easiest to observe.

GREENHOUSE EFFECT: A warming effect that follows when light and heat can pass through a barrier into an object, but not out again.

GREENWICH MERIDIAN: See: Prime Meridian

H

HELIUM (He):The second element in the periodic table, has 2 protons, 2 neutrons, and is a noble gas. It has an atomic mass of 4.00260 amu, and is inert. Helium was discovered in 1825 by Sir W. Ramsay of England, and N. Lenget and P.T. Cleve of Sweden. Helium melts at -272.2 degrees Celsius, and boils at -268.934 degrees Celsius. Helium has a density of 0.1785 g/L at STP (20 degrees C, 1 atm)

HYDROGEN (H): The first element on the periodic table, is a gas, and has an atomic mass of 1.00794 amu. It was discovered in 1766 by Henry Cavendish of England. Hydrogen is also special because it is the most common element in the universe. "Hydrogen" is Greek for "forming water," as hydrogen makes up 11% of water. All heavier elements are built from hydrogen, as well as helium. It is debated whether hydrogen is a metal or a nonmetal, and has been placed both above lithium and next to helium in the periodic table, while sometimes it is off on its own. Hydrogen melts at -259.34 degrees Celsius, and freezes at -252.87 degrees Celsius. The density of Hydrogen at STP (20 degrees C, 1 atm) is 0.0899 g/L

HYDROGEN SULFIDE (H₂S): Hydrogen Sulfide is a colorless, very flammable gas. In low concentrations it smells like "rotten eggs" however the sense of smell is lost after 2-15 minutes of exposure making it impossible to smell dangerous concentrations. It is heavier than air. It is considered to be a very toxic gas. When H₂S burns it produces another very toxic gas - Sulfur Dioxide (SO₂). H₂S is used in metallurgy, the preparation of phosphorous and oil additives, as well as a reagent in chemical analysis. During the recovery and processing of crude oil, H₂S can contaminate the atmosphere and become a major health hazard

HYDROSPHERE:The layer of liquid water on the surface of a planet.Hydrosphere is the study of water and how it interacts with the atmosphere and the surface of a planet.

I

INCLINATION: The angle between the plane of the orbit of a planet and the ecliptic.

INFERIOR CONJUNCTION: The position of a planet when it is between the Sun and the Earth.