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
G - I
J - M
N - R
S - Z
D E F
DEEP SPACE OBJECT (DSO): Galaxies, star clusters, and gaseous nebulae generally fall into this category. Almost all DSOs require a telescope to see in any detail at all, though some can be detected with just binoculars, and a few can be barely seen with the unaided eye. Many DSOs are only fully revealed through long-exposure photography.
DEGREE: Distances on the sky (sometimes called the celestial sphere) are measured in angular degrees rather than inches or feet. It is 180 degrees from one horizon to the opposite horizon (90 degrees from horizon to the zenith).
Here are some handy gauges for estimating distances in the sky:
The moon and sun are almost exactly 1/2 of a degree (30 arc minutes) in diameter.
DENSITY: The mass of an object divided by its volume.
DIAMETER: The length of a line passing across the widest part of a circle or a sphere and through the center of the circle or sphere (as confusing as that may sound).
DIRECT MOTION: Moving counterclockwise (common sense tells us that clockwise motion should be direct but that is untrue).
DOBSONIAN: Named for John Dobson of The San Francisco Sidewalk Astronomers (who prefers to call these "Sidewalk Telescopes"), this is a design which allows for very large apertures at very affordable prices. The trade-off is that they are mounted on altazimuth mounts, instead of equatorial ones, which makes them essentially useless for astrophotography, but an inexpensive alternative if you only plan to do visual work. These are light buckets. If you are planning to build your own telescope, you might want to consider a Dobsonian. Note: That this design is now the #1 Design seen at many Star parties.
EARTH-GRAZER: An asteroid that approaches Earth more closely than Venus ever does. Such asteroids can be potentially dangerous.
ECCENTRICITY: Degree to which the ellipse of an elliptical orbit is flattened.
ECLIPSE: The covering from view of an object in the sky by another object that moves in front of it. In principle, the same as anoccultation, though the term is most often used for lunar/solar events.
ECLIPTIC or ECLIPTIC LINE: An imaginary line in the sky that represents the plane of the earth's orbit around the sun. Since it closely matches the plane of the solar system itself, most all solar system objects (planets, asteroids, moon, sun, etc.) will appear close to the ecliptic in the sky. The ecliptic line passes through all twelve constellations of the zodiac.
ELECTROMAGNETIC FORCE: Basically, it's the electrical force by which an electron is attracted to an atomic nucleus but repelled by another electron.
ELECTRON: A tiny, negatively charged particle in the outer region of an atom.
ELLIPSE: A plane curve that looks like a flattened circle.
ELONGATION: This is the distance a planet is from the sun in the sky, and is expressed in degrees . As an example, a planet with an elongation of 180 degrees would be exactly opposite the sun in the sky (i.e., one setting in the west while the other rises in the east). A planet with a 0 degree elongation would be lost in the glare of the sun, and thus, invisible. Greatest elongation is used to describe the inner planets (i.e., Mercury, Venus) when they are farthest from the sun, and thus, most easily visible in the dawn or dusk sky.
EQUATOR: A circumference that lies halfway between the two poles of a spinning object.
EQUATORIAL BULGE: The extra thickness of a planet in the equatorial regions due to the centrifugal effect of its rotation.
EQUATORIAL MOUNT: An equatorial mount is set to the current latitude, and is polar aligned (pointed at the North Pole in the Northern Hemisphere, the South Pole in the Southern Hemisphere) and then moves only in Right Ascension and in Declination. This takes a while to get used to, but offers the wonderful side effect of being able to track the astronomical objects you are looking at as they move across the sky (which is very visible motion at telescopic magnifications) by moving in only one direction (Right Ascension). Most equatorial mounts come with motor drives that take care of this for you.
EQUATORIAL PLANE: The projection of the earth's equator out into space
ESCAPE VELOCITY: The speed which an object must move in order to escape the gravitational pull of another object, such as a planet or a star.
EXIT PUPIL: This refers to how wide the beam of light exiting the eyepiece is, and is equal to the aperture divided by the magnification. If it is bigger than the size of your pupil in the dark (7mm when you are young, 5 or 6mm when you are over 40, as a general rule) you will not be taking in all the light available, effectively, you will be using a smaller aperture telescope than you have.
EYEPIECE: This is the thing you actually look into. Almost all telescopes separate the Optical Tube (the telescope proper) from the eye piece. Essentially, the telescope makes a really tiny image of what it's pointed at. The eyepiece acts as a magnifying glass to allow you to see the image bigger than it would otherwise be. The magnification is the focal length of the telescope divided by the focal length of the eyepiece. Eyepieces are described by the diameter of the barrel, always expressed in inches (.965", 1.25" and 2" are the sizes in common use) and the focal length always expressed in millimeters (4mm - 40mm is the usual range). Short focal length eyepieces are also termed high power, long focal length are low power.
Also significant with eyepieces is the apparent field of view (expressed in degrees) and eye relief (expressed in millimeters). The apparent field refers to how big the circle of space you see in an eyepiece appears. Bigger is better. Eye relief is a measure of how far from the eyepiece you can have your eye and still see. If you wear glasses to correct astigmatism, you will need fairly long eye relief (the focus knob will correct for almost all vision problems except astigmatism).
There are several types of eyepiece designs. The most popular are Kellner (inexpensive, most popular for cheap telescopes, short eye relief and narrow fields of view. Good to avoid if you can afford better); Orthoscopic (good price/performance compromise); Erfle (wide field of view, expensive); Plossl (perhaps the best all-around eyepiece. Some moderately expensive versions available); and Ultra Wide (very expensive, almost double the number of lenses as other designs makes for more light loss in the eyepiece, large exit pupils. Can cost more than a small telescope. Not a good place to spend your money when you are just starting out).
F/10, F/6.3: See Focal Ratio
FINDER SCOPE: The finder scope is a low power telescope attached to the telescope you are using. Because most telescopes show such a small portion of the sky, it is virtually impossible to locate anything just by looking through them. So you look through the finder scope to center the object you want (the finder has crosshairs) and then you can use your real telescope on it. Note that you can ignore all the claims about big finder scopes. You almost certainly don't care. All you need is to be able to point your main telescope at something in the sky. Finder scope size only matters when you are starhopping through fairly dim stars (where the larger aperture allows you to see dimmer stars). This will not be an issue for you for quite a while (if ever). Many people use a Tetrad sight, which is simply a red LED you can sight on- you get absolutely no more aperture than your naked eye. The finder scopes are usually advertised as 8x50 (or such). The eight refers to the magnification, the 50 to the aperture in millimeters-just like binoculars.
FIRST POINT of ARIES(): Arbitrary direction in which the inertial axis points for the Solar System. 2000 years ago on the vernal equinox the sun rose in the constellation Aries. The zodiacal sign for Aries is and for historical reasons the symbol of the Ram continues to be used to define the principle axis in celestial coordinate systems.
FOCAL LENGTH: This is the length of the light path, from the objective to the focal plane. The magnification is the focal length of the telescope divided by the focal length of the eyepiece. See also focal ratio below.
FOCAL PLANE: The plane that the telescope (or eyepiece) focuses on. When you turn the focus knob on the telescope, you are moving the eyepiece back and forth until you make the two focal planes coincide.
FOCAL RATIO: Also referred to as the "speed" of the telescope, is the ratio of focal length to aperture, and is always expressed as an f/number. Thus an 8" telescope with a 2000mm focal length is f/10 (because 8" is 200mm, and 2000 / 200 = 10). An f/10 telescope is "slower" than an f/4.
Fast telescopes give wider, brighter images with a given eyepiece than slower ones (but note that at a given magnification, the images are-assuming identical optics-exactly the same: what you see through a f/6.3 telescope with a 12mm eyepiece is identical in width and brightness to what you would see through a f/10 telescope with a 19mm eyepiece).
In general, the slower the telescope the more forgiving it is of optical errors in the objective and eyepiece. A telescope of f/10 is fairly forgiving, f/6.3 much less so.
FOCUS (plural, foci): One of the two points inside an ellipse. The two foci are at equal distances from the center of the ellipse and on opposite sides, along the major axis.
FOCUS: To adjust the setting of a lens (or eyepiece) to produce a clear image.
FOCUSER: This is the thing that holds the eyepiece. It moves in and out so you can focus the telescope. It is always included with the telescope when you buy one. The size, almost always .965", 1.25" or 2" refers to the barrel diameter of the eyepieces it accepts.
FORK MOUNT: A fork mount is a type of mount where the telescope is held by two arms, and swings between them. A fork mount can be either alt-azimuth or equatorial (through the use of a wedge). Fork mounts are most commonly used with Schmidt-Cassegrain telescopes, and are almost always equatorial.
FREE RADICAL: A molecule that does not have all the electrons between its atoms paired, but has one or more unpaired electrons.