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. Hypertext links (blue words) throughout the CAS web pages are directed to this glossary page as needed.

APERTURE: The diameter of a telescope's primary (largest) optical surface (lens or mirror), usually expressed in inches or meters. Larger apertures allow the telescope to collect more light and see fainter objects.

ARC MINUTE; ARC SECOND: These are finer divisions of a DEGREE, which is the primary unit of measure in the sky. A degree is subdivided into minutes and seconds, much like an hour is. Thus, 60 arc minutes equals one degree and 60 arc seconds equals one minute. The size of a planet in the sky is usually given in arc seconds (Ex: Jupiter is about 38 arc seconds in diameter).

ASTROLOGY: Ask Nancy Reagan.

CONJUNCTION: When two or more celestial objects are very close together in the sky, they are said to be in conjunction. They appear close only because of our perspective, or line of sight. In reality, they are usually very far apart in space. When one object actually crosses in front of the other, it is called an OCCULTATION.

CONSTELLATION: An apparent pattern, or grouping, of stars in a region of the sky that man has given a name. Modern astronomers maintain these ancient names and boundaries as a convenient way of mapping the regions of the sky.

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). A handy gauge for estimating distances in the sky in knowing that your outstretched hand (lengthwise) takes up about 5 degrees in the sky. The moon and sun are almost exactly 1/2 of a degree (or, 30 ARC MINUTES) in diameter.

ECLIPSE: In principle, the same as an OCCULTATION, though the term is most often used for lunar/solar events.

ECLIPTIC LINE or ECLIPTIC: 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.

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.

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.

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

LIGHT POLLUTION: The illumination of the night sky caused by artificial light sources on the ground (streetlights, billboards, etc.). Both the light and the loss of contrast make it difficult to find fainter stars and nebulae. This is becoming a bigger problem for both amateur and professional astronomers.

MAGNITUDE: Astronomers use magnitudes to describe the brightness of an object in the sky. It is a non-linear, numerical scale where the smaller numbers represent brighter objects. Generally, the unaided eye can see stars down to magnitude 6 in good skies. Pluto is magnitude 14, and thus, is invisible without optical aid or photography. Venus has a magnitude of about -4 and the Sun is -27.

MERIDIAN: An imaginary line in the sky passing from the due south horizon, through the ZENITH, and on to the due north horizon. Objects are highest in the sky, and thus least affected by atmospheric distortions, when on the meridian.

NEBULA (plural NEBULAE): The term originally applied to any extended (i.e., fuzzy, non-stellar) object in the sky. More recently, it is used to describe clouds of gas in space. Some nebulae are illuminated by nearby stars (bright nebulae), while others remain dark and are only seen if they obscure a brighter object (dark nebulae).

OCCULTATION: When one celestial body passes in front of, or covers, another in the sky, it is called an occultation. A solar eclipse is really just a specific type of occultation (the moon occulting the sun).

OPPOSITION: When a planet is exactly opposite of the sun in the sky. This means that the object will cross the meridian exactly at midnight. Planets are usually best viewed when at opposition. Venus and Mercury never reach opposition because their orbits lie closer to the sun than Earth's.

REFLECTING TELESCOPE: A telescope that uses a mirrored, concave, surface to bring the image to a focal plane. The most common type is known as a "Newtonian" reflector - named after Sir Isaac himself. These are popular with amateurs due to their low cost, ease of construction, and good images.

REFRACTING TELESCOPE: A telescope that uses a clear (glass) objective to refract, or bend, the light to form an image at the focal plane. Binoculars are really just two small refracting telescopes. Small astronomical refractors are common, but larger ones (4"+) can get very expensive.

SEEING: This term has a specific meaning when referring to observational astronomy, and it is a description of the stability in the atmosphere. Thermal turbulence in the atmosphere is usually the main factor that limits the resolution, or fine detail, that can be seen on the planets or the moon. When the seeing is good, planetary detail is distinct and has better contrast. Bad seeing is characterized by a dynamic blurring, or even severe motion of the entire planetary disk. Seeing can be specified on a 1 to 10 scale called the Pickering Scale, with a 10 being the best..

ZENITH: The point in the sky that is directly overhead for the observer.

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.