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Absolute Magnitude

A scale for measuring the actual brightness of a celestial object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth.

Angular resolution

The ability of an instrument such as a telescope, to distinguish objects that are close to each other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two neighboring objects as two separate objects. The angular resolution of the human eye is about a minute of arc, while it can get up to 0.7' arc min for people with an extreamly good eyesight.

Angular Size

The apparent size of an object in the sky, or the distance between two objects, measured as an angle. Your index finger held at arm’s length spans about 1°, your fist about 10°. 

Aperture
מפתח הטלסקופ. ראה הרחבה במאמר לבחירת טלסקופ.
Aperture

The diameter of a telescope’s main lens or mirror — and the scope’s most important attribute. As a rule of thumb, a telescope’s maximum useful magnification is 50 times its aperture in inches (or twice its aperture in millimeters).

Aphelion

The point in its orbit where a planet is farthest from the Sun.

Apogee

The point in the orbit of the moon or a satellite, at which it is furthest from the Earth

 

perigee versus apogee

Illustration: perigee ‘Supermoon’ versus an apogee ‘Minimoon’

Apparent Magnitude

The apparent brightness of an object in the sky as it appears to an observer on Earth.

Arc minute

One arc minute is 1/60 of a degree of arc. The angular diameter of the full moon or the Sun as seen from Earth is about 30 arc minutes - 1/2 arc degree.

Arc second

One arc second is 1/60 of an arc minute = 1/3600 of an arc degree. The angular diameter of Jupiter varies from about 30 to 50 arc seconds, depending on its distance from Earth.

Asteroid

A small planetary body in orbit around the Sun. Smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets.

Asteroid belt

A region of space between Mars and Jupiter where the great majority of asteroids is found