Apparent Magnitude

Apparent magnitude describes how bright a star appears. Sticking to the convention of the celestial sphere, apparent magnitude assumes all stars are the same distance away from the earth. Absolute magnitude is how bright a star really is and some of our dimmer stars are really very bright, but very distant, stars.

The brighter the star or planet the smaller the magnitude number. Some are so bright that they have negative magnitudes. Jupiter has a magnitude of between -2.1 to -2.6, Venus is -4.4 to -4.7 and the sun -27. The 6000 stars visible to the naked eye have a magnitude of 6.2 to 6. Not all are visible at the same time. On a good night you might see about 2000 stars with the naked eye.

When stars are low in the sky their light travels a long way through the earth's atmosphere. The scattering of their light by the atmosphere is at its greatest and they look quite dim (see Figure 8.17). As they travel higher in the sky their light takes a shorter path through the air and the stars appear brighter. Do not confuse this with a star's magnitude.

8.17 A Star's Brightness Varies with its Altitude and Visiblity Planets

Planets lie in a great arc across the sky with the sun and the moon and do not twinkle like stars. Not all planets are visible at the same time. Seen through moderately powered binoculars, they appear as small discs.

Jupiter and Venus are always much brighter than the stars, and a bright star seen occasionally near the morning or evening sun is as likely to be Mercury as Venus.

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