## Finding The South Pole

In the southern sky, the southern Cross acts as a pointer to the celestial pole. Extend the long axis of the Cross fivefold to reach the southern pole, an area bereft of any stars of note. As a further rough guide, the south celestial pole forms a triangle with the bright stars Canopus and Achernar.

1 First, locate the familiar shape of Crux, the Southern Cross (highest on April and May evenings). It is one of the smallest constellations but also one of the most distinctive. Then locate the two brightest stars in Centaurus, Alpha and Beta Centauri, known as the Southern Pointers.

2 Imagine a line connecting the top to the bottom of the Southern Cross. Now imagine that line extended by five times. Next imagine a straight line going from halfway between the Pointers to cross the line leading from the Southern Cross. The pole lies where the two lines intersect.

angles in the sky

The position of an object in the sky can be described by its altitude and azimuth. Its altitude is its distance above the horizon in degrees, 0° being on the horizon and 90° directly overhead. Azimuth is the object's compass bearing from north. Due north is azimuth 0°, due east is 90°, due south is 180°, and so on back to north. An object's altitude and azimuth change as Earth rotates. A planetarium-type computer program will tell you the altitude and azimuth of an object at any given time and from any desired location.

MEAsuRiNG ALTiTuDE

You can learn to estimate the altitudes of celestial objects above the horizon with your arms. The altitude of the object shown here is 45°, halfway from horizon to zenith (the point directly above you).

### MEAsuRiNG AziMuTH

Azimuth is measured clockwise around the horizon from north, and can also be estimated with your arms. The object shown here is in the northeast at an azimuth of 45°. If it were in the northwest, its azimuth would be 315°.

sizing things up

The sizes of objects on the celestial sphere, and the distances between them, are measured in degrees and parts of a degree. A rough-and-ready ruler to help size things up is literally on hand. Your finger at arm's length is about 1° across, more than enough to cover the half-degree width of the Sun or Moon. A hand at arm's length has a width of about 10°, while a hand with splayed fingers is about 16° wide. Everyone's hand is different, so measure up the distances between bright stars and the sizes of various constellations for yourself.

FiNGER-wiDTH

The Moon or Sun, both of which are only half a degree across, can easily be covered by an index finger at arm's length.

half-handspan

The back of a closed hand is about 10 degrees across, the measure ofthe Big Dipper in the night sky.

whole handspan

An outstretched hand with splayed fingers (here seen against the Square of Pegasus) is about 16 degrees across.

star brightness

The brightness of a star as it appears in the sky is termed its apparent magnitude, and this is shown on star charts by dots of different sizes. Star brightness can now be measured very precisely, but originally astronomers divided stars into six broad groups, from 1st magnitude (the brightest) to 6th magnitude (the faintest visible to the naked eye).

### CLASSiFiCATiON

Each step on the magnitude scale is equal to a brightness difference of about times, so a 1st-magnitude star is 100 times brighter than a 6th-magnitude one. Stars brighter than 1st magnitude are given zero or even negative values. Sirius, the brightest star, is of magnitude -1.44. A star's apparent brightness is affected both by its actual light output and by its distance from us.

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