Astronomy Lesson

This is 4-hour nighttime exposure of the southern sky at Bea Creek camp, just east of Tom Price, in Western Australia. Set the camera up, clicked the button, and went to bed.

It appears as if all of the stars in the night sky are rotating around the Earth. In actuality, it is the Earth which is rotating underneath the stars. But since my camera is fixed to Earth, we get the illusion of celestial rotation. Since the stars made about 1/6 of a circle in the sky, the exposure lasted 1/6 x 24 hours = 4 hours. The shutter stayed open until the batteries gave out.

The color and capturing the Southern Celestial Pole in the sky (the spot around which all of the stars seem to be rotating) were beautiful surprises. If you look carefully, you can spot the Southern Cross (Crux).

The Southern Cross (in green) is one of the most recognizable constellations in the southern hemisphere. You’ll find it on the flags of Australia and New Zealand. In the picture, the Southern Cross is on its side. The top of the cross is at about 4 o’clock, while the bottom is at about 10 o’clock.

With the lack of a significant pole star in the southern sky (Sigma Octantis is closest to the pole, but is too faint to be useful for the purpose), two of the stars of the Southern Cross (Acrux and Gacrux) are commonly used to mark south. Following the line defined by the two stars for approximately 4.5 times the distance between them leads to a point close to the Southern Celestial Pole.

Explorers used to use the Southern Cross for navigation. To find due south, you can:

1. Draw an imaginary line between the two long axis stars of the Cross. Extend this line four and a half times times “down” (to lengthen the longer lower leg of the cross). You are now at the South Celestial Pole. Drop directly down to the horizon. This marks due south.
2. Draw an imaginary line between the two long axis stars of the Cross and continue it. Connect a line between the two pointer stars, then draw a long line at right angles to the center of this line. Extend this long line to where it would intersect the line from the Southern Cross. Where these two lines cross, (the South Celestial Pole), drop to the horizon. This marks due south.
3. Get comfortable and watch the stars for about 3 hours. Find the spot on the sky around which all the stars seem to be rotating. Draw a line directly down to the horizon from the south celestial pole. This marks due south.

Alpha Centauri (purple) and Beta Centauri (blue) are also visible. As a pair, Alpha and Beta Centauri are called the “pointer stars” because they point to the Southern Cross.

Alpha Centauri is actually a star system made up of two stars, Alpha Centauri A (slightly larger than our Sun), Alpha Centauri B (slightly smaller than our Sun). Near these two stars is a third star, Proxima Centauri, which is too small and dim to be visible to the naked eye. At 4.24 light years from the Sun, Proxima Centauri is the closest star. It has been the closest for about 32,000 years and will be so for about another 33,000 years, when a star called Ross 248 will pass closer.