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Terms and Queries


Dark adaption.
When you first wander out into the night, your eyes are still used to the light indoors. After a while, you will notice that you can see more stars. This process of increased sensitivity is called dark adaption. After a sufficently long time you can see quite dim objects. However, even brief exposure to white light (like a torch, or a neighbour turning on an outside light), can ruin your dark adaption.

How do I find east (west, south, north)
How do I find east (west, south, north). Strangley, the simplest way may be a street directory. Street directories have compass bearings on them, and you can work out the orientation of your local landmarks from that. For those in the bush, ordinance maps and compasses work well.

You can use the position of sunrise and sunset, but this is actually complicated. East is more or less where the sun rises, and west is more or less where the sun sets. If you can locate the position of Sunrise and Sunset, then if you stand with your right hand pointing to the west (sunset) and you left to east (sun rise) you are facing south, and north is to your back. However, sunrise and sunset are offset from due east by an amount that varies with lattude and season. In December, sunset is nearly 6 handspans to the left of due west in Melbourne, and 3 handspans to the left of due west in Darwin. In July, sunset is over 4 handspans to the right of due west in both Melbourne and Darwin. In May and October sunset in Melbourne is closest to due west.

Of course, once you can find the Southern Cross, five cross lengths from Acrux (the brightests star of the cross) along the long axis of the cross brings you to the south celestial pole. Alternately, you can use the intersection of the line drawn through the long axis of the cross, and a line drawn perpendicular to a line between the two pointers from halfway between them.

An index of brightness of the stars, with the higher numbers being fainter. Each unit of magnitude increase roughly corresponds to a 2.5 times fall in brightness. The faintest visible stars are 6.0, and rarely with very dark skys and ideal conditions 6.5. The brightest stars, excluding the Sun, are of magnitude -1.0. The planets can be brighter still, with Venus reaching -4.0. For handy examples, let's look at the pointers and the Southern Cross. Alpha Centauri is mag -0.1, beta Centauri is magnitude 0.6 alpha Crucis is 0.9, beta Crucis is 1.3, gamma Crucis is 1.7, epsilon Crucis is 3.4 and iota Crucis is 4.7.

Meteors should not be confused with meteorites. Meteors are the objects that flash across the sky and meteorites are objects that have fallen to the ground. The origin of most meteor shows is the debris from old comets. In contrast, most meteorites are the debris from collisions of objects in the asteroid belt. There is little if any evidence that any meteorites result from meteor showers. The words meteor/meteorite are commonly interchanged but it is good to remember they mean different things. (Thanks to Glenn Moore for this bit).

When one planetary body passes in front of another this is occulation. Occulations of planets and naked eye stars by the moon are relatively uncommon and quite interesting to watch. These events are not only interesting for their rarity, but also provide valuable scientific information.

Why the red cellophane over the torch?
If you want to write notes to yourself (like how many meteors per hour you have seen), use the Southern Sky Watch guide or read a Sky Atlas, you want to be able to use a light without destroying your night vision. Red light is great for this, and simply taping some red cellophane (or any other red, transperent film) over your torch is a simple way to achieve this.

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Created: Thursday, 2 April 1998, 5:37:47 PM
Last Updated: Thursday, 2 April 1998, 5:37:47 PM