Know your constellations

July 30, 2015

Being able to recognise and point out constellations is a sweet skill that anyone can learn.

The bad news is that learning all the constellations is ridiculously difficult. The good news is that you don't need to. If you learn the three easiest constellations you'll always be able to find at least two of them and that's probably enough to impress your friends.

The constellations to learn are Orion, Scorpius and the Southern Cross (the astronomical name for the cross is "Crux").

Orion and Scorpius move a lot throughout the year and are not always visible from New Zealand, so it's easiest to start with Crux which is always visible. It's a small constellation accompanied by two "pointer" stars that make a line to the top of the cross.

Different constellations move in different ways as seen from Earth. Crux appears to move in a circle around a fixed point in the sky known as the South Celestial Pole. This point is directly south and about half way between the horizon and directly overhead (zenith).

You can imagine Crux being the hour hand of a clock, in fact if you know the annual cycle well enough you can use it as a clock.

At this time of year Crux is visible near the top of the circle in the early evening, at which point it is more or less the "right" way up. Throughout the night it travels clockwise half way round the circle, flipping over as it goes until it's upside down at the bottom of the circle around 5 am. During the day it travels back to the top of the circle.

In addition to the 24-hour cycle, each night these positions all move slightly clockwise as part of the annual cycle.

In order to be able to find constellations easily you need to practice. Make a habit of looking up every time you go out at night. Look towards the south, find the cross, and imagine the circular path it travels. The more often you see it in different positions, the easier it becomes.

Your Night Sky, July/August 2015