Constellations in the northern hemisphere
Firstly, we need to be aware that the North Star, or Polaris, is unique in the night sky as it sits almost exactly at celestial North Pole. Unlike any other star, the sun or the moon, the North Star is always in the same position in the sky. It therefore offers an excellent way of determining the cardinals of the compass at night. The North Star is a moderately bright star with an apparent magnitude (brightness) of 1.97, however it is easy to find because it is by itself in a black patch of sky. If you focus on the patch, your eye will automatically fix on the North Star. To signpost our way to it we use one or more of the five circumpolar constellations (a constellation being a group of stars forming a pattern or shape which can be made out in the night sky). The five are:
• The Big Dipper/The Plough (Ursa Major)
• The Little Dipper (Ursa Minor)
• The King (Cepheus)
• The Queen (Cassiopeia)
• The Dragon (Draco)
These constellations rotate anti-clockwise around the North Star and never set, meaning that they are always visible on a clear night.
Finding True North
1. Find and face The Big Dipper. Draw an imaginary line from the so called ‘pointer stars’ that form the leading edge of the dipper.
2. Continue in a straight line, about five times the distance between these two stars, until you get to the next bright star (it will be slightly right of your imaginary line); you have found the North Star.
3. Confirm this is the North Star by finding The Queen (Cassiopeia) which is opposite the Big Dipper (it has five stars that form a shape like a ‘W’ on its side).
4. Directly facing it, point up to it with both arms outstretched, hands together. Slowly lower your locked arms to the horizon – this is true north.
Near the Equator the North Star can sometimes be below the horizon. Then The Queen (Cassiopeia), since it is on the other side of the pole, will be up. An imaginary line that bisects the angle between the more open of the two V’s comprising the letter W of Cassiopeia, points to the North Star.
Determining Magnetic Declination Using The North Star
In the northern hemisphere, if you can see the North Star, the declination can be determined as the difference between the magnetic bearing and the visual bearing on the polestar. The North Star currently traces a circle 0.75° in radius around the north celestial pole, so this technique is accurate to within a degree. At high latitudes, nearing the North Pole, a plumb-line is helpful to sight the North Star against a reference object close to the horizon, from which its bearing can be taken.
Constellations in the southern hemisphere
The North Star is not visible anywhere south of the equator. In addition, there is no direct equivalent of it in the night sky, so determining south is a little more complicated. Fortunately, there is the Southern Cross, which includes one of the southern hemisphere’s four first-magnitude (very bright) circumpolar stars, Acrux. From latitude 33° S southwards, the cross is above the horizon at all times.
Finding The South
There are three ways to find due south (also called celestial south) using the Southern Cross constellation. Finding the Southern Cross for the first time can be tricky as it is relatively small and looks more like a kite than a cross. In addition, there are a few other constellations which look like crosses. Fortunately, Rigel Kentaurus (Alpha Centauri) and Hadar (Beta Centauri), a pair of adjacent pointer stars, help to confirm that you have located it. No other cross has a similar signpost.
1. Draw an imaginary line between the stars Acrux and Achernar (see diagram), both of which are first-magnitude.
2. Due south is halfway along this line.
3. Directly facing it, point your hands, one at Achernar and the other at the closest of the pointer stars to the Southern Cross (Beta Centauri).
4. Bring your hands steadily together until they meet. They are pointing at the celestial South Pole. Drop your hands straight down to the horizon. That marks due south from where you are.
Constellations visible in both hemispheres
The only constellation that can be used for navigation, in either hemisphere, is Orion, sometimes called The Hunter; it is located on the celestial equator and visible for most of the year in both hemispheres. Orion is also permanently visible from the Equator and is one of the most recognisable and visible constellations in the night sky.
1. Find Orion and locate Orion’s Belt – this is 3 stars in a row.
2. Above the belt three stars form his shoulders and neck – the middle star is called Meissa.
3. From the centre star on his belt, called Alnilam, draw an imaginary line that just goes to the right of Meissa.
4. This is the general direction of True North.
Planets in our solar system are not used for this type of navigation, despite reflecting the sun’s light and the fact they can look like stars. The way to tell the difference is that planets, which are far closer to us, do not twinkle. The twinkle is the result of particles interfering with the light from the star as it travels millions/billions of kilometres.