Probably the simplest way to use the stars to find north is by finding Polaris, also known as the Pole Star or the North Star. Polaris isn’t a particularly bright star but it has the distinct advantages of being over the north pole and not appearing to move in the night sky, 2 features that we can use to our advantage.
I say ‘appears’ not to move, because of course it is moving. First off we need to remember just how far away Polaris is from us, a colossal 434 light years away; that’s 27 million times as far away from us as we are from the sun! So no wonder it isn’t particularly bright from our vantage point.
The reason it doesn’t appear to move is because the Earth’s rotational axis stays the same. It points to the same region in space, so the north pole always points to the same spot, by and large where Polaris is located. And because Polaris is rotating around the galactic centre at almost the same speed and angle as our own sun, it stays in the same relative position, and thus giving the appearance of being stationary.
We can use 2 constellations to help us find Polaris, Cassiopeia and The Plough.
The Plough is actually an asterism (a pattern of stars) rather than a constellation. Regardless of astronomical terminology, for many it is one of the most familiar objects in the sky and one that many of us can recognise from its shape (although I always think it looks more like a shopping trolley with really long handles!).
Bear in mind, however, that The Plough rotates in an anti-clockwise direction. So whilst Figure 1. below is a good representation of The Plough, it only shows how it will look at a particular time; I could equally have drawn it at a different position in the sky, such as in Figure 2.
The thing to remember though, is that wherever The Plough might be in the sky, you can still use it to find Polaris.
The 2 stars at the ‘end’ of The Plough are commonly referred to as ‘the pointers’; if you draw an imaginary line that connects ‘the pointers’ and carry it on, it will take you to Polaris.
Polaris is about 5 times as far from ‘the pointers’ as they are from each other. This is useful to know because even if Polaris is obscured by clouds, if you can see The Plough, you can figure out where Polaris (and therefore north) will be.
According to Greek mythology, Cassiopeia was an Ethiopian queen who boasted about her beauty to water nymphs. The nymphs complained to the gods, who had her daughter Andromeda tied to some rocks as a sacrifice. Anyway, it’s a useful constellation that we can use to find Polaris. Cassiopeia is often described as being ‘w’ shaped, and this is the case, but as it moves across the sky sometimes it will be more like an ‘m’ or even an ‘∑’.
Once you’ve located Cassiopeia, draw an imaginary line that bisects the angle made by the first ‘v’ of the ‘w’ shape; this will lead you to Polaris.
This last diagram shows the relative positions of The Plough and Cassiopeia to each other. Note that they are more or less on opposite sides of Polaris, so even if there is cloud cover, you should be able to see one of them and either find Polaris or estimate where it is.
Once you’ve found Polaris, drop an imaginary line to the horizon and you have north.
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