The sky looks different, and moves differently depending on where you’re looking from. A crucial first step in learning astronomy is figuring out where you are with respect to how things are moving. Once you know this, you’ll be in a position to understand how the sky moves over days, months and years.
Let’s focus on Earth for a moment. To describe any location on Earth, you need to use reference points. These reference points can be co-ordinates, physical landmarks, or directions between a starting point and an end point – they are just something to help you determine where you are compared to where something else is.
To find objects in the sky, we need to define some ‘sky reference points’. An easy way to do this is to just draw parallels with our familiar Earth reference points. Since we live on a big sphere, it’s useful to imagine the sky as another sphere that encompasses Earth, with stars stuck on it like glitter. We call this imaginary sphere the Celestial Sphere, and when we talk about the movement of the sky, we imagine that the Earth is not spinning, but this big sphere we are inside is the thing that’s moving.
N.B: The sky is not actually a sphere with stars stuck on it – it’s just useful to think of it this way to orient yourself. In reality, the stars are all at varying distances from us.
The Earth has three key reference points to help us determine where we are on it. These are the North Pole, the South Pole, and the Equator.
We use latitudes to identify these points, where the Equator is 0º, and the North and South Pole are 90º North and 90º South respectively.
In a similar way, we can give our Celestial Sphere three reference points to help us navigate. Let’s call these the North Celestial Pole, the South Celestial Pole and the Celestial Equator, and place them directly above their earthly counterparts.
The ‘latitude’ of the Celestial Sphere is labelled declination (Dec), and is also measured in degrees. It’s pretty much exactly the same as the Earth’s latitudes. The only subtle difference is that we take Northern declinations to be ‘+’ and Southern declinations to be ‘-‘. So instead of saying ’23º North’ or ’75º South’, we say ‘+23º Dec’, and ‘-75º Dec’.
The North Celestial Pole is 90º (+90º) Dec, the South Celestial Pole is -90º Dec, and the Celestial Equator is 0º Dec, just like our earthly poles and Equator.
The North Celestial Pole is located almost exactly where the star Polaris is (this is why it’s called the North Star!). Finding the South Celestial Pole is a little more intricate – you have to draw lines through the Southern Cross and its pointers, but we’ll touch on this later.
Now we all know that the Earth goes around the Sun, and not the other way around. But if we are thinking of the sky as a sphere that holds us in, and we stretch our imagination a little more, we can imagine that the Sun is going around the Earth on this sphere.
The line that the Sun traces as it “goes around the Earth” is called the ecliptic.
The Earth spins at a tilt of 23.5º – its spinning axis is tilted with respect to the Sun. Because of this tilt, the Sun’s declination is not fixed – it varies between -23.5º and 23.5º over the course of the year.
We now have four sky reference points to help us orient ourselves while looking up – the North Celestial Pole, the South Celestial Pole, the Celestial Equator and the ecliptic. Technically, the Equator and ecliptic are lines and not single points, but as long as you know where they are in the sky, this doesn’t matter too much.