The sky looks different, and appears to move differently when observed from different places on Earth. A crucial first step to learning the sky 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 the course of a night, a few months, and a full year.
To describe a location on Earth, you need to use reference points. These reference points can be co-ordinates on a map, physical landmarks, or a set of directions between a starting point and an end point. Reference points are also critical for finding objects in the sky.
To make getting our bearings a little intuitive, astronomers draw parallels between Earth reference points and sky 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. We call this sphere the Celestial Sphere.
N.B: I want to emphasise that the sky is not actually a sphere with stars stuck on it – it’s just useful to imagine it that 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 – the North Pole, the South Pole, and the Equator. We use latitudes to identify these, where the Equator is 0º, and the North and South Pole are 90º North and 90º South respectively.
Similarly, we give our Celestial Sphere three reference points to help us navigate – the North Celestial Pole, the South Celestial Pole and the Celestial Equator, directly above their earthly counterparts. The ‘latitude’ of the Celestial Sphere is called declination (Dec), and is also measured in degrees. The Celestial Equator is 0º Dec, just like the normal Equator is 0º latitude. The only subtle difference is that directions North of the Celestial Equator are considered positive, and directions South of the Celestial Equator are negative. So the North Celestial Pole is 90º (+90º) Dec, and the South Celestial Pole is -90º Dec.
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, it’s also useful to imagine that the Sun is going around the Earth in this sphere. The line that the Sun traces as it “goes around the Earth” is called the ecliptic.
The Earth moves around the Sun on a flat plane, but spins at a tilt of 23.5º. Because of this tilt, the Sun’s declination varies between -23.5º and 23.5º over the course of the year.
We now have four things to help us orient ourselves while looking up – the North Celestial Pole, the South Celestial Pole, the Celestial Equator and the ecliptic. For now, we’ll focus on the first three – the ecliptic will make itself useful later on.
From a single point on Earth, we can only see half the Celestial Sphere at any given moment.
Imagine standing exactly at the North Pole of Earth.
Here the North Celestial Pole will be directly above you, and the Celestial Equator will be where your horizon is, so this is the half you’ll see:
Now imagine standing at the Equator.
Here the North and South Celestial Poles will be at their respective sides of the horizon, and the Equator will pass over your head, as a line joining East and West.
To test your understanding, look at the illustrations below and imagine standing at the South Pole and at 45º latitude respectively. Try to figure out where the Celestial Equator and each Celestial Pole will be with respect to you. Also work out the declination at the horizon and directly overhead.
Repeat this exercise for your current latitude. Drawing it out helps!