Learning the Night Sky: How the Stars Move

In this post, I use ‘stars’ to refer to anything that isn’t the Sun or a planet. The Sun and planets will be covered in a future post.

The stars we see in the sky change over the course of a night, and also over the course of a year.

How the stars move over one night

The Earth’s rotation is responsible for the rising and setting of stars.

1. Stars rise in the East and set in the West.

This is because the Earth moves from West to East.

To visualise this, imagine your head as the Earth, and your screen as a star in the sky. If you keep your eyes fixed on the screen and turn your head to the right, the screen appears to move to the left. The same thing happens with the Earth – as it spins from West to East, the stars appear to move from East to West in the sky.

orientation_1_illustrations-copy-01.pngorientation_1_illustrations-copy-021.png

2. Stars move parallel to the Celestial Equator.

Although stars rise in the East and set in the West, the path they trace in the sky is not always a direct line overhead. Over a night, they trace lines like the orange trails in the diagram.

Orientation_1_Illustrations copy-03             orientation_1_illustrations-copy-041.png

Let’s see how this looks from different points on the Earth.

If you read the previous post, you should be able to tell where the NCP, SCP and CE are. Knowing where you are relative to these will tell you how the stars will move over the course of a few hours.

For example, at the North Pole, the North Celestial Pole is directly overhead and the Celestial Equator is at the horizon. Over a night, the stars will trace circles around the North Celestial Pole.

orientation_1_illustrations-copy-05.png     orientation_1_illustrations-copy-06.png

Here’s a long exposure of what you would see looking directly above you:

north_pole_star_trails.jpg

Let’s imagine a different situation. Standing at the Equator, the Celestial Equator is directly overhead, so the stars will trace lines going over your head.

orientation_1_illustrations-copy-07.pngorientation_1_illustrations-copy-08.png

A long exposure of what you would see looking overhead:

equator_star_trails.jpg

Now imagine standing at a declination of -45°. The South Celestial Pole will be 45° above the Southern horizon, and he Celestial Equator will trace out an East-West line 45 ° above the northern horizon (the diagram is a little easier to understand). The sky will move like this over the course of a night:

Orientation_1_Illustrations copy-09  Orientation_1_Illustrations copy-10

Here are the star trails you would see from this declination:

-45dec_star_trails.jpg

Note: Where the poles are above the horizon, some stars never set, they are always up in the sky. Such stars are called circumpolar stars.

Why the stars are different at different times of the year

The Earth’s orbit around the Sun explains why the sky changes over the course of a year:

The revolution of the Earth explains why we see different stars and constellations at different times of the year. The Earth orbits the Sun over a period of 12 months.

orientation_1_illustrations-copy-11.png

At different times of the year, the “night” side of the Earth faces a different part of the sky.

orientation_1_illustrations-copy-121.png

This explains why the night sky in January looks very different to that of July, or even April. In fact, the stars in the sky actually change on a daily basis. On average, stars rise 4 minutes earlier every day.

Summary:

Over the course of a night, stars rise in the East, move parallel to the Celestial Equator, and set in the West. This makes different star trails depending on where you look from. This is because the Earth rotates on its axis.

Over the course of a year, the stars look different because the Earth faces a different part of the sky. This is because the Earth revolves around the Sun.


P.S: There is another movement of the Earth called precession, which I haven’t talked about here. Precession is a wobble of the Earth on its axis, but is only significant over a period of 13,000 years, so it’s safe to ignore for the purpose of this post.

 

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