The Circling Stars

Stars circle around the North Pole

We are always moving. The Earth spins on its axis once a day and orbits the Sun once a year.

The stars are far, far away - so far that their motion compared to the position of the Sun is only detectable to the most sensitive of instruments. Their apparent motion in the night’s sky is therefore dominated by the Earth’s rotation and orbit.

You can see that for yourself any clear night. The stars slowly move across the sky, rotating around a point where there is one star that appears fixed: Polaris. This is often called the North Star as it is almost directly above the Earth’s North Pole, and so the sky appears to rotate around it.

The best way to find Polaris is to follow the Great Bear or Ursa Major, but that’s a good subject for another blog post.

In the mean time, how to take a photo like the one above? While one way would be to take a very long exposure - but the photo above shows the motion over about 5 hours, and it would be hard to get the stars to appear as the background would be so bright. So in this case I took a whole set of photos with the same parameters and the camera fixed in position on a tripod. I basically set up the Sony A7iii’s intervalometer and went to bed!

Each image was 24 seconds long, using a wide angle 24mm lens and f/4 with ISO 100 and I ended up with nearly 800 images. These were processed:

• in CaptureOne to give a constant white balance and enhance contrast etc.

• some frames showed aircraft flying across, and these were removed

• the remaining images were then stacked into a single image using Affinity Photo’s maximum merge feature.

The images were connected together to make this video:

What I found interesting was how an image like this focusing on the area around the North Pole shows that Polaris isn’t fixed but does actually move, if only very slightly.

Everything is moving: nothing is fixed.