To start off well in this I will give a brief explanation of something that complicates a little the taking of photographs and that is the apparent movement of the stars in the sky; the reality is that it is the Earth that moves on its axis and not the stars.

Polaris, the ancient reference point


It is important to know that there is a single star in the Northern Hemisphere that never moves in the sky like the others and it seem it's always fixed. This is Polaris (or commonly known as the North Star) and is the apha star in the Ursa Minor constellation. It is called alpha because it is the brightest star (I explain the classification nomenclature for the Greek alphabet in this article). Even if it is the brightest star in the constellation, compared to others like Sirius in the constellation of Canis Major it is less brighter.

Both Ursa constellations are large, but what identifies them is that the brightest stars form a kind of "dipper".

For this reason, in order to locate the Ursa Minor and Polaris when there is little luminosity, it is necessary to rely on other more luminous stars, such as those of the Ursa Major. As a reference we first find the Dubhe star which is one of the brightest. Then we look a little to the right in a northeasterly direction and there you will find Polaris.

The importance of Polaris for astrophotography in the northern hemisphere
The important thing about this is that the Earth's axis is pointing towards this star and therefore gives the appearance that it is fixed and that all the others are rotating around it. There are many devices such as equatorial type mounts and tracking motors, which have an axis that must be pointed towards this star and is fixed. The other axis follows the natural movement of the stars in the sky.

It is thanks to this movement that we can take photos with the trajectory of the stars that are the famous circumpolar photos. In fact what you have to do is simply point the camera towards Polaris one and let it take pictures and then join them so that you can see the effect of lines around it.