Practical astronomy projects for every level of expertise DIY ASTRONOMY

Create a 24-hour polar star trail photo

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Merge a series of photos to make a spectacular image showing Earth's rotation

Star trails are relatively simple to do yet produce very striking results, especially when centred around the north or south celestial poles. These images capture the movement of Earth as it rotates on its axis, producing star trails that are comprised of fragments of concentric rings. If you are in the polar regions during the winter and have clear, dark skies for 24 hours, the trails will form complete circles, including the pole star Polaris because it is not perfectly aligned with the celestial pole. Most of us will never experience a polar winter, but it is still possible to create a 24-hour star trail photograph by merging together a stack of many images taken with the same setup, from exactly the same spot, on different nights throughout the year.

Although taking a star trail image on a single night is straightforward, replicating it exactly on multiple dates, then merging them together successfully, is the challenge here. First, the camera location must be identical. We placed marks on the ground to ensure the tripod was in exactly the same spot, but attaching the camera to a fixed structure would be even better. Focus must also be the same, otherwise the width of the star trails will differ, and they won't line up perfectly. Circumpolar star trails can be created under moonlight, but doing so will affect the fainter stars, so try to ensure the Moon's brightness - its phase - is consistent between sessions. Also, when you blend your final stacked images, the overlapping regions will be brighter, so only stack what's necessary to complete the full circle.