The Sky Guide Challenge
Find and picture the point in the sky that marks Earth’s North Celestial Pole
Finding the North Celestial Pole.
Our challenge this month is to see how many stars you can image or see between Polaris and the North Celestial Pole (NCP). The NCP is one of two positions in the sky representing the projection of Earth’s spin axis. Imagine sitting on a swivel chair, spinning around and looking straight up. The spot on the ceiling that barely moves as you spin is the upper projection of the chair’s spin axis. The star Polaris is special for the inhabitants of the northern hemisphere because it marks this position in the sky.
Polaris doesn’t quite sit on the NCP. It’s known as the Pole Star because at mag. +2.0 it is the closest naked-eye star to this position. A long exposure centred on Polaris shows stars moving concentrically around the NCP, with Polaris also describing a tiny arc around this position. This begs the question as to whether Polaris really is the closest star to the pole. Here, wide-angle star-trail photographs don’t tend to have enough image scale to show the faint stars closer than Polaris.
A simple solution is to use a telescope. If you have one that’s on a polar-aligned mount, turn the mount so that its polar axis points either east or west. With the mount’s drive turned off, you can now point a cameraequipped telescope at the pole with ease. A low ISO setting will avoid over-exposing the sky too much, but as always, review your shots and experiment to ensure you get the best results.
Polaris sits about 40 arcminutes from the NCP, so an image scale giving a 2° short dimension on a DSLR frame and with the NCP centred, is ideal. Be prepared to take a few test shots to identify where the centre of rotation actually is. For a full-frame DSLR a focal length of around 600mm works well. For a typical non-fullframe DSLR (for example an APS-C sensor) 400mm will give a similar result.
Rotation can be obtained by taking long exposures of 10-20 minutes each or, if sky brightness is an issue, by taking shorter exposures of between 1-5 minutes. Adding them together in an image editor, layering them and setting the blend mode of the upper layers to lighten will combine the trails. Alternatively, specialist star trail software such as StarTrails (www.startrails.de/html/software.html) can be used to achieve a similar effect.
Shorter exposures do have the advantage of making star identification easier, allowing you to match the result in a star chart in an attempt to identify the true pole star. The freeware Cartes Du Ciel is great for doing this. Bear in mind, however, that your result won’t hold forever because the NCP moves, taking around 26,000 years to complete a circle of radius 23.5° in the sky.