There’s no getting away from it - The Earth rotates.

Once every 23hrs and 56 minutes it spins right the way round, and that leaves us humans standing on its surface watching everything appear to whizz past us.  The Sun rises in the East and it sets in the West, as does the Moon and all the stars and planets.

So what?  Well, the problem with this situation for the astronomer is that if you want to look at one of these celestial bodies through a telescope, you will very quickly find that it moves out of the field of view, so you have to keep chasing it across the sky.  This problem has, of course, been solved for hundreds of years now, with driven mounts on telescopes, which allow the user to keep the object they are viewing in the centre of their telescope, even though the object insists on trying to get out of the way.

The same also applies to photographing the night sky – a pastime which we can heartily recommend.  Let’s face it – just about everybody these days has access to a digital camera of one sort or another, and if you start looking through the settings on your camera, you may find that it has the ability to take long-exposure photographs.  By long, we are talking about 5 seconds or more.  If you can do that, then you should be able to photograph the night sky.  You will need to make the camera as sensitive to light as possible (a high ISO number, like 800 or 1600) and set the exposure time to as long as you can.  Steady the camera by mounting it on a tripod, or if you don’t have one then a small beanbag should give enough support too. If you are able to set the focus distance, then set it to “infinity”.  Point your camera skywards and away you go.

But this article is about taking star trails.  So, what exactly are they?  Well, if you have a go at taking long exposure photos as described above, then you will start to notice that as you increase your exposures beyond about 20 seconds, that each of the stars on the picture starts to stretch out in to a line, or trail.  This is more noticable if you camera is pointing away from the pole star (where angular rotation is much less). These star trails are a record of how far those stars moved in the sky during your exposure.

So we now have the ability to do a little scientific demonstration. We are going to take a star trail photo which includes the Pole Star in its field of view.  The Pole star (or North Star, as it is often known) is the one that can be found by using the two end stars of The Plough, and imagining a line northwards.  The first reasonably bright star you come across is the pole star, so called because the Earth’s North pole is pointing more or less directly at it, and so everything in the sky will appear to rotate around that point.

If you can, you want to try to take an overall exposure of exactly 1 hour (yes, you heard correctly – 60 minutes – 3600 seconds).  The problem with exposures of this length is that unless you are in a really dark site, under a cloudless, moonless sky, then the photo may become “bleached out” with other artificial light sources from nearby streetlights etc. The picture here shows how bad this can be. What should be a black background becomes a washed-out orange from the sodium streetlights.

There are ways around this problem though, and it doesn’t involve you having to move to middle of the Sinai Desert for the dark skies either (although you would get even better results if you did…) Instead of taking one very long exposure, you can take a number of shorter exposures (say about 30 seconds each, in which case you would need to take 120 images) and then stack them on top of each other in a photo editing package afterwards, using the “lighter” mode or similar for stacking layers. There are also some dedicated star trail packages which are free to download – one nice on is StarStax.  As long as you have managed to keep your camera fixed in the same place throughout, they should fit quite nicely on top of each other.

So – back to our demonstration. Assuming you have managed to take one hour’s worth of photos and have got them stacked together, then you could take a measurement of the angle of the star trail.  Print your picture out, or display it on your computer screen and then take a protractor (yes – remember them?) and measure the angle that any one of the stars in your photo has travelled during the one hour exposure.  You should find that every star trail in the picture is exactly 15 degrees long – the amount that Earth rotates in one hour.  And so, there you have concrete, visible evidence that the Earth does indeed rotate, and proof of how fast is does it.  This a great little demostration for kids to do.  Have a go.

If you really get in to taking multiple images to make up star trails, then you could also animate them in to a short film.  The clip shown here was taken by York AS member Martin Whipp, who decided that it would look rather nice if he put the top of the spire of a local church right where the Pole Star sits in the sky.  If you want more information about how this effect was created, then email us on info@yorkastro.org.uk