Star trails that show the rotation of Earth require patience but are easy to create - see how with our top tips
Our planet is always moving – and it’s possible to prove it in an image. Earth spins on an axis and at either pole it points into space. Take a series of identical photos and in every one the stars will be in a slightly different position. Layer those photos on top of each other and you’ll see a spectacular star trail! Light pollution and moonlight have a major effect on the brightness of the stars and therefore of the star trails in your image, but it’s not a dealbreaker. A dark sky location will give you a more impressive image, but you can easily create a perfectly good-looking startrail from home.
Here are some of our top tips to get you started…
1 Where to point your camera
You can point your camera anywhere in the night sky and take a long exposure image. After about 25 seconds the stars will begin to visibly trail in your image. That’s because the Earth is rotating at about 1,000 miles per hour. If you’re in the northern hemisphere then to get the most impactful image of all – concentric circles of stars – you must aim your camera northward. Do that and the stars will appear to not only trail, but to circle around a star called Polari, also called the North Star. It just happens to be very close to the point in the northern sky the Earth’s axis points at: the northern celestial pole.
You can create an identical image in the southern hemisphere by pointing your camera south, though there’s no South Star as such. For a rough position you can look for the Southern Cross. The alternative from either hemisphere is to point away from the celestial pole toward the celestial equator (so, south from the northern hemisphere and north from the southern hemisphere). What you’ll get are long and gently curved star trails.
2 Finding the centre
You don’t have to have Polaris dead centre in your image, but if you don’t then the curves won’t be perfect and your circle star trails may look a little warped (though that may be exactly what you want). Even some experienced astrophotographers struggle with finding Polaris, but it’s actually very simple to find with your own eyes from anywhere in the northern hemisphere. Find the Plough/Big Dipper in the northern sky. It’s high up in spring and lowdown in autumn (“spring up, fall down”). Find the two stars at the end of the bowl – Merak and Dubhe – and note the distance between them. Then take a line that goes through them and continues about five times that distance. Polaris is the first bright star you come to. It’s simpler than it sounds.
Although you may be tempted to take a startrail in landscape orientation to get as much of the sky in as possible it’s definitely worth trying portrait astro-photos, which are less common and therefore more impactful.
3 Scouting a location and time
Since the effect you’re after is in the night sky surely it doesn’t matter what else is in your image, right? Wrong! If you just point your camera north without thinking about your foreground then it’s going to look very dull indeed. You need to think like a landscape photographer; by day go look for an interesting scene that offers a subject in the foreground and plenty of sky, taking into account the basic ‘rule of thirds’ composition technique. A regular compass is useful to begin with for finding a scene that’s orientation will roughly work, but the best way to find a specific place to put your camera is to use the PhotoPills app. Its Night AR mode will use your smartphone’s camera and overlay the position of Polaris.
A startrail is best done during the moonless ‘astro window’, which stretches from Last Quarter Moon to a few nights after the New Moon (about 12 nights) each month. You can produce a startrail from anywhere at any time, but certainly don’t bother making a big effort to go somewhere dark if the Moon is up (check the Moon phase). Try ISO 400 if there’s a lot of moonlight and ISO 800 if not.
4 The basic technique
The basic technique for taking a startrail is to craft a beautiful nightscape image with a shutter speed of 30 seconds, then take it 20-400 times with about 2 seconds between each shot. A two-hour shoot will get you a good star trail, but a five or six hour startrail will give you lusciously long trails and more colourful stars. It’s down to patience and ambient temperature because you’re going to have to wait around until the unit finishes! You then use freeware to stack those exposures to create one image. Some astrophotographers take three-minute exposures, so produce fewer images to stack, but sticking to 30 seconds is easier. With a wide-angle lens on your camera, and using a sturdy tripod and an in-camera or external intervalometer (or a remote shutter release cable that you can lock), you should experiment with changing the aperture (f2 to f4) and ISO (800 or 1600 to keep the image noise-free).
Just don’t touch your tripod! During the shoot you’ve got to leave your camera and tripod alone so it can take identical images over and over.
You need to get a sharp focus on the stars. A good habit to get into is to switch to manual focus, take a ‘throwaway’ shot of the night sky and then zoom-in to see if your stars are sharp in the centre of your image. You can use Live View on newer cameras to adjust your focus in real-time by bumping-up the ISO, while on older cameras it could be trial and error (test shot, zoom in, manually adjust focus, repeat). Shoot in RAW.
5 Foreground and clean-up
Once you’ve got your images you need to think about exposing for the foreground (unless you want a silhouette), which you can blend later in post-processing. Firstly, if your foreground subject is close to you then you’ll have to shine a light on it and manually refocus your camera … without moving the tripod. Secondly, you’ll need to take a photo of the foreground that allows you to see it clearly on your camera’s LCD. That means a very long exposure of several minutes on ‘bulb’ mode. To find the correct settings ramp-up your ISO to something really high – perhaps ISO 128,000 – and expose for a few seconds. That’s your test shot. If it works and you can see your foreground, go back into PhotoPills, enter your camera and lens details into ‘Spot Star’ then go into ‘Exposure’ and enter your test settings. It will give you the equivalent settings for a much lower (and, therefore, much cleaner) ISO.
Although exposing for 30 seconds works well for star trails, if you stick to 20 seconds then you get a bonus – a time-lapse! Instead of merging all of your images into one you can instead create a short movie showing the stars moving around Polaris.
6 Creating the finished image
You’re going to need two pieces of software to create your startrail image; the free StarStaX to stack the images and Adobe Photoshop to blend your foreground image. The first step is to post-process your night sky images; open them all in Photoshop, correct one until you’re happy with colour and cleanness of the sky then apply your settings to every image. Output them in JPG then drag them all into StarStaX, enable the ‘gap filling’ mode and wait for five minutes as it gradually builds a star trail image. It outputs as a JPEG. You then need to use Photoshop to blend it with your foreground shot if you have one.
It’s slightly easier to create a startrail composition the closer you are to the equator. That’s because the position of Polaris indicates your latitude. For example, from London in the U.K. Polaris is 51º above the northern horizon, whereas from the Canary Islands it’s 28º. From lower latitudes you can choose to have the startrails significantly lower in your image – or even behind a foreground element.
- AuthorJamie Carter
Jamie Carter is a journalist and author focusing on stargazing and astronomy, astrophotography, and travel for Forbes Science, BBC Sky At Night magazine, Sky & Telescope, Travel+Leisure, and The Telegraph.View all articles