Planes, satellites & a meteor: decoding London’s night’s sky

I love watching the night’s sky, there’s always something going on, and it keeps getting busier up there. Every week there seems to be a new launch - this week has seen another Falcon crewed Dragon capsule launch to the International Space Station (ISS) and tonight (fingers crossed) there’ll be another OneWeb satellite constellation launch.

This week has also been the peak of the Lyrid meteor shower, so I thought I’d have a go at spotting them. Meteors are particles of dust shed from a comet that falls into our atmosphere, burning up in a spectacular flash of light. But they can be hard to see in areas like London with a lot of light pollution.

The Lyrids appear to radiate from a point in the sky near the constellation Lyra, towards the north-east at this time of year. Only a couple will be seen in an hour and they flash quicker than anyone can take a photo, so the best way was to set the camera up in time lapse mode.

I used an exposure of 2 seconds, which is relatively short for this type of work, but means I could get an idea of how fast things were moving or lasted in the sky. I used a Sony A7iii camera with wide angle 24mm f1.4 lens, set up pointing north, configured to automatically take 2,000 images.

The next day, using Premier Pro, I converted these 2,000 images into a video that looked a bit like this:

Hope you can see that - it helps to watch on a computer screen in a dark room! With 2 - 3 seconds between image this appears sped up by a factor of 50 - 75 times.

There’s a lot going on - so what is everything? Are there any meteors there?

Let’s first look at the start of the video. I used Affinity Photo to stack the images in sets of 50 and from the first few stacks made the following:

The first thing to note are the lines curving around a point towards the top. These are star trails: a photographic record of the stars which appear to rotate around the North Pole - though of course it is actually the Earth that is rotating. The North star is the bright dot at the centre of the dark patch that all the other stars are circling around.

Then there’s the wiggling dashed line with one loop around the North star: this is a helicopter. You can tell this not just from the erratic route, but also the changes in colours, reflecting its navigation lights as it manoeuvres around in the sky. The line is dashed due to the way the camera takes the 2 second images with a slight pause before it takes the next one. From the length of line you can get an idea of how fast it was moving in the sky - the shorter the line the slower it was appearing to go, the longer the greater its angular velocity.

At the bottom is the track of an aircraft: you can see dots not lines and some of these are coloured. These represent the flashes of the aircraft’s navigation lights.

There are also a couple of satellites, but to make them clearer here’s another example stack of images with no aircraft, just satellites:

This shows two satellites, but how can I tell? Well there are a couple of clues:

• The length of the line. This, as mentioned, is linked to the angular velocity of the object, and these line lengths are typical for 2 second exposures for satellites in low Earth orbit (LEO).

• The line is also straight - no wiggling like a helicopter

• There are also no flashing lights or colours - there are no navigation lights on satellites (yet!)

• You can also see how the dotted line towards the top right seems to fade out - this is typical of a satellite entering the Earth’s shadow

• Also, given the time of night, aircraft would be too low to reflect the sun’s light, so they must be much higher, i.e. in orbit.

Hence I’d say they are satellites.

It’s also worth noting that the lines for the satellite on the left are longer than that for the one on the right (at the same elevation angle). This means it must have a higher apparent angular velocity. Assuming both are in close to circular orbits, then this means the satellite to the right is in a higher orbit, as angular velocity decreases with orbit height.

These satellites would look like a dot of light moving across the sky. But some satellites are tumbling, typically because they’re old or out of control. In these cases they appear not as a continuous dot of light but a flashing one, with repeats as in this example, with six short lines from top right to bottom left:

It’s also possible to deduce how quickly its tumbling. In this case the flashes are about the same distance apart as four of the non-tumbling satellite’s lines. So maybe rotating a full 360 degrees ever 10 seconds - or tumbling at something like 30 to 40 degrees per second.

So that’s star trails, helicopters, aircraft and satellites. But what is this?

In the image at the top this is towards the lower half, slightly to the right.

Some things to note:

• There is just a single line: there are not multiple dashes as per the satellites

  • So it must have been a quick flash - under 2 seconds

• It is a long line, much longer than the dashes of the satellites

  • So it appeared to be going fast, with higher angular velocity than a satellite

• It has a diamond shape, brightest in the centre

• There are no navigation lights.

Hence this looks like a meteor! Also, it’s angled so that if you continue back toward the right it would head towards Lyra.

Combining all these elements we can identify all the objects in the night’s sky over London:

This image is a stack of the last 500 or so images from the video, hence it doesn’t include the helicopter - and that would be rather distracting!

So next time you’re out at night, look up and see what you can spot. I can’t promise any meteors, but hopefully you’ll be able to distinguish between satellites and aircraft and even guess which satellite is in a higher orbit than another.