The recent meteor shower was a great celestial occasion for both viewing and photography. In this post I hope to explain what causes the ‘shooting star’ displays and illustrate how they appear to radiate from a particular point in the night sky.
I’ve been waiting for a clear sky on the peak night of the Perseids for several years and this year provided me with the closest yet, almost 2 hours of clear sky with patchy moving cloud. So let’s start with a close-up picture of a shooting star.
40s f4 24mm ISO1600
Shooting star is a colloquial name for the trail of a meteoroid as it passes through our planet’s atmosphere. At this point it is more correctly called a meteor, if any fragment manages to meet the ground it will then be called a meteorite. Meteoroids are solid chunks of material moving through space with a size ranging from a spec of dust up to 10 meters across. When such an object enters Earth’s atmosphere, at speeds up to 40 miles per second, it heats up and usually disintegrates at about 30 miles of altitude. This fiery death happens in a mere second and is what we see from the ground. The colours seen in this display, can give scientists a clue about the composition of the meteor. In the picture above the blue/green suggests copper and the yellow/orange suggests sodium. If the brightness of a meteor in our skies is greater than that of the planets, then it is considered a fireball or bolide. We were lucky to see one such fireball last night, high in the north-east, above Dyfi Forest.
The picture above shows a wider field of view, the Perseid meteor can be seen top left, whilst one of our neighbouring galaxies, Andromeda, may be seen in the bottom right. These images were taken using a Canon 5DMkII & 24-105L lens on a normal photographic tripod. With an exposure time of 40 seconds, this is long enough for the Earth’s motion to cause star trails, which are indeed very evident. To avoid this one could mount the camera on a German equatorial mount, as per the picture at the bottom of this blog post.
A meteor shower is said to happen when many meteors are seen in the sky over a short period of time and they all appear to radiate from the same point. This is caused by a stream of cosmic material colliding with Earth’s atmosphere. The Perseids are just one such stream of material. The comet Swift-Tuttle travels through our solar system on a 130 year orbit, occasionally it loses matter and this is left as a stream of cosmic debris that our planet passes through in the fist half of august each year.
So why do the meteors in a shower appear to radiate from the same point and can we illustrate it?
Having been dropped by a speeding comet, all these cosmic dust particles are travelling in parallel with each other and at very similar velocities, perspective vision demands that they appear to originate from the same point. For example, imagine you are standing in the middle of a long straight road. Far in the distance 2 motorbikes appear from the same point. In truth one is on the left of the road & one is on the right. As the motorbikes approach you, they will appear to diverge away from the same starting point and eventually one will pass one side of you and the other will pass the other side of you. The motorbikes have never moved apart, yet they appeared to originate from the same point. This is how it is with the meteors but on a much larger, cosmic scale.
The Perseids appear to radiate from a point in the constellation of Perseus, hence the meteor shower’s name. To illustrate this I took the picture below:
This was taken with a Canon 40D and 15mm f2.8 fisheye lens, mounted on an equatorial mount to avoid field rotation and star trailing. A series of 170 35-40 second images were taken sequentially. Satellite & aircraft trails were filtered out, as were cloud laden exposures. The remaining exposures were further filtered for meteor activity and then composited to form the above image. Six meteors can be seen streaking across the night sky. To the left one can see the Plough (Big Dipper) asterism ; to the right the constellations of Cassiopeia & Perseus can be seen. Each of the six meteors’ paths can be traced back to within the constellation of Perseus.
The orientation of the above image is looking directly north, over southern Snowdonia. Bottom left is Foel Crochan, Aberllefenni, with some cloud cover.
I hope you saw some meteors, if not, better luck next time. If you are hoping to photograph meteors, remember that the burst of light is typically only 1 second long and that’s what you’re trying to image. So use a fast lens and maximise the light gathering potential of your camera. Expose for long enough that the sky appears slightly brighter than black. Use a focal length of your choice, wider gives you more chance of catching one, longer may give you more detail.