How to watch the Perseid meteor shower in 2016
By Pete Lawrence
The annual Perseid meteor shower shows activity from 23 July through to 23 August, reaching its peak during daylight for the UK on 12 August. The best nights to watch out for a Perseid meteor streaking across the sky will be the nights of 11/12, 12/13 and 13/14 August, as the rates will be considerably higher than at other times. Activity tends to be highest between 01:00 BST and the start of dawn. This year there is a prediction for enhanced activity around 1am BST but there are no guarantees!
A meteor shower’s zenithal hourly rate, or ZHR, gives the number of meteors you could expect to see if the point in the sky where they appear to emanate from was overhead, the sky was perfectly dark and clear and you could see the entire sky in one go. In reality, the number of meteors you get to see (on average) is known as the visual rate and this will typically be between 20-50% of the value of the ZHR. For the Perseids, the ZHR is normally around 100 meteors per hour, so you could expect to actually see 20-50 meteors over the course of an hour, depending on how clear and dark your sky is. The enhanced activity predicts this figure may double for a short period. The first quarter Moon will be low and set in the early hours so it shouldn’t interfere with the show.
Of course meteors don’t operate by the rules and it’s fairly common to go for long periods where nothing appears to be happening at all. Then, just as you’re about to give up, a bright meteor will flash across the sky and you’ll be hooked again.
The particles that eventually go on to produce Perseid meteor trails are all travelling in parallel orbits through space. When they enter Earth’s atmosphere, a perspective effect makes it look as if they are all radiating away from a single point in the sky. This point is known as the shower’s radiant. The radiant isn’t a static point but moves slowly over time. The Perseid shower gets its name because it’s most active when the radiant happens to be in the constellation of Perseus, the constellation that represents a mythological Greek hero.
So if you see a meteor streaking across the sky, does that make it a Perseid?
The answer to this is not necessarily. The Perseid shower is the strongest shower active at this time of year but there are others happening too and these can add to the number of meteors seen. They all peak on different dates and although most have very low ZHR values, together they all add up.
If you photograph a streak of light, is that likely to be a Perseid?
Again, for the reason stated above, not necessarily and there’s another pretender that can invade meteor photographs masquerading as a meteor trail, and that’s the light trail of an artificial satellite.
How can I recognise a Perseid?
To determine whether a trail belongs to the Perseid shower, it must follow some simple rules. First, and most important, the trail must appear to be coming from the Perseid meteor shower radiant. If you’re not sure where this is but can recognise the ‘W’ shaped constellation of Cassiopeia, then this can help. Extend a line from the middle star of the ‘W’ through the next one to its left (imagining the ‘W’ the right way up). Extend that line for about 3x the distance again. The radiant position during peak activity is quite close to this position as shown in Figure 1.
One easy way to tell whether a meteor trail lines up with the radiant is to first identify where the radiant is in the sky, and then, immediately after the trail has passed, hold up a ruler or piece of string with two hands, and line this up with the trail. If the line passes back to the radiant, then that’s one Perseid tick box ticked. If a trail lines up but the meteor was travelling toward the radiant, then it’s not a Perseid.
Meteors tend to vapourise in a well-defined layer of the atmosphere. The result of this is that the trails tend to be of the same length. When you see a Perseid close to the radiant, its trail looks short because it’s travelling towards you. A meteor that occurs exactly on the radiant will be travelling straight towards you and appear as a point of light. This is called a pin-point meteor.
The further from the radiant a trail begins, the longer the meteor trail will appear to you. So if you see a meteor that appears to be travelling directly away from the radiant but starts close to the radiant position and produces a long trail, that’s unlikely to be a Perseid. Similarly, if you see a really short meteor that lines up with the radiant but a long way away from the radiant position, that’s less likely to be a Perseid. You can see this effect clearly in Figure 2 which is a composite shot – combining a number of individual photographs carefully aligned on the stars.
Perseid meteors result from small grains of dust left around the comet of 109P/Swift-Tuttle. They all enter our atmosphere at a speed of 58km/s. After a while, if you’ve seen a number of Perseid meteors streaking across the sky, you get used to this speed. If a meteor appears which fulfils all of the previous criteria but is travelling very much slower or very much faster than normal, then this is less likely to be a Perseid.
Satellites posing as meteors
If you attempt to capture a meteor with a camera by taking a lot of 30s or longer exposures in quick succession, you’re very likely to record things which mimic meteor trails. The most common tricksters are artificial satellites. There are so many of these orbiting the Earth that it’s now very common to see them if your sky is good and dark.
When a satellite passes through your camera’s frame, it’ll record as a line on an exposure running for a number of seconds. One way to prove it’s a satellite is to look at the frames before and after the one that has recorded the line. If the trail continues across two or more shots, it’s very likely to be a satellite.
Other clues are shape and colour. Meteors caught with a colour camera will typically show colours; starting green and then changing to pink. A really bright meteor will cause its image to overexpose and the colour may be lost, appearing as white. However, a really bright meteor tends to be easy to recognise and should still show colour at the start of its trail.
Satellite trails appear white or sometimes yellow-orange if they have been photographed low down and close to the horizon. A trail caused by a satellite which has not changed brightness as it passes across the sky will look like something has drawn a regular line across your photo.
Many satellites change brightness as the angle they make relative to the Sun alters as they move along their orbit. This can cause a satellite to flare and increase in brightness significantly. The brightest satellite flares can be so bright that they cast shadows!
If you catch a satellite flaring with your camera this can look very much like a meteor trail (see Figure 5). The false meteor appearance can be further enhanced if the camera shutter closes part way through the flare. A meteor trail will typically look narrow at the start and widen as the trail gets brighter. If you catch a brightening satellite which is then truncated because the shutter closes on it, this also starts narrow and gets wider. Look closely at the truncated end of the trail and you’ll see that it’ll have the appearance of being cut off. And don’t forget to check to see if the trail passes through the next photo in the sequence.
So with this information to hand, go out and enjoy the Perseid meteor shower but remember, not everything that you see streaking across the sky may be what you think it is!