The Perseids Meteor Shower is without doubt one of the highlights of the astronomy calendar. It has a really broad spread of activity, which starts around 23rd July and continues until around 16th August. The peak of the activity is spread over a couple of nights around 12th August, but rates really begin to ramp up around 8th August. This year the height of the peak is expected around 2am BST on the morning of 13th August and with the Moon out of the way, it looks like a potentially great year for the Perseids.
As you can see from the chart below, the Zenith Hourly Rate (ZHR) for the Perseids has been anywhere from 50 – 200+ meteors per hour over the past 16 years, (200 is the number that the press will no doubt be quoting near to the peak!). However, this number is slightly misleading as it is not the number of meteors you will actually see. The ZHR assumes the radiant is directly above your head (more about the radiant in a moment), that you have no light pollution, and it refers to the number of meteors across the entire sky. It’s rare that we have the radiant above our heads, or that we have pristine, cloud-free skies, and even if we did have all that, we don’t have eyes in the back of our head, so a more realistic number to have in mind is between 30 and 60 meteors per hour from a dark sky site. Rates will increase during the early hours of the morning as the Perseids radiant climbs higher in our skies, and also because Earth naturally “scoops up” more debris on the dawn side of the globe. Obviously meteor numbers will decrease the closer you get to a light polluted location. But meteors are totally random, so sometimes you might not see anything for several minutes, then several will come along at once. Although it may not have a zenith hourly rate as high as the Geminids, there is something to be said for being out in the garden on a balmy August night compared to being surrounded by 6” of frozen snow on a December night (which was the conditions I was out in during the Geminids last December!)
The principle meteor showers are all named after the constellation which contains the radiant for that shower. Meteors during a shower may occur anywhere in the sky, but if you trace back the path the meteors appear to have travelled along, meteors belonging to a particular shower will all appear to originate from one place in the sky, and that point is called the radiant. In the case of the Perseids, the radiant is located in the constellation of Perseus, not far from the Double Cluster. If you see a meteor which doesn’t follow a path back to this point, then it is probably a sporadic meteor rather than a Perseid.
Almost all of the principle meteor showers are caused by the Earth passing through the debris stream left behind by a visiting comet, with the two exceptions being the Geminids and the Quadrantids which are caused by asteroids. In the case of the Perseids, the parent comet is 109P/Swift-Tuttle which is a periodic comet that visits the inner solar system once every 133 years. The last time it visited the inner solar system was 1992. As these tiny pieces of debris burn up in the atmosphere, it causes a bright “shooting star” or meteor. If you get a larger piece of debris burning up, then it can cause a fireball. Perseids debris usually burns up around 80 km up in the atmosphere. The ZHR for a shower will increase if the parent body has recently made a close approach and the rates did in fact increase during the early 1990s. During comet Swift-Tuttle’s visit back in 1865, a new filament of dust was pulled off the comet and this gives rise to a mini-peak the day before the main peak of the shower. It is definitely worth observing the Perseids a few night before and after the peak.
The great thing about observing meteor showers is that you don’t need any specialised equipment; all you need is your eyes. Get comfortable on a sun lounger and don’t forget to take out a blanket because while we may not have ice and snow in August, it can still feel really chilly during the early hours when you’re sitting still for prolonged periods. Give your eyes 20 minutes to become fully dark adapted, then just lie back and watch the sky. To increase your chances of seeing more meteors, make sure you are looking at least 45 degrees away from the radiant and about 45 degrees up from the horizon. If you observe in pairs or groups of more than two, make sure that you’re all looking in different directions to maximise sky coverage. And here is where you can do some actual science; try to make a note of the time each time you see a meteor and the direction it was travelling in. How fast was it moving? Did it have a persistent trail? What colour was it? Perseid meteors are often characterised by a long, bright trails which are green in colour. If you have more astronomy experience, see if you can estimate how bright the meteor was by comparing it to the magnitudes of the nearby stars. The Perseids are often rich in fireballs and these are always spectacular to observe! It will help to pre-prepare your observing sheets before you go outside, and have one for each allotted time period. Or you could use a Dictaphone or voice recorder to record your observations. Once you collate all of your observing information, you can submit it to UKMON, BAA and the SPA where it will be compared with data from elsewhere. So you will be contributing data to the professional astronomy community.
If you want to photograph the Perseids, all you need is a basic camera that is able to do long exposures. Focus the camera on a bright star, then point the camera about 45 degrees up and 45 degrees away from the radiant. Set the camera to around ISO-800 at the lowest f number your lens will allow, then use a remote shutter cable to shoot 15 – 20 second exposures on continuous. Then you can just leave the camera running and focus your attention on the visual observations. If you do see a meteor within the field of view of your camera, note down the time to help you find the photo later. And if you manage to photograph a bright meteor, don’t stop shooting because it’s sometimes possible to pick up the slowly dissipating ionization cloud which occurs after a bright event. Just leave the camera running, then if you have captured an ionisation cloud you can create a timelapse video of it.
We have had so many clear nights here in the UK recently, so let’s hope that they continue during the peak of the Perseids. If they do, we could be in for a real observing treat!