Newbury Astronomical Society

We talk about meteor ‘showers’, but the word shower is something of a misnomer as far as meteors are concerned. Throughout recorded history there have only been a handful of examples of the kind of display that many people would imagine as a shower.

Even at the peak of the Perseid meteors, if you expect to see at least one meteor a minute you will often be disappointed. Expecting to see a meteor every few minutes is far more realistic but, just like London buses, it's not unusual to wait ages and then find several come along at once! If you go out expecting to see 5 meteors an hour and manage to see 10 you'll be delighted; expect to see 100 but only see 10 and it’s a flop.

When people talk about meteor numbers what they are usually quoting is the Zenithal Hourly Rate (ZHR). This is an idealised figure which assumes excellent sky conditions with the radiant (the area from which meteors appear to come from) directly overhead. This allows useful comparisons of meteor activity from hour to hour, day to day and across the years.

Using the maximum ZHR we can see that some showers, such as the Geminids in December, have strengthened significantly over the last century; while others, such as the Leonids in November, show large variations in activity from year to year. The rate quoted in the headlines is usually the maximum for a shower but unfortunately the rate can vary by the hour so very often there isn't even a single number which applies to the whole night!

The visual rates are mostly calculated from data gathered by small bands of dedicated amateur observers who spend many hours meticulously recording the meteors they see, along with enough information to arrive at (amongst other things) the ZHR. These are the real heroes of #Meteorwatch because without their work we wouldn't know what to expect, and even the best models still need observations to check the results. If you would like to help with this work visit the BAA meteor section pages and the International Meteor Organization (IMO) website.

So how does all this translate into what the average suburban dweller might see at maximum?

Lets start with someone at a reasonably dark site. Typically they would see around 5 random, or sporadic, meteors per hour on any moonless night of the year. If we then get a shower with a predicted ZHR of (say) 100, you might think they would get to see around 20 times more meteors than on an average night.

However, this assumes the radiant is overhead. Unfortunately the lower the radiant is the fewer meteors will pass through the visible area of sky. For the Perseids, at midnight in the UK, the radiant is only 45 degrees up so this reduces the likely rate to about 70 meteors per hour - even under the darkest skies.

Now allow for the light pollution that we all live under which affects the limiting magnitude, that is the faintest stars we can see. There are generally many more faint meteors than bright ones, so orange-lit skies can rapidly reduce the numbers visible. Even in a semi-rural area where you can clearly see the Milky Way this can knock enough off the limiting magnitude, to reduce the rate to 30 per hour.

Move to a small town such as Newbury and you could lose almost another magnitude, so the rate would drop to around 15 per hour. By the time you get to the London suburbs only the brightest meteors are visible and the rate could be as low as 5 per hour - but the ZHR figure is still 100 per hour!

This is not intended to put anyone off but rather to emphasise that, if you go to a secure, dark location, you can dramatically improve the number of meteors you will see. The brightest meteors are the ones people enjoy most, and many of these will still show through light polluted skies - so all is not lost even in the suburbs.

Also don't forget that predictions are just that, we don't have a detailed enough knowledge of where all the debris that causes meteors is, so there is always room for pleasant surprises if we hit a denser patch.

So – if you want to see more meteors – go to the darkest spot you can find (away from street lights), on the nights around the predicted maximum, look up and see what there is to see!

A useful guide to sky brightness in your area is the night sky simulator on the Needless website.

In theory, imaging meteors could hardly be simpler, just point the camera at the sky and leave the shutter open for long enough. In practice you will have to take a lot of images to get more than a handful of meteors. Meteors are so brief and move so fast that there isn't time to expose much of the film or digital sensor, so it is usually only the brightest fireballs that produce impressive images. What appears to the eye as an obvious meteor barely shows up on an image, a good example of just how powerful the human eye can be.

One of the problems with imaging meteors today is that the sky is now so full of man-made objects that you are far more likely to record them than a meteor. To the eye a meteor is obvious because of its fast movement and short duration. On a photograph this shows up a streak of light, but so do satellites and it can be easy to mistake a satellite trail for a meteor, particularly if you weren't watching at the time.

Things like planes are easy to pick out, the red and green or flashing lights are a bit of a giveaway. Mostly satellites show as a steady track, across several frames if a series is taken, and are fairly easily recognised. Sometimes though it isn't quite so obvious so it pays to check, here are a few examples of what to look out for:

A bright Iridium flare This was an Iridium satellite flare with the full path showing how it brightened and faded. At its best this was much brighter than Venus and has been recorded more strongly than an equally bright meteor would have done because of the slower motion. A bright Iridium flare	Partial satellite flare This image from the Perseids last year seemed almost to good to be true, and it was. The trail points back to the radiant but there is something not quite right about the look of it. On closer examination it turned out to be a satellite flare truncated by the end of the exposure. Partial satellite flare	The trail of a small satellite This is a satellite trail through the same region. It wasn't as bright but has recorded more strongly than the meteor. The trail continued on the following image leaving no doubt about what caused it. The trail of a small satellite	ISS passing through Orion This was a pass of the International Space Station which can appear as bright as Venus. For the southern UK there will be some early morning passes of the ISS during the few days around Perseid maximum. ISS passing through Orion
ISS going into earth shadow Visually the ISS can't be mistaken for a meteor, taking several minutes to cross the sky. However catching the trail on camera as it enters earth shadow can give a deceptive image. ISS going into earth shadow	A Genuine Perseid This genuine Perseid was seen as an obvious 1st magnitude meteor and comparable to some of the brightest stars in the sky, but the trail on the image isn't very strong. The green colour is distinctive although it wasn't as prominent as this to the eye. A Genuine Perseid

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