AnnMarie's Blog

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.

click image for a larger view …

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:

click on image to view a larger version …

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.

Have fun.

We’re lucky enough to have several RSPB  reserves locally; the nearest one being Ynys-Hir . I try to visit them as often as possible but probably don’t make as much use of them as I might. And I guess that’s what this post is about, a quick mention for Ynys-Hir to say how well worth a visit the reserve is.

Below is a picture that I took a couple of years ago, from the window of the Ynys Eidiol hide – presented as a scrollable panoramic:
(You’ll need either Quicktime or as I recommend the DevalVR plugin then just click the play button; try scrolling & fullscreen)

You can see another picture that I took of sunset from the Breakwater hide, Crimson Flight, over on my galleries.

By visiting Ynys-Hir not only will you have the opportunity to enjoy the landscape & nature of Mid-Wales but you’ll also be helping the RSPB to conserve this resource.

Spent a few enjoyable hours recording bat calls last night – here’s a report on some of our insect eating friends, including some recordings to listen to: 

Date: 3rd August 2010 

Sunset: 9:05pm 

Duration: 9:00pm to 12:10am 

Location: Foel Friog  

Weather: 100% Overcast, quite a dark night; 14degC; occasional spots of rain; little breeze, almost still. 

Introduction 

Bats account for about 1/3 of all the mammal species on this planet. They often seem to have had bad PR, yet they are remarkable creatures that are generally beneficial. 

In the UK, all of our bat species eat insects and we have 17 species that breed here. If you go for countryside hedgerow walks late on a summer’s evening or watch around your house night light, then you may be lucky enough to see bats flying around. The group of species most likely encountered in these circumstances are the Pipistrelle . These are small fast flying bats with great acrobatic ability, turning quickly to catch midges & the like. 

Many bats use ultrasound to echo locate  their prey. This sound is too high pitched for human ears to hear (save for a few low notes from Noctules (in the UK) that may be heard by young people with good hearing). This is where a bat detector can be a fun & useful piece of equipment. There are several different types available but that is outside the scope of this article. Suffice to say that the following recordings were made using a frequency division  detector linked to my netbook computer. Analysis was done using Batscan v.9 and Audacity v1.3. 

A few Pipistrelle facts: 

• Main food is small insects esp. midges; a Pip can eat 3000 midges in one night
• Can live for up to 12 years but 5 years is more common
• Typical wingspan is 20cm with a 4cm long body
• 3 species in UK, Common, Soprano and Nathusius
• Typical weight is 6g but may be more
• Covered in brown fur, more reddish above & yellowish below
• One of the most common bats in Britain
• Mating season is Aug – Sept, when males have a territory
• Young are born in July and leave the roost within 4 weeks

The following sections may seem a bit techy but I hope there’s a little bit of interest for everyone who might be interested in bats … 

Soprano Pipistrelle (Pipistrellus pygmaeus) 

Recorded hunting rapidly up and down a mixed woodland edge at just above head height, some 25m from the river. The details of the ultrasonic calls can be seen on the sonogram below: 

This recording was made at approximately 10:20pm. Further analysis of the call showed the Peak Frequency to be 54.95kHz +/- 0.05kHz. The pulse repetition rate was measured as varying from 8.75 pulses/s up to 18 pulses/s. The peak frequency analysis can be seen below: 

The peak frequency of 54.95kHz is very typical of the Soprano Pipistrelle (average often quoted as 55.5kHz). 

To listen to these Soprano Pipistrelle bat sounds … 

Common Pipistrelle (Pipistrellus pipistrellus) 

Recorded hunting along hedgerow above our hide night light. Lights attract insects at night and this makes it a great zone for bats to hunt in; it’s also a good place for you to watch them. The sonogram below shows how the Common Pip’s call is similar to that of the Soprano Pip but at not quite such a high frequency. 

This recording was made at approximately 11:30pm. Further analysis of the call showed the Peak Frequency to be 46.70kHz +/- 0.05kHz. The pulse repetition rate was measured as varying from 7.75 pulses/s up to 14.5 pulses/s. The peak frequency analysis can be seen below: 

As previously mentioned the Common Pip’s peak frequency is slightly lower, measured here at 46.70kHz and often quoted to average around 46.5kHz. 

To listen to these Common Pipistrelle bat sounds … 

The call of Pipistrelle bats is sometimes said to be a ‘hockey stick call’. This is because the call starts at its highest frequency and then falls in pitch whilst gaining power; this creates a sort of hockey stick shape when seen in a detailed sonogram , as below: 

A Noctule Bat was also recorded in concert with the Pipistrelles but a clean solo recording was not made. 

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I hope this has helped to interest you in our British Bats. 

May I take this moment to suggest Batbox detectors and either NHBS or Alana Ecology as suppliers of wildlife & field equipment; thanks to them for the good service they provide.