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Posts Tagged ‘Perseids’

Today (the 30th of June) is the annual “meteor watch day”, and because of this I am on live TV this evening talking about meteors. I haven’t been able to find out why today (the last day of June) is designated as “meteor watch day”, but as the day seems to be American in origin I’d normally suspect that the greeting card industry were behind it! But, in this case, I cannot see too many cards saying “happy meteor watch day” being sold, so maybe on this occasion it is not an invented holiday by the US greeting card industry 😉

What is a meteor?

I have discussed meteors several times before, for example here and here. But, just so this background information is all in one place, I’ll repeat myself. A meteor is simply a bit of (natural) space debris which enters into the Earth’s atmosphere. Before it enters the Earth’s atmosphere it is called a “meteoroid” (or, a really big one would be called an “asteroid”), but upon entering the atmosphere a meteoroid becomes a meteor.

The reason a meteor appears bright is because it is burning up in the Earth’s atmosphere due to friction. Most meteors are very small, no larger than grains of sand, but about once a day something the size of a basketball enters the atmosphere, and about once a week something about the size of a car. Larger and larger meteoroids are less and less common, so for example something the size of the meteor which exploded over Chelyabinsk in southern Russia last year (which is thought to have had a size of 17-20 metres as it entered the Earth’s atmosphere) probably enters the Earth’s atmosphere once every 50 or so years. The larger the meteor, the longer it will take to burn up, so most meteors (the sand-grain sized ones) burn up in less than a second.

A “meteorite” is a meteor which makes it to the ground, or at least of which fragments make it to the ground. So, for example, the meteor which exploded in the atmosphere over Chelyabinsk in February of last year (2013) led to several fragments landing (as I discussed <a href="“>here), and these fragments are meteorites.

Not surprisingly, it is only the larger meteors which make it to the ground as meteorites, although precisely what size a meteor needs to be to make it to the ground as a meteorite depends on a number of things including the composition of the meteor, the angle at which it enters the Earth’s atmosphere and the speed at which it enters the Earth’s atmosphere.

As I’ve also mentioned before, there is now strong evidence that it was the impact of an asteroid (remember that is just the name we give to large meteoroids) which wiped out the dinosaurs and most of life on Earth some 70 million years ago. Although the statistics are very paltry and therefore unreliable, we believe the Earth gets hit by an asteroid large enough to cause a mass-extinction event about once every 150-250 million years. Although undoubtedly rare, such events are cataclysmic to life on Earth, and so it is not wasted money to spend time and resources looking for such large “near Earth” objects.

Meteor showers

There are many meteor showers during the course of each year, they occur when the Earth passes through some debris in its annual orbit about the Sun. This debris is often the material blown off of comets. Some of the better known meteor showers are

Some well-known meteor showers
Name of shower Month in which it occurs
Lyrids late April
Perseids mid August
Orionids late October
Leonids mid November
Geminids mid December



The naming convention is quite simple, they are named after the constellation from where the meteors appear to emanate. So, for example, the Perseids meteor shower in mid-August appears to radiate from the Perseus constellation, as this figure shows.



The Perseids meteor shower in mid-August appears to radiate from the Perseus constellation

The Perseids meteor shower in mid-August appears to radiate from the Perseus constellation



The best way to see a meteor shower is not to use a telescope or binoculars, but rather to just look up with your eyes. Although when traced back they appear to emanate from a particular part of the sky, they can appear anywhere and using a telescope or binoculars will restrict your field of view, leading to your possibly missing a meteor.

Lying on one’s back on the ground (or a rug or mat preferably; or reclining in e.g. a deck chair are very effective ways to view a meteor shower. But, be warned that if you are observing one in the winter months it will probably get pretty cold, so have some warm clothes and blankets with you, and a warm drink.

Also, meteor showers are best viewed after midnight. This is because after midnight your particular part of the Earth is facing in the direction of the Earth’s motion about the Sun, and so the meteoroids enter the atmosphere at a steeper angle with a higher speed, and are more numerous than earlier in the night. The image below shows a wonderful image of the Geminids meteor shower, taken in 2012.



A wonderful image of the Geminids meteor shower, which occurs each December.

A wonderful image of the Geminids meteor shower, which occurs each December.



Fireballs

If you are very lucky you may see an extremely bright meteor, which is called a “fireball”. I have never seen one, but I have a story of how much luck is involved in seeing one. One November, when I was working at Yerkes Observatory, we arranged a public viewing of the Leonids meteor shower. Several of us working at the Observatory were out for a few hours with members of the public, and we saw several dozen small meteors. But nothing spectacular. My boss had been in his office working all evening, not taking part in our public observing. He lived in George Ellery Hale’s old house (the Observatory’s “Director’s House”), which was all of 2 minutes walk from the Observatory, and at about 9pm he popped home for a break from his work. As he walked the 150 metres or so to the house, he saw a great big fireball streaking across the sky, and the rest of us all missed it as we were inside warming up with a hot drink!



A fireball streaking across the sky

A fireball streaking across the sky



The Perseids meteor shower is in mid-August, and is one of the most popular meteor showers as it occurs during the Northern Hemisphere summer when many people are on their summer holidays and away from city lights. If you get a chance to observe it this year, I highly recommend it, but bear in mind that you do need to be in a dark place. Most meteors are quite faint, and you will miss all but the brightest ones if you are in a city.

Which is the most spectacular meteor shower you have seen? Have you ever seen a fireball?

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Back in November (2011), I wrote a blog on the planets which would be visible over the winter months. I thought it was about time, being over a week into the official summer, that I wrote a blog about the planets visible over the summer months this summer (2012). Unfortunately, there aren’t many planets visible this summer, Saturn and Mars is your lot.

This summer, Mars is to the Western side of the constellation Virgo, and is transiting at the moment (in early July) at 18:24. This means that, by the time it gets dark, which in Wales is not before 21:30 this time of year, Mars is quite far over to the West and on its way down in the sky. On the 3rd of March, Mars was at opposition, which means the Earth was at its closest to it. As a consequence, not only is Mars quite low (25 degrees above the horizon) by the time it gets dark, but it is also not very close to us. These two things combined mean Mars will be quite an unspectacular sight through a telescope.

Mars through a small telescope. If you are very lucky, you may see signs of the polar caps.

The other planet visible this summer is Saturn. Saturn is transiting at the moment (early July) at 19:55, so is reasonably high (30 degrees) in the sky after it has got dark. It is also to be found in the constellation Virgo, but over towards the constellation’s Eastern end, just to the North of the constellation’s brightest star Spica.

Saturn and Titan through a small telescope. Even with quite a small telescope, you should be able to see the rings and Titan quite easily.

Seeing Saturn for myself never ceases to excite me. Even through quite a small telescope one can clearly see the rings, and usually Saturn’s brightest moon Titan. If you want to see either Mars or Saturn this summer, then you really need to do so over the next few weeks, as by August they really will be setting too early to be able to see at all.

Although there aren’t too many planetary highlights this summer, there is still a lot to see in the Summer sky. One of the easiest things to find is the summer triangle, which is an asterism made up of Vega, Deneb and Altair (the brightest stars in the constellations Lyra, Cygnus and Acquila respectively).

The Summer Triangle, which is made up of the stars Vega, Deneb and Altair.

One of the other hightlights of the summer sky is the Ring Nebula, Messier 57. It is, in fact, what is called a Planetary Nebula. These are nothing to do with planets, but are in fact dying stars. Their name comes from the fact that, through 17th Century telescopes, they resembled the gas giant planets Jupiter and Saturn.

A planetary nebula is an object where the central star has thrown off its outer layers, and the remaining core (which we call a White Dwarf), is the remains of the once active star. The gases glow due to the electrons in the gas being excited by the energetic ultra violet light coming from the white dwarf. The white dwarf at the centre of the Ring Nebula is quite clearly visible through a medium-sized telescope.

The constellation Lyra (the harp), showing the location of Messier 57, the Ring Nebula

Messier 57, the Ring Nebula, one of the best planetary nebulae in the sky.

Our own Sun will end its life as a planetary nebula and white dwarf, as it is not massive enough to become e.g. a neutron star or a black hole. For a brief period (about 50,000 years), what hydrogen which the Sun will throw off during its asymptotic giant branch phase will glow in the sky, before fading from view as the white dwarf remains of the Sun slowly cools over time.

Update

I am going to be on BBC radio this Friday (13th of July 2012) talking about the summer sky. In preparing for this interview I realised that Jupiter is, of course, visible in the morning sky. It is to be found in the constellation Taurus, which is itself an easy constellation to find with the bright star Aldebaran in it. Jupiter is currently (mid July) rising at 02:45, so over the next few months is actually the best planet to see, by mid-August it will be rising about 00:45 and my mid-September by about 22:45.

Jupiter is in Taurus at the moment, just to the north of the bright red star Aldebaran, and to the East of Capella, “the Shepherd’s star”, which is in the constellation Auriga.

Jupiter is well worth looking at in a telescope. As I commented in my blog about the 2011/12 Winter sky, one can nearly always see the Galilean moons of Jupiter through a small telescope, and if one is lucky one can also see the bands and the great red spot. So, if you are out looking at the sky over the summer, don’t forget to stay up late (or get up early) to catch a glimpse of Jupiter.

Venus is in the same constellation. It is only some 5 weeks ago that Venus transited the Sun, but already it has moved to the West of the Sun in the sky so that it is now rising before it. Venus will appear as a large crescent at the moment, as it is on the near side to us in its orbit.

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