The Perseids!

The radiant for the Perseids.  Click for a expanded chart you can use to locate the constellation Perseus.  Credit: thenineplanets.org

The radiant for the Perseids. Click for a expanded chart you can use to locate the constellation Perseus. Credit: thenineplanets.org

One of the best meteor showers of the year is about to reach its peak. The Perseids is a shower not to be missed.

Meteor showers, in a nutshell, come from the debris trail of comets. As a comet moves through its orbit, it can shed bits of debris for various reasons from dust size on up. In particular when a comet gets close enough to the sun for it to warm slightly and become “active” and the volatiles and dust are streamed away creating the familiar comet tail from the comet tail. What remains is a rather persistent trail of debris. We see meteor showers when the Earth passes through these debris trails.

Not all debris trails are created equal in the amount of debris they contain as you might expect, nor does the Earth always hit the debris trail fully. Therefore some showers are better than others and sometimes a particular shower can vary from year to year.

A couple more things, meteor showers are generally named for the area they seem to come or radiate from (or near), hence the term “the radiant”.

We should define the terms meteor and meteorites. We get a little sloppy and use the two interchangeably, a meteor makes the streak we can see (aka falling star) and a meteorite is a meteor that makes it to the ground. By the way a meteoroid is the little (or big) chunk of iron or rock orbiting around in space. Don’t worry that much about it though not many will ‘call you out’ on it, I certainly won’t.

So the progression is: A meteoroid hits the atmosphere and creates a bright streak and becomes a meteor, most burn up, those that don’t and survive to the ground is a meteorite.

The Perseids shower is made up of the debris from the comet 109P Swift-Tuttle. 109P Swift-Tuttle is a periodic comet, the period of the comet is a little more than 133 years. The last perihelion for this comet was in 1992 so we know the comet is heading away from us and in fact it is something like 34.7 astronomical units away by now.

The radiant in the case of the Perseids is the radiant is the constellation Perseus. If you are wondering why Perseus is the radiant, a look at the static image below from the JPL Small-Body Database Browser showing the path of the comet (the blue arc) might help explain things. Image: JPL

JPL109P

Perseus is a northern constellation and with some planning may be viewable from the mid-latitude regions of the Southern Hemisphere. To get a chart you can use to find Perseus, click the image at the top of the post.

Generally the better viewing is going to be late on 12-August and the following hours early on 13-August. Don’t worry too much you should see activity. It should go without saying you need clear and pretty dark skies. Go out and get comfortable orienting yourself towards the constellation Perseus and enjoy. No, don’t worry about being exact, close is close enough you will see the radiant. Just be sure to remember the progression of the radiant is to the west as time goes on, so if you go out a little while before daybreak (as I LOVE to do) the radiant will have moved.

The Perseids are nice because you can face north and look straight up and see them. One year I did most of my observing looking straight up out of a hatchback on a car while traveling home from a class (learning initiation and administration of intravenous therapy of all things).

You DON’T have to wait for the peak which is 12 – 13 August to see the Perseids, even tonight you can see them.  They build slowly over time to the peak and also diminish slowly so they are not “all or nothing”.

Go on out and enjoy the show!

2 thoughts on “The Perseids!

  1. Very cool summery! I recently tried to find all these pieces of information and is was really a pain in the arse.

    This will be my first conscious observation of an meteor shower and I really look forward to it.

    Just two minor quibbles:
    1. I just checked it with “Cartes du Ciel” and at least here in the northern hemisphere Perseus rises in the evening in the North (or something 20 degrees to the East) and then moves to the *North-East* (not to the West) *and* at the same time Perseus moves to the Zenith. It comes close to East and Zenith a bit after sunrise.

    2. The peak is supposedly rather sharp:
    http://media.skyandtelescope.com/images/Perseids_2012_plot.jpg
    http://www.skyandtelescope.com/observing/highlights/218037521.html
    But yes, there is enough activity before and after that date.

    As for me:
    The weather here is not good. The forecast says I might be SOL here and have to look tonight (the 11th) because on the 12th and 13th it might be cloudy and rainy. Nooooo!

    But at the same time light pollution is very low here at the moment – my local community is battling with an “invasion” of flies every year, and these insects swarm in masses around the street-lights. So every year they switch off the street lights for a couple of days during this “fly season”, which is now! And yesterday evening the clouds briefly broke up and revealed a gorgeous sky.

  2. Ah yes. The movement is actaully in a circular fashion around so depending on the time of observation could be in either direction and indeed for you it would be north-east and for the same local time period for me it would be west. I need to think globally I guess LOL. Cartes du Ciel is of course correct and is a great program BTW. I am going to do a post on it shortly.

    Oh and the peak is rather well defined, the slow build up I mentioned is really slow drips and drabs and meteors can be seen for some time before (days) but NOTHING like the peak.

    Oh I hope you do get good skies! I am hearing clouds are going to move in on me.

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