• Choose a month

  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Get ready for the 2009 Perseid meteors – but don’t expect a shower!

When:
The nights of August 11-12 and August 12-13 starting an hour after sunset.

Where:
Anyplace you have a clear and dark sky – with as much horizon visible as possible.

Look:
Northeast – while a Perseid meteor can appear anywhere in the sky, your best chance to see  several will be to scan the sky to the northeast in the general vicinity of the W of the Cassiopeia.  However,  you don’t have to fixate on one region. Get comfortable, look high in the northeast, and from time to time look around to different sections of the sky to enjoy the sights and stay alert.

The W of Cassiopeia, rising in the northeast after sunset, is a good place to start looking for Perseids. The actual radiant point is below it in the constellation of Perseus. Click for a larger image. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

The W of Cassiopeia, rising in the northeast after sunset, is a good place to start looking for Perseids. The actual radiant point is below it in the constellation of Perseus. Click for a larger image. (This chart uses a screen shot from Starry Nights software which I have then annotated.)

What can you expect to see?
More meteors than normal, but far fewer than you would in a year when the Perseids are at their best.  I will be happy if I see one every five to 10 minutes. For everyone everywhere, the intensity of the annual Perseid “meteor shower” is in a downswing and will be further reduced this year by competition from a bright moon. In North America the predicted shower peak comes at mid-day when, of course, none will be seen because it’s – well, day! That’s why it’s recommended to look either the night before or the night after the predicted peak of 18 hours Universal Time August 12.

The moon is especially bothersome because it will be near last quarter – bright enough to drown out many meteors – and will rise around midnight.  The shower is normally best seen after midnight because it is then that its radiant point is highest in the sky. So when the most meteors would normally be seen, there will be the most interference from the moon.

So, should you just not bother with the Perseids this year? No.  I certainly intend to take advantage of it. If the sky is clear on either night I intend to make them part of the reason to observe, but not the only reason. I won’t lie in a beach chair looking up most of the night as I have done other years.  Instead, if the skies are clear, I plan to head for the darkest spot nearby – Allens Pond Bird Sanctuary – and from there I will do three things:

  • Enjoy the summer Milky Way and the Perseids for the hour or two before the moon comes up and the skies are genuinely dark. During that time I will lie back in a rotating beach chair, binoculars at the ready, and look for Perseids.
  • After the moon rises I intend to enjoy a night full of planets and double stars, as well as the moon.
  • And as the icing on the cake, I’ll continue to keep my eye out for more meteors than I can expect to see on a typical night.

So what’s a meteor and do they really shower?
Meteors and meteor showers are fun if for no other reason than they are a chance to see something happening in the sky. Most of what we look at doesn’t change – or rather changes so slowly we don’t notice the change. Meteors, on the other hand, demand that you be looking in the right place at the right time. Only on the very rare, very bright meteors do we actually have time to warn others and have them turn their heads and see what we see.  But what we see is a space event happening closer to us than any other.

Meteors – “falling stars ” – can be seen any time. You don’t have to wait for a shower like the Perseids; you just have to be lucky. But they are most frequent at certain times in the year when the Earth happens to be plowing through a meteroid-rich area.  We call this occasion a meteor shower. (For your dictionary: A meteroid is a small bit of space rock that becomes a meteor when it collides with our  atmosphere and heats to incandescence as it descends towards Earth. When it gets here – which is rarely as anything except fine, incinerated dust – it is a meteorite. )

The reason for a shower such as the Perseids is that we are passing through the debris trail of a comet. Think about it. The general model for a comet is a “dirty snowball,” and as that dirty snowball nears the Sun it melts, and as it melts it leaves a trail of dirt particles behind it – particles that remain in orbit until something like the Earth sweeps by and captures some of them with its gravity.

The comet itself can vanish entirely – but the result is a river of space dust – a river that is most intense nearest where the comet actually was.  That’s why there are some years – the 1990s in the case of the Perseids – when the meteor shower is more intense than others.  Now we are in a period when we are passing through the trail of the comet that creates the Perseids at a point where that trail is sparse – so there will simply be fewer Perseids.

That trail is not encountered all over the sky. It collides with our atmosphere near a particular point in our sky. That point is called the radiant – you might think of it as a hole through which the Perseids fall – and in the case of the Perseids it appears to be in the constellation Perseus.  But we don’t see all the meteors at this point. We see a meteor only when its collision with our atmosphere is intense enough to make it burn up. The faint meteors we see are made by a speck of dirt about the diameter of a pencil lead. The brightest ones are caused by something about the diameter of the pencil’s eraser.  In either case it will, for all practical purposes, burn up entirely in our atmosphere – 50 -to-75 miles up – and nothing significant will remain for anyone to find on Earth. But exactly where it burns up is another thing. That’s why we will see a sudden flare – a falling star – anywhere in the sky.

And that’s awesome! Think about it a moment. If someone struck a match 50 miles away would you see it? Evena big, wooden kitchen match? How about amile away?  Yet a grain of sand, hurtling into the atmosphere, shows us such a brilliant light we can’t miss it!

When you are watching for Perseids you don’t have to look near the radiant point, though you will see more there.  A meteor can flare up suddenly anywhere and appear to draw a short (usually 5-10 degrees long) straight line across the dome of the sky. (Bright ones may actually leave a trail, which you can see for a few seconds with the naked eye or binoculars.) If we trace a line backwards along the meteor’s trail we will see it comes from the area near the radiant point.

In the early evening that Perseid radiant point is low in the northeast. That means nearly half the meteors that are radiating from it are happening below our eastern horizon. That’s why the shower is best in the early morning hours when the radiant is high in our sky. If the radiant is overhead, then we have nearly doubled our chances of seeing  a meteor.

There are many meteor showers in the course of a year and some are better than others. The Perseids is one of the most reliable ones and happens to come at a convenient time for northern hemisphere observers when it is comfortable to be out at night, lying on the ground, and looking up.

Personally, I don’t like the word “shower.” It immediately gives the impression that what we are going to see will be more intense than what most of us actually experience. I prefer calling this a meteor “event.” But, we have been calling such events “showers” for years and too often they are hyped in the press and then people are disappointed when nothing like a shower occurs.

In the final analysis there’s only so much time you can spend lying on your back gazing at the starry sky: though I very much enjoy that time it’s made much more enjoyable by knowing that at any instant there’s a heightened likelihood that you will see a bright meteor.  That – and the summer Milky Way – make looking for Perseids in a dark and moonless sky always worth the effort for me.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: