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The Lyrid Meteor Shower[/COLOR]
The oldest known Meteor Shower graces our skies this month, on April 22nd as the Lyrids kick off the 2007 meteor season! While the waxing crescent moon will cause some interference, the sky should be dark enough to make many of these bright meteors easily visible. All you need is a clear sky, away from city lights… No telescope is needed!
What is a Meteor Shower?When comets make their slow journey around the Sun, the shed off tiny dust particles which continue in the same orbital path, much like dirt falling off a truck on the highway. Each year, as the Earth travels around the Sun, our path intersects the same path that comets take, and like the windshield of our car, our atmosphere slams into this cosmic dirt, producing brief steaks of light commonly called “shooting stars”.
Photo of Lyrid Meteors:http://www.astropics.com/lyrids/4lyrids.htm
Here's a photo taken by the MSX satellite showing the meteors hitting the atmosphere from above!http://members.cox.net/starsgalore/leonidsfromspace_msx_big.jpg
While intense Lyrid displays are not unheard of, they are rare. In fact, the Lyrids are better known for their longevity than for their dazzle. Lyrids have been observed for at least 2600 years, according to Chinese records from 687 BC describing "stars that fell [like] rain." This makes it the oldest recorded meteor shower in history. In 1982, people witnessed a rate of 90 meteors per hour for a brief time – but this is not usual. On an average year, you can expect 1 or 2 bright meteors every few minutes. These showers are unpredictable, and no one can say what you will see exactly.
Where did the Lyrids come from?The Lyrid meteor stream is associated with periodic Comet C/1861 G1 Thatcher, which loops around the Sun every 415 years in an elliptical orbit tilted nearly 80 degrees with respect to the plane of the solar system. Because the comet spends most of its time well away from the planets, it is nearly immune from significant gravitational perturbations. This is probably the reason why the debris stream has remained stable and the Lyrid shower has been observed for 2600 years.
What causes the streaks of light?The comet debris that we are plowing into strikes the earth at a velocity of 49 Kilometers per second (almost 110,000 MPH!). The ram pressure from these tiny collisions heats and ionizes the air around them – creating an intense, but brief glow. Once they enter our atmosphere, these tiny comet fragments are called meteors and if one should be large enough to actually make it to the Earth’s surface before disintegrating it is called a meteorite.
Where in the sky are they?Since the Earth slams into this comet orbit each year, the exact point of entry on Earth is the same. In the spring sky, this will be in the Constellation Lyra, near the bright star Vega in the Eastern sky. From a Northern Hemisphere site look low in the Northeast just after dark, but after midnight look higher up in the East. Vega is the brightest star in that part of the sky.
[SIZE="Small"](screenshot using Stellarium on Mac OSX)[/SIZE]
This point is called the radiant (in yellow above)and all the meteors will appear to come from this point. This spot is really an optical illusion. All the meteors are striking the Earth parallel to each other. The lines heading to a point are like the vanishing point of train tracks converging to a point in the distance. Since the exact size of the meteor is random, some will not begin to streak until far from this point – but all streaks will lead back to this radiant point.
You will occasionally see a meteor shoot across the sky from a different direction than this radiant. That is a sporadic meteor, simply a piece of space debris left over from the creation of the Solar System.
How should I observe the shower?Lyrid meteors are about as bright as the stars in the Big Dipper. You should get away from city lights as much as possible to truly observe the shower. You won't need binoculars or a telescope, the naked eye is all you need, as they often streak half way across the sky! Occasionally you may see a meteor travel all the way to the horizon, and about 15% will even leave behind a dust trail high in the sky! If you are lucky, a meteor will be large enough to break apart into smaller pieces or steak across in a blazing fireball - occasionally loud enough to be heard!
The best thing to do is to dress warmly, bring some hot chocolate or coffee and a sleeping bag. Then lie down and simply look up! If you have several people with you, have them all laying in a different direction, so you all cover the whole sky. The display is right overhead!
When should I observe them?You can expect to see this meteor shower anytime from the evening of the 21st of April to sunrise on the 22nd. You may occasionally spot some Lyrids a few days before this night, and a few days after – but the real shower will be between midnight and sunrise on Sunday Morning, 22 April. Go out after dark on the 21st, and you may start to see them. But early morning, just before dawn and you will be at the peak of the Lyrid shower.
The reason that you need to observe before dawn is due to the fact that this is the time when your observing point on Earth is hitting the meteor stream head on. Think of being in a car driving into a cloud of flies! At night, just after sunset you are peering out the REAR of the car. Just before dawn, your night sky overhead is looking straight ahead in the direction that the Earth is moving. At that time of the night (morning?) you are looking out the windshield of your “car” – and the meteors are slamming into your Earth head on!
[SIZE="Small"](C)2007 Peter Kennett[/SIZE]
For a fun twist on observing meteors, tune your AM radio to an unused frequency. Radio waves from stations half-way across the planet can bounce off the ionized meteor trail and allow you to receive distant stations just for a few seconds!
Are we in any danger?No. This debris is very tiny, most are the size of grains of sand or small pebbles. As they strike the Earth’s atmosphere they disintegrate into dust. In fact, Earth receives about 1,000 tons of space dust each year!
Can I Photograph this event?Capturing meteors as they burn up across the sky is probably the most exciting photographic experience in astronomy. Ideally you want to try to capture a meteor shower when the date of the shower happens to line up with a period with no Moon in the sky. A bright Moon will make viewing and certainly photographing meteors difficult if not impossible.
The easiest way to capture meteors is to use a film camera that allows you to lock the shutter open indefinitely, such as a Single Lens Reflect camera (SLR) with a locking cable release. You should use a standard 50mm or wider lens, with its aperture opened all the way. Use a fast film, and a steady tripod. Point the camera straight up and be sure to have the camera focused on infinity. Lock the shutter open, and leave it for several minutes without moving the camera – a tripod helps. If you see a meteor cross the sky in the same area your camera is pointing at, you should stop the exposure and start another. If you have very dark skies, you can continue the exposure and try to catch more meteors, but remember that the longer you expose the more your stars will trail due to the Earth’s rotation, and the more light pollution you will get. Most areas will want to stop within 20 minutes per exposure, but very dark skies can go longer.
I recommend a 35 to 50mm lens. A longer lens will catch fewer meteors, and those it does catch will often be so long that they get cut off. A shorter lens will not show the really faint meteors as well.
If you have a digital SLR, you can take hundreds of shots (10 second exposures) and combine them into a fantastic collage. Probably the best example of this technique is by Fred Bruenjes. His incredible composite image consisting of over 160 meteors is a must see image!
http://www.moonglow.net/ccd/pictures/other/
Enjoy!
Peter Kennett
http://www.PetesAstrophotography.com
