Wednesday, June 15, 2011

Meteor shower tonight, and why they have calendar dates and constellation-names

Tonight (June 15) is the peak of this year's June Lyrid meteor shower, which is not necessarily the most spectacular meteor shower or the brightest, but does tend to produce up to fifteen faint meteor streaks per hour, which is nothing to sneeze at (for comparison, a list of meteor showers throughout the year can be found here).

Here's a link to a short writeup from Universe Today, which is also the source for the video above showing images of the meteors from last night.

Meteor showers take place around the same day each year because they are caused by the passage of the earth through debris in space along its orbital path. The current consensus is that this debris is most often the particles of frozen ice and other frozen elements such as ammonia that detaches from comets on their orbit of the sun. These detached particles of "comet dust" continue to orbit the sun as well, along the same general path that their "parent comet" took, except more slowly. Therefore, when the earth comes across one of these "comet trails" of debris, the particles enter the atmosphere and burn up, creating visible streaks across the sky.

Since these trails are located at specific points on the earth's orbital path, they can be seen on the specific day each year that the earth begins to pass through that location. Also, they are associated with a constellation that is visible from that point in earth's orbit where the meteors will seem to originate each year (the June Lyrids originating from a point in the constellation of the Lyre, and the spectacular August Perseids originating from the constellation of Perseus, for example).

If you think about the mental model of the earth orbiting the sun inside your dining room, which was described in this previous post on Orion, as well as in the recent interview on Red Ice Radio, you will remember that at certain points on its orbit, the walls visible at night to the observers on that earth would have different pictures on them depending on where in the room the earth was on that night. If you took two chalkboard erasers and clapped them together at a certain point in your dining room, then when the earth that was circling inside your dining room reached that cloud of eraser dust, the inhabitants on it would see a meteor shower that night when they were facing away from the sun (which is located in the center of your dining room, in the middle of the dining room table, perhaps in the form of a candle or a flaming ball of cotton).

It stands to reason that if there was a picture of a lyre on your wall in the direction that the observers would be looking when the earth entered the eraser dust cloud, then they would see that meteor shower emanating from that picture of a lyre (perhaps it is a Guinness poster on your wall that they would see, which features a harp rather than a lyre). The point from which the meteors appear to emanate is know as the "radiant" because they radiate from that location in the sky.

When the earth in your dining room passed out of the cloud and proceeded onward, it would pass through other clouds that are located along its track, but when it got back to the same point a year later, the eraser chalk dust would still be there, lingering in the air, and the inhabitants of the tiny earth in your dining room would see it originating from the region of the harp or the lyre on the wall of your dining room again.

The constellation of Lyra (the Lyre, not to be confused with the zodiacal constellation of Libra, the Scales) contains the brilliant star Vega, the fifth brightest in the sky and the first star ever photographed (in 1850), according to H.A. Rey (38). It is part of the easily-located "Summer Triangle" visible high in the summer sky in the northern hemisphere during the months May to October or so, and which consists of the stars Vega, Deneb (in the Swan) and Altair (in the Eagle). Both the Swan and the Eagle are in the Milky Way. Deneb can be found using a line from the back two stars in the bowl of the Big Dipper as "pointers," rather than the front two stars of the bowl (which are used as pointers to the North Star, Polaris).

Although the moon is full again and will be doing its best to drown out the stars in the night sky around it, try checking out the June Lyrid meteor shower tonight.