Friday, October 21, 2011

The Orionid meteor shower

The earth has reached a special point on its orbit, a point it passes through every year at about this time, when the Orionid meteor shower is visible.

The Orionids are not as spectacular as the Perseids in August, numbering around 30 per hour in contrast with the almost one hundred-per-hour that the Perseids can muster, but they are remarkable for three reasons.

First, they can be seen to radiate from a point near the constellation Orion, one of the most recognizable and dramatic constellations in the night sky, and one which plays a very important role in ancient mythology (see this previous post for more detail).

Second, the Orionid meteor streaks themselves are dramatic, often having long tails and exhibiting distinctive colors before they burn up.

Finally, the space debris that causes the Orionid showers (burning up in the earth's atmosphere as our globe plows through the debris field) is left by one of the most famous denizens of the solar system: Halley's Comet.

To understand why we encounter this particular debris at this particular point in our orbit each year, and why it is associated with the constellation Orion, visit this previous post entitled "Meteor shower tonight, and why they have calendar-dates and constellation names."

The diagram above (admittedly a rather rough sketch) may help in conceptualizing the Orionids. As earth makes its annual circuit around the sun, different constellations are seen in the night sky by observers when the daily rotation on the axis turns them away from the sun (although those constellations that are "on the ceiling" in the direction of the north pole for those in the northern hemisphere, or "on the floor" in the direction of the south pole for those in the southern hemisphere, are visible at night year-round).

As earth began approaching the point where Orion would become seen again, he was first visible rising in the east just prior to the sun rise: look at the diagram above, note the direction the earth is turning around its axis, and envision Orion being revealed just above the eastern horizon right before the turning revealed the sun. However, as earth progressed, he rose above the eastern horizon a bit earlier each 24-hour period, and he is now making his way pretty far across the sky before the turning of the earth brings the sun back up to drown out the night stars.

As earth progresses around its path, it runs into various clouds of debris left by comets and other phenomena, and each October it reaches the trail of particles left by the famous comet Halley (for more on the importance of comets as clues to the geology of the earth, see the discussion in this post, and the other posts referenced in that one). Because of the angle that the earth plows into this field, the stars in the background for an observer gazing into the heavens looking for meteors happen to be the stars of Orion, as shown in the drawing (of course, Orion's stars would be very far "into" the surface of the drawing; he looks too close in this drawing because to make his stars easy to see I had to draw them big -- he would look farther away if his stars were drawn as tiny dots but then you might not notice them very easily).

The path of Halley's Comet and the debris that it leaves is shown by a cloudy trail of dots in the above diagram. Note that the earth crosses through this track of debris again in late April / early May, when Halley's dust is again responsible for a meteor shower, this one seen with Aquarius in the background, giving rise to that meteor shower's name, the Eta Aquariid shower. The Eta Aquariids also have tails with colorful streaks just as the Orionids do.

One thing to note that doesn't come out perfectly clearly in the rough sketch is the fact that Halley's comet has a path which is not on the same plane that the earth and the other planets follow (it is not on the ecliptic plane). Its plane comes "up" from the south pole side of earth, goes around the sun, and then exits back "down" towards the direction that the south pole is pointing, not at a 90° angle to our ecliptic plane but at a shallower angle of attack, as shown in this helpful diagram.

Here are a couple of articles that explain how to view the Orionids this weekend: