Saturday, April 21, 2012

April Lyrid meteor shower for 2012, what causes it, and how to see it

The April Lyrid meteor shower takes place each year as the earth reaches the portion of its orbit designated by the dates April 20 through 26. This meteor shower is the product of the dust trail left by the long-period Comet Thatcher (C/1861 G1). The earth reaches the heaviest part of the residue cloud on April 21 or 22 each year, and this year it should be on April 21. Since these meteors can generally only be seen at night, the night of April 21 is probably the best night to see the shower, so make your plans now!

Here is a previous post featuring a rough sketch I drew to help illustrate the connection between the paths of comets traveling through the inner solar system and the meteor showers that are associated with different nights of the year. That diagram shows the meteor showers created by the debris from Comet Halley, the October Orionids and the Eta Aquariids (which take place around May 4). Here is a link to a helpful web page listing the meteor showers of the year.

Note that Comet Halley in the diagram can be seen to approach the inner solar system and earth's orbit from "below" the ecliptic plane (the south-pole-side of the plane of the ecliptic), and then break above the plane for a short time before diving back down on its way out. This diagram shows the plane of Halley's Comet more clearly.

In contrast, the image above from NASA's Jet Propulsion Laboratory shows that Comet Thatcher spends most of its time "above" the ecliptic plane (on the north-pole-side, that is) and dives in from a fairly steep angle. In these diagrams, the path of the comet is light blue when it is above the ecliptic plane of the earth, and darker blue when it is below it.

The image also clearly shows another striking aspect of Comet Thatcher: it is a long-period comet, with an aphelion over 110 astronomical units from the sun (an astronomical unit or AU is a unit of measurement corresponding to the mean earth-sun distance). In contrast, Halley's Comet is a short-period comet, with an aphelion of only 35.1 AU. Comet Thatcher only comes by every 415 years or so, while Halley's Comet appears every 75 or so years.

The images below show Comet Thatcher's path from a closer and closer vantage point (to give a full appreciation for the amazing orbit of this far-traveling space object: after you stare at those for a few minutes to let it really sink in, we can go on to discuss the meteor shower that its trail causes each year):

As the earth passes through the line marked on the images above representing the path of Comet Thatcher, the debris left by the comet causes the April Lyrid meteor shower (you can see why Comet Thatcher only causes one meteor shower each year, while Halley's Comet causes two if you compare the third image above with the image of Comet Halley's orbital path).

This post from last year describing a different meteor shower (the June Lyrids, which also appear to radiate from the region of the constellation Lyra, but which are caused by a completely different comet trail and are not as strong as the April Lyrids nor as anciently attested) gives a mental image you can use to explain the predictable meteor showers of the year to your friends (it is also a helpful mental construct for understanding the important phenomenon of precession, as discussed in this post and in greater detail in the Mathisen Corollary book).

As that post from last year's June Lyrids explains, meteor showers are named for the constellation from which they appear to radiate (they may be seen all over the sky, but they will seem to be coming from a certain point called the radian, and their tails will generally appear to point back towards the radian). Meteor showers named Lyrids feature meteors which appear to streak away from a point in the constellation Lyra the Lyre. This diagram of the night sky for the early morning hours of April 22 gives a good depiction of that concept.

The Lyre is a small but easily-identified constellation, because it contains the star Vega, the fifth-brightest in the sky. Also, for viewers in the most heavily-populated regions of the northern hemisphere, Lyra is visible every night of the year (although sometimes only during the post-midnight hours when most people are fast asleep). For observers in San Luis Obispo, California (W120°39'36.0", N35°16'48.0") on the night of April 21, 2012, the bright star Vega rises at 9:14 pm (and about four minutes earlier each night after that). For New Paltz, New York (W 74°04'48.0", N41°44'24.0"), Vega rises at 10:23 pm on the night of April 21 (and about four minutes earlier each night after that).

Vega is one of the three stars of the brilliant Summer Triangle, discussed in this previous post. That post features several diagrams to help you locate Vega and the Lyre (in conjunction with the Swan and the Eagle and the Milky Way).  You can also use the interactive sky chart available at Sky & Telescope which enables you to enter different locations and times and view the sky and constellations for those different places and times (you can get to that by starting at this Sky & Telescope article about this month's Lyrids).

Because the moon will be at New Moon on April 21 (between the earth and the sun and hence out of the sky during the night), it should be an ideal night to try to observe the Lyrid meteor shower.

In fact, the conditions are good enough that NASA scientists will be attempting to photograph some of the meteors from the International Space Station, and simultaneously from locations on the earth (which may enable them to create a "3-D view" of the meteor if they can catch one from two directions at once). This article from NASA's Science News page has some helpful advice from NASA scientist Bill Cooke, head of NASA's Meteoroid Science Office, who will be staying up all night on April 21-22 to chat with the general public about the shower at this URL (thanks to the astute Mr. Mark D. S. for alerting me to this!).

Here's hoping that this year's April Lyrids are a positive and memorable event for all observers. As you observe a few of the meteors, think about their origin in that lonely ball of ice orbiting far, far beyond Pluto right now (see top diagram). If you want to learn more about the difference between long-period and short-period comets, and the possibility that this bi-modal distribution of comets may support the hydroplate theory of Dr. Walt Brown, check out this and other previous posts about comets.