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May 5th, 2012:

Perigee Moon

Perigee Moon.jpg

To watch the red perigee moon rise over the first city lights of evening is sublime; to understand why it is so broad and so bright is the particular pleasure of being human.

Sky Show Lookahead

Some interesting things are coming up in terms of sky spectacle, and this would be a good time to post a reminder, especially as the first will be happening…tonight.

There’s a full Moon tonight, and it’s being called a “supermoon” because it will be larger (by 14%) and brighter (by 30%) than an average full Moon. (I learned the phenomenon as a “perigee Moon” many years ago.) It will be the brightest full Moon we’ll see this year, though most years have at least one supermoon.

14% wider isn’t a lot, and in truth you’ll be hard-pressed to notice its greater size if you don’t watch the Moon frequently. For the greatest effect, watch it rising over whatever the Moon rises over in your corner of creation. But brighter, yeah. That thing will be a searchlight, and if you ever wanted to go dancin’ in the moonlight without tripping over your own feet, this would be the night to give it a try.

Then on Sunday, May 20, we’ll get something completely different: An annular eclipse of the Sun. There’s a boggling coincidence in the apparent sizes of the Sun and the Moon from here on Earth. Both are approximately 30 arc-minutes in diameter. This means that on occasions when the orbits of Earth and Moon align such that the Moon passes in front of the Sun, the Moon just barely covers the Sun. Phenomena like prominences and the solar corona that would be hidden by a Moon with a larger apparent diameter are thus revealed to us.

Now, the next time the Moon passes near the Sun, it will pass in front of the solar disk, causing a solar eclipse. But this time, the apparent diameter of the Moon will be close to its minimum, and thus the disk of the Moon will not quite cover the disk of the Sun. This provides us with an annular eclipse, meaning a ring of bright Sun with the dark disk of the Moon at its center.

The eclipse of May 20 is significant for many of my readers because the annular phase touches the western US. The eclipse happens late in the day in the US, and by the time the umbra (the darkest part of the Moon’s shadow) reaches west Texas, the Sun is setting. If this is hard for you to envision, see the wonderful animation (even more wonderful because Flash is not required) on the eclipse’s Wikipedia page.

One of the best places to see the eclipse will be Albuquerque, which is almost bang-on the center of the umbra’s path. The eclipse’s maximum extent will occur at 7:33 PM local time, just over the western horizon. A partial eclipse will be visible throughout the US except for the east coast.

All the usual cautions for observing the Sun apply. The high road is to use a small telescope to cast the image of the Sun on a piece of white foamcore. That’s generally what I do. Just be careful not to look (or allow others to look) through the telescope. Pinholes can work remarkably well, and I remember projecting one partial solar eclipse onto the sidewalk in front of a restaurant through the holes in a saltine cracker. You may glimpse a small sunspot or two during the partial phase, though this cycle’s spots are the weakest of any I’ve seen in my lifetime. We’re within a year of the Cycle 24 sunspot maximum, and the pickins are still very slim.

Then we move to something not only completely different but vanishingly rare: The transit of Venus across the face of the Sun on June 5/6, 2012. There was no transit of Venus in all of the 20th century, and there will not be another until 2117. In the continental US, it will happen before and then after sunset; i.e, Venus will still be crossing the solar disk when the Sun goes below the horizon. To see the whole thing, you’ll need to be west of Hawaii and east of central Australia.

I’ll have more to say about the transit of Venus later in May. However, in the meantime, Venus is going into its crescent phase, which is very bright and easily seen through binoculars–and glorious in even the smallest telescope. Look west after sunset and you can’t miss it; it’ll be the brightest thing in the sky after the Moon.

Quite a show coming up, starting tonight. Don’t miss it!