Thursday, December 11, 2008

Biggest Moon, Smallest See

This week, for your observing pleasure, I give you the biggest Moon to moon over and the smallest sea to see.

Just after sunset on Friday, December 12, look east to watch the Full Moon rising. This month, the Full Moon occurs only a few hours after it reaches perigee, which makes it the biggest Moon of the year. Perigee is the point in the Moon’s orbit where it’s closest to Earth. The Moon’s orbit is an oval-shaped ellipse, not a circle. At one narrow end of its elliptical orbit, the Moon is at perigee. At the other end, it’s at apogee, the point in its orbit where it’s farthest from Earth.

Image source: NASA

As with any object, the closer the Moon is to us, the larger it appears. In particular, Friday’s Full Moon will appear about 14% bigger and 30% brighter than it did when it was near apogee, back in June. Of course, you won’t have the apogee Moon hanging in the sky next to the perigee Moon to aid you in size comparison. You’ll just have to take my word for it and enjoy the show as the super-sized Moon climbs in the sky.

Appearance of Moon at perigee and apogee in 2007

Take advantage of the jumbo Moon to look for the smallest naked-eye lunar feature that most people can see: the Sea of Crises or Mare Crisium (MAH-ray CRISS-ee-yum). A mare (MAH-ray) is a crater that filled with lava that afterwards cooled and solidified into basalt. Although Mare Crisium is not the smallest sea on the Moon, it is a mere 260 miles in diameter. Spotting it is quite an accomplishment.

Image from Wikipedia

Look for Mare Crisium soon after moonrise on the 12th, while the sky is still bright with twilight. I don’t have the sharpest eyesight, and I find it easiest to pick out that feature when the Full Moon is not yet blindingly bright, as it will certainly become after twilight fades and the sky darkens. Mare Crisium will be near the upper limb of the rising Moon; the limb is the outer edge of the Moon’s disc. Look for a small, dark dot just above the “ears” of the Rabbit in the Moon.

If you can’t look on Friday evening, watch for the moonrise on Saturday evening, about an hour after sunset, and try it then. After Saturday, you won’t be able to spot Mare Crisium, as that edge of the waning (shrinking) Moon will be in shadow. You’ll have to wait until a couple days after the next New Moon on December 27 to try to spot the little sea in the first illuminated slice of the waxing (growing) crescent Moon.

If you observe the biggest Moon and/or if you spot Mare Crisium, post a comment on this page and share your success!