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X Marks the Spot: The Lunar X
updated: Feb 16, 2013, 11:00 AM

By Chuck McPartlin

On Sunday, February 17, between noon and 4 PM, go out and take a look at the Moon with binoculars or a telescope. It's a tad past its First Quarter phase, and the illumination is just right for seeing something called the Lunar X.

Right along the terminator, the boundary between light and shadow, you'll be looking at the places where it's the dawn of the lunar day. You may see some isolated points of light in the shadow - the peaks of lunar mountains reaching up into the sunlight.

Except for the interior of some craters at the poles, any place on the Moon gets 14 Earth days of sunlight, followed by 14 days of darkness. There is no permanently "dark side" of the Moon, just a far side.

Because the Moon is so much smaller and less massive than the Earth, the force of gravity that you would feel on its surface is only about one sixth of what you feel at the Earth's surface. This means that the lunar terrain can be much more extreme, with higher peaks and deeper pits before the rocks slump and collapse.

Asteroid impacts have left lots of craters on the Moon, and without wind and rain and gophers, these craters last until they are erased or deformed by subsequent impacts. This jumbled terrain makes for interesting effects along the terminator.

Follow the terminator up about one quarter of the lunar diameter from its lower limb. You should see a small bright X extending into the darkness, as shown in the image below, from the NASA Goddard Space Flight Center's Scientific Visualization Studio.

The Lunar X is formed when sunlight hits the high intersecting rims of four craters: Purbach, about 73 miles across and 9,800 feet deep, Werner, about 44 miles across and 14,000 feet deep, LaCaille, about 42 miles across and 9,200 feet deep, and Blanchinus, about 36 miles by 42 miles and 13,800 feet deep.

Because the Moon is tidally locked to the Earth, its rotational period matches its period of revolution around the Earth, and we always see pretty much the same face of the Moon. Because its orbit is elliptical, and its orbital speed varies, it does seem to wobble a bit back and forth, so we actually see about 60% of its surface. Because of this wobble, called libration, the Lunar X does not appear at every lunation.

For Santa Barbara in 2013, we'll be able to see the Lunar X on:
April 17, from about 4 to 7 PM PDT
August 13, from about 1 to 4 PM PDT
October 11, from about 2 to 4 PM PDT
December 9, from about 4 to 7 PM PST

Links for a cloudy day:

NASA GSFC Scientific Visualization Studio

The Hitchhiker's Guide to the Moon


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