CFHT, Current Image of the Week


July 10th, 2000

The Moon, Earth's satellite

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The Moon, Earth's satellite

Credit: Image by J.-C. Cuillandre, CFHT

Image description:

This picture of the moon was taken in April 2000 with the CFHT wide-field CCD camera, the CFH12K. Just a few days after the new moon phase, we observed it as a crescent low on the horizon. The top image shows the full width of the camera's field of view (8,196 pixels, only 3,000 pixels out of the 12,000 pixels of the camera's length are showed on that top image) and the two images below are regions of that same image. The bottom image is at full resolution. Details of about a half-mile are distinguishable here.

The most prominent structure in the top image is the "Mare Crisium" (Sea of Crisis) in the left center. The bottom image focuses on two craters to its left: Cleomedes on the left (named after a Greek astronomer, 50 B.C.) and Macrobius on the right (named after Ambrosius Aurelius Theodosius Macrobius a Roman writer C. 410). In this picture, Macrobius appears in the "terminator", the region delimiting the areas of the moon exposed to sunlight (i.e., from the moon's perspective, a sunrise marking the start of 15 Earth-days of daylight.) The shadows increase the contrast in this region, revealing great details of the shape of the moon's tormented surface. The diameter of the Macrobius crater is about 40 miles and the Mare Crisium is 261 miles. The moon's diameter is about 2100 miles, or one quarter of Earth's diameter.

The moon's actual color is rather white, due to sunlight reflecting on its grey surface. The orange color in the image is artificial, but simulates the effect of light scattering by Earth's atmosphere at moonrise and moonset. This is a monochromatic image, made from a set of 0.4 second exposures.

Moon Facts:

The analyses of rock samples brought back by American astronauts from the Apollo missions date the formation of the moon to about 4 billion years ago. The moon's appearance has not changed a lot over the last 3 billion years, after intensive bombing by meteorites and volcanism ended at the age of 1 billion years.

The many craters are caused by explosive impact of meteorites on the moon's surface, giving a circular shape to the craters regardless of the angle of the meteorite's impact. The large flat and soft looking areas (called "Mare", meaning "sea" for its similarity with a flat ocean) are actually lava flows that have covered previous meteorite impact sites. The few craters seen in the "seas" were created by more impacts which followed this volcanic activity.

The moon orbits around Earth in 27 days but since the Earth orbits around the Sun, it takes a couple more days for the moon to align with the Earth-Sun axis and reach the next full moon phase. It is this period of 29 days we use for the lunar calendar. Between two consecutive full moon configurations, the moon appears to us to go through a set of declining and increasing phases. Half the surface of the moon is always illuminated by the sun at any time. We see only a fraction of this illiminated surface, as the Sun, Earth and moon are constantly changing position with respect to each other. The same side of the moon always faces Earth. The other side, the dark side of the moon, can only be seen by space exploration projects. The following diagram (not to scale) provides an idea of the way the moon looks to us at various times during the 29 day period.

The moon is represented by the smaller disk. The half exposed to the sun is represented in yellow. The white dot is a fixed point on the surface to show how, relative to Earth, the moon does not spin on an axis. When the moon is between the Sun and Earth, the moon is invisible to us, totally lost in the Sun's glare. But when a precise alignment occurs, a solar eclipse can be seen from Earth: the shadow of the moon projects on us and hides the Sun for a couple of minutes.
Check out the moon phases for this month.

The moon calendar defines the operations of our telescope: when the moon is up, its glare increases the sky brightness in the optical wavelengths and it becomes difficult to observe faint and distant objects. Fortunately this glare is not present in infrared. In these wavelengths, the sky appears dark regardless of the phase of the moon. That's why you can notice on our semester schedule that the CFH12K CCD mosaic, our main optical instrument, is always on the telescope around the new moon while KIR, our infrared imaging detector mounted on the adaptive optics system, is on the telescope during the full moon periods.

Elsewhere on the Web, you can find more technical information on the moon and this lecture on the moon.

The moon has been an endless source of inspiration in many forms of art: poetry, photography, movies... Here's a favorite, a radically new view of the moon by N. Park: Wallace and Gromit's first adventure in A Grand Day Out ("Everybody knows the moon is made of cheese.")

next week: To be announced soon.

editors: François Ménard & Jean-Charles Cuillandre
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Copyright © 2000 by CFHT. All rights Reserved.

CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.