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Astronomical Observatory: Cool Images

Images: The Crescent Moon (2/28/1)

Crescent Moon

Contents: The crescent Moon (29% illuminated in this image) is probably the most recognizable of all astronomical sights. The crescent shape is a consequence of illumination of the spherical Moon by the Sun. For an interactive tutorial on the geometry of lunar phases and how to use the Moon to tell time, see the Lunar Phases Web Tool.

Beyond the crescent shape, three distinctive types of features are noteworthy in this image. The first is an intrinsic color variation: the large, smooth darker patches are known as mare. They may be seen with the unaided eye and have been known since ancient times. They formed when dark, low viscosity lava filled in enormous crater basins over three billion years ago.

The second is a consequence of surface relief: the raised rims of smaller, circular craters catch the sunlight on the left inside of the rim and cast a shadow on the right inside. As the orientation of the solar lighting shifts through the month, the bright and dark parts will swap sides. This observation and interpretation were first reported in 1610 by Galileo Galilei, who went on to conclude that the Moon must therefore be more like the Earth (which has relief in the form of mountains) than like the perfectly spherical celestial bodies made of ether suggested by ancient Greek astronomers, and hence was more consistent with a Copernican view of the solar system (with the Earth orbiting the Sun instead of the other way around).

Third, the Moon is generally much brighter near the limb (the right edge) than near the terminator (the line on the left separating the day and night sides). This is a more subtle consequence of the illumination geometry. Near the limb, the lunar surface directly faces the Sun, and so receives more sunlight per square meter than near the terminator.

This gradient in intensity is in principle visible to the unaided eye as well, but is hard to see at night because the stark contrast between the black sky and the bright Moon make light variations across the Moon seem negligible. The gradient is much easier to see in daytime observations of the crescent Moon when the surrounding sky is only slightly less bright than the Moon itself. (Try this at home! If you are not sure when and where to look for the crescent Moon in the day, check out the Lunar Phases Web Tool.)

Processing: This monochrome image was made by freshman Peter Schrock. He combined 9 separate images obtained between 8:30 and 9:00 pm on February 28, 2001 using a Johnson B (blue) filter and 0.7 second exposure times. He set the brightness scales the same for each image with CCDSoft, mosaicked the images together using Microsoft Paint, and then applied unsharp masking to the result in CCDSoft.

Orientation and scale: North is up and East is to the left. The Moon's angular diameter in this image is 31.2 arcminutes. (Its linear diameter is 3476 km, and its distance from the center of the Earth at this time was 386,771 km.)

Content updated 3/16/01


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