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Jon Cruchala
Jon Cruchala
Astronomy Meets Climatology
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Take a close look at the next crescent Moon after sunset and notice how the "dark" side actually appears to be glowing with pale gray light. What you're actually seeing is "Earthshine"; that is, light reflected from the Earth's daytime side onto the Moon's dark side. If you were standing on the Moon looking up, you would see the Earth shining brightly against the black star-filled background of space.
The August 2004 issue of the Bulletin of the American Meteorological Society reports that scientists have been measuring variations in the intensity of Earthshine as a new way of monitoring the reflectivity of the Earth's atmosphere. (The total reflectivity of the Earth's atmosphere is known as it's "albedo", generally in the range of 35%-40%). This reflectivity, or albedo, is directly related to the Earth's overall average cloud cover. The greater the cloud cover, the more of the Sun's light is reflected back into space. Global variations in average cloud cover, in turn, provide scientists with an important tool in the study of climate. Thus, astronomy meets climatology, with Earthshine as the common link.
Of course, we can't discuss reflectivity and albedo without mentioning the planet Venus. With an albedo of very nearly 50% and a relatively close proximity to the Earth, Venus appears brighter than any other object in the sky except for the Sun and Moon. The large albedo of Venus is due to the fact that this neighboring planet is closer to the Sun and is completely shrouded in clouds. Venus is truly the "diamond in the sky", shining brilliantly as either the "evening star" or "morning star" depending upon its position relative to the Sun and Earth.
Venus is interesting in another regard. Because it circles the Sun in an orbit inside of Earth's, Venus exhibits phases - just like the Moon. Contrary to expectations, perhaps, Venus is brightest when in its crescent phase. This is due entirely to its distance from Earth. When Venus appears "full", its distance from Earth is over 150 million miles. In its crescent phase, however, Venus is a "mere" 25 million miles distant. This tremendous decrease in distance more than compensates for the reduced visible surface illumination of the crescent phase. The phasing of Venus is not distinguishable to the naked eye, but becomes readily visible in a small telescope.
When the next "new" Moon occurs (generally about two weeks after the full Moon), begin looking for the crescent Moon and its Earthshine after sunset during the following few days. Note how the intensity of the Earthshine appears to change each evening as the Moon moves out of its crescent phase (i.e. "waxes").
Don't forget Venus: As the days become shorter, you may now be arising before the Sun. If so, take a look at the eastern sky as you head off to work or school. That beautiful bright "star" shining against the morning sky is our highly reflective neighbor, Venus.
Related Products: Explore concepts presented in the above article with these ScientificsOnline Products.
Solar System Simulator
Celestron StarHopper 10 Dobsonian Telescope
Celestron CGE 1100 (XLT)Telescope
A Space Colony - Expedition to Mars
Venus and Mars Globes
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