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Moon Transits the Sun
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Moon Transits the Sun

March 4, 2007

On Feb. 25, 2007 there was another kind of eclipse of the Moon when it crossed the face of the Sun - but it could not be seen from Earth.

This sight was visible only from the STEREO-B spacecraft in its orbit about the sun, trailing behind the Earth. NASA's STEREO mission consists of two spacecraft launched in October, 2006 to study solar storms. The transit started at 1:56 am EST and continued for 12 hours until 1:57 pm EST. STEREO-B is currently about one million miles from the Earth, 4.4 times farther away from the Moon than we are on Earth. As a result, the Moon will appear 4.4 times smaller than what we are used to. This is still, however, much larger than, say, the planet Venus appeared when in transited the Sun as seen from Earth in 2004.

This alignment of STEREO-B and the Moon was not just due to luck. It was arranged with a small tweak to STEREO-B's orbit last December. The transit is quite useful to STEREO scientists for measuring the focus and the amount of scattered light in the STEREO imagers and for determining the pointing of the STEREO coronagraphs. The Sun as it appears in these the images and each frame of the movie is a composite of nearly simultaneous images in four different wavelengths of extreme ultraviolet light that were separated into color channels and then recombined with some level of transparency for each.



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