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Saturday, March 3

STEREO catches a lunar transit

This is something you just won't see from Earth. The moon transiting across the face of the sun. On the rare occasions that happens from Earth we see a solar eclipse.

NASA's Stereo probe (or one of them, Stereo B) is about a million miles from the Earth and therefore the moon appears much smaller against the sun. The probe was therefore able to take this amazing image of a 'smaller' moon transiting the face of the sun.

There's also a video of the transit on the Stereo website.


  1. Anonymous2:29 pm

    The nearest we get to a transit from earth is an annular eclipse. The question which arises in my mind is: "How much of the sun's disc must be covered for a transit to become an annular eclipse?"

  2. hanks for the comment Dave. I was thinking the same thing as I wrote the post, so I did a bit of research on the web. I fact it looks like the whole eclipse naming thing is a bit of a minefield as this quote from Wikipedia indicates.

     "The term solar eclipse itself is strictly speaking a misnomer. The phenomenon of the Moon passing in front of the Sun is not an eclipse, but an occultation. Properly speaking, an eclipse occurs when one object passes into the shadow cast by another object. For example, when the Moon disappears at full moon by passing into Earth's shadow, the event is properly called a lunar eclipse. Therefore, technically, a solar eclipse actually amounts to an eclipse of the Earth."

    Except of course it wouldn't be an eclipse because the moon's shadow does not cover the entire Earth. It is similar to the shadow of one of Jupiters moons crossing the surface of the planet. That's just called a transit of the shadow.

    All very confusing, and of course doesn't anwser you original question !

  3. Looking at that image, it struck me that some of the transiting exoplanets would look similar to this if we had telescopes with sufficient resolving power. The moon appears about 1/6 of the diameter of the moon so 1/36 of the area of the disc. This means the light from the sun would drop about 3% as the moon transited. This is similar to the changes caused by some of the the transiting "hot jupiters" discovered.