Posts Tagged ‘Transit’

What an eclipse looks like from outer space

20 June, 2009

Before we go into an awesome video, a bit of terminology first. When an object appears to be visually smaller than another body (e.g. the Sun) and passes in front of it, that’s called a transit. If the object appears to be visually bigger than the other body, and blocks it out completely, that’s an occultation.

An example of a “transit” is the Space Shuttle Atlantis on its mission STS-125, transiting the Sun. (I really just wanted an excuse to bring up this awesome picture again.)

Another example is this image of a lunar transit taken by the NASA STEREO-B spacecraft. STEREO A and B are satellites specially launched in 2006 for solar observation.

The description that goes with the video goes:

No human has ever witnessed a solar eclipse quite like the one captured on this video. The NASA STEREO-B spacecraft managed by the Goddard Space Center, was about a million miles from Earth, February 25, 2007, when it photographed the Moon passing in front of the sun. The resulting movie looks like it came from an alien solar system.

The fantastically-colored star is our own sun as STEREO sees it in four wavelengths of extreme ultraviolet light. The black disk is the Moon. When we observe a lunar transit from Earth, the Moon appears to be the same size as the sun, a coincidence that produces intoxicatingly beautiful solar eclipses. The silhouette STEREO-B saw, on the other hand, was only a fraction of the Sun.

The Moon seems small because of the STEREO-B location. The spacecraft circles the sun in an Earth-like orbit, but it lags behind Earth by one million miles. This means STEREO-B is 4.4 times further from the Moon than we are, and so the Moon looks 4.4 times smaller.

This version of the STEREO-B eclipse movie is a composite of data from the coronagraph and extreme ultraviolet imager of the spacecraft.