In the Shadow of the Moon

Moon's shadow
LRO turned to image the Earth four times during the solar eclipse on 20-21 May 2012; in this view the Moon's shadow is seen passing over the Aleutian Islands. Annotated NAC Image E192199689L [NASA/GSFC/Arizona State University].

A solar eclipse occurs, from the Earth’s perspective, when the Moon passes directly between the Earth and the Sun. This alignment results in a shadow of the Moon passing across the Earth. In a total eclipse, the Moon blocks the entire disk of the Sun, and viewers on Earth witness only the the Sun's faint corona that extends thousands of miles into space. However, in an annular (or "ring of fire") eclipse like the one that occurred on 20-21 May 2012, the apparent size of the Moon is smaller than that of the Sun, so terrestrial viewers can see a bright ring or annulus of the Sun around the Moon. Which type of eclipse you experience, total or annular, depends on where the Moon is in its orbit. The Moon's orbit isn't perfectly circular, so sometimes it is closer to the Earth, and bigger in the sky (resulting in a total eclipse), and sometimes it is farther from the Earth and smaller in the sky (an annular eclipse).

Annular eclipse, E. Speyerer
Photograph of annualr eclipse taken from Kanarraville, Utah, on 20 May 2012, by E. Speyerer.

What does a solar eclipse look like from the Moon? The LROC NAC captured four images of the Earth, two on each of two successive orbits, during this solar eclipse. In these images you can see the Moon's shadow passing over the Earth over a period of about two hours. The image above shows the eclipse as it progressed over the Aleutian Islands, below you can see it a bit earlier as the Moon's shadow passed over Japan. 

The first of four images captured by the NAC during the annular eclipse of May, 2012, as the Moon's shadow passed over Japan. Annotated NAC image E192192490L [NASA/GSFC/Arizona State University].

The LROC NAC cannot easily acquire images of the Earth, and acquiring Earth views requires a significant amount of planning. The NAC is a line scanner, meaning that it has only one row of 5064 pixels per camera. Instead of snapping a single frame, an image is built up by the motion of the spacecraft in orbit about the Moon (about 1600 meters per second). To obtain an image of the Earth the spacecraft is turned 180° to face the Earth, then the spacecraft is pitched as quickly as possible (one-tenth of a degree per second), so that the image is built up line by line. You can see that two of the frames in the animated image below are slightly clipped, because LRO's timing wasn't perfect and the NAC ran out of lines before completing the scan (the NAC buffer is filled up after 52,240 lines, which is 256 Mbytes of data).

A compilation of the four images collected by the LROC NAC during the solar eclipse of May, 2012. Two images were collected during each of two successive orbits. NAC images E192192490L, E192192869L, E192199689L, E192200072L [NASA/GSFC/Arizona State University].

Because it was an annular eclipse, the shadow isn't totally dark; some sunlight still made it down to viewers of the eclipse as it passed over. The image below provides a zoomed in view of the Moon's shadow.

Zooming in on the Moon's shadow during the solar eclipse. NAC Image E192199689L [NASA/GSFC/Arizona State University].

The eclipse was spectacular from the Moon, but it was also quite a view from within the Moon's shadow!

Just barely, E. Speyerer
Photograph of partial eclipse taken at the same time as today's Featured Image (2012-142 00:33:41.036) from Kanarrville, Utah. The full eclipse had not yet quite reached Utah, thus Moon is seen as blocking only a small portion of the Sun (photograph by E. Speyerer).

View the full resolution NAC eclipse image.

Revisit Earlier NAC images of the Earth:

Americas from the Moon

Earth from the Moon



Posted by Brett Denevi on May 25, 2012 10:19.