Text on screen - 'LRO, Lunar Reconnaissance Orbiter.' Shot of the moon, gradually falling out of shadow.
NARRATOR:
If you looked at the moon over the course of a few weeks, you'd probably notice that it looks slightly different every day. The change in its shadow is based on where the moon is in its orbit.
Animation showing the sun, then the earth, and, shaded behind it, the moon. Another animation shows the moon revolving around the earth with the sun shining from the left. An arrow points from the earth to the moon, labelled 'Sightline'. An insert shows how the moon looks from this sightline, correlating to its position in orbit in the animation. The shadows on the moon's face form the various phases of the lunar cycle.
NARRATOR:
We call this cycle 'the phases of the moon' and it occurs roughly once a month. At least twice a year, however, something quite different happens. The moon passes through the shadow cast by the earth, causing it to look extremely unusual for a short period of time. From the earth, the moon will appear to darken and turn a deep red before eventually returning to normal. This is called a 'lunar eclipse'. If we were to look at what happens from space during an eclipse, it would go something like this. First, the moon passes through what's called the 'penumbra', where the sun's light is only partially obscured. This results in only a slight darkening of the moon. As the moon continues along its path, however, it enters what's called the 'umbra', where all direct light from the sun is blocked. But if the sun is blocked, why does the moon turn red? When light from the sun goes by the side of the earth, it passes through a long and thick layer of Earth's atmosphere. Shorter wavelengths of sunlight, like blue, are scattered by the atmosphere, so by the time the light has finished its trip to the moon more of the longer wavelengths, like red, are left over. On the earth, the same thing happens at sunset, as the ground you stand on gradually passes into night. As the eclipse ends, the moon leaves the umbra, returns to its normal colour and then leaves the penumbra, brightening and resuming its original cycle. Overall, the whole process lasts only from a few minutes to a few hours, so you'll have to be quick if you wanna see it.
Text on screen - 'Total Lunar Eclipse, June 15, 2011.' The moon is shown in five stages of eclipse, a red moon at the centre. The first stage is listed as occurring at 22:13 UT, the final stage at 18:13 UT.
NARRATOR:
But, as long as you're willing to stay awake, you'll catch the moon as you won't see it too often.
Text on screen - 'NASA. Goddard Space Flight Center. www.nasa.gov/goddard'