Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2000 October 13 Eclipse Moon Trail Credit & Copyright: Joe Orman

Explanation: Tonight, Friday the 13th, October's big, bright, beautiful full Moon will be in the sky, rising as the sun sets. A time exposure of this evening's full Moon would show a brilliant circular arc or Moon trail tracing its celestial path. In fact, this single, four hour long exposure from the evening of January 20 shows a full Moon trailing through hazy skies above Phoenix, Arizona, USA. Of course, the picture also shows something you won't see tonight -- a total lunar eclipse. A lunar eclipse is caused when the full moon enters Earth's shadow and as the eclipsed Moon's light grows steadily fainter, the Moon trail becomes narrow and dim. The total eclipse phase, when the Moon passes completely within Earth's shadow, occurs near the middle of this Moon trail arc. But even during totality, the Moon trail is visible and noticeably red. Normally illuminated by sunlight which falls directly on its surface, during a total lunar eclipse the Moon is still illuminated by sunlight filtered and refracted through Earth's atmosphere. The refracted light lends the eclipsed Moon a dim and reddish appearance.