Montana Earth Science Picture of the Week

This series of photos was taken by Fred Espenak during a total lunar eclipse in 2000. Montanans shared in the viewing of a similar eclipse on the evening of Thursday, May 15, 2003.

A lunar eclipse happens when the moon passes through the Earth’s shadow. Earth’s shadow has a darker inner part, called the umbra, and a lighter perimeter, called the penumbra. When the moon completely enters the umbra, as it did on May 15, 2003, the eclipse is considered to be “total”. A lunar eclipse can only occur when the moon is “full”. Even though a full moon occurs every month, a lunar eclipse does not happen that often because the moon’s orbit around Earth is tilted 5 degrees compared to Earth’s orbit around the Sun. Therefore, the full moon usually passes above or below Earth’s shadow.

Here’s what happened on the night of May 15, 2003. At 7:05 pm the moon moved into the the lighter outer shadow (penumbra). In Montana we didn't see this part because the moon did not rise above the eastern horizon until 8:37 pm, shortly before it got dark. As darkness settled in, Montanans were able to see the next phase. By 9:14 the moon moved completely into the umbra. We saw the moon's surface darken and then take on a burnt orange color. This happened because our atmosphere filters out shorter wavelengths of sunlight and changes the direction of the longer red and orange waves, causing them to reflect off of the moon’s surface. The most interesting part of the eclipse was watching the moon move out of the umbra, beginning at 10:07 pm. As this happened we were able to see the curved shadow of the Earth on the surface of the moon. Centuries ago, this curved shadow, observed during lunar eclipses, was used as evidence to support the idea that the Earth was not flat.

8:37 pm: moon rose above eastern horizon

9:13 pm: entered darker portion of shadow, became burnt orange color (umbra)

9:40 pm: middle of eclipse: moon was in middle of shadow; darkness enhanced viewing (see middle image above)

10:07 pm: moon began to move out of umbra; Earth's shadow was vivible.

11:18 pm: moon passed through lighter part of shadow (penumbra)

12:14 am: moon left penumbra

Terms: umbra, penumbra