Lunar Eclipse Activity (Western Hemisphere)

[Ad Astra]

Hi Folks,

I'm posting this particularly for my friends on the Beginner's Forum who enjoy following along with my astronomy class and trying your had at the lab activities! The eclipse will be visible in the southwestern USA, Mexico and across the Pacific region. Sorry, I know many of you in the UK are out of luck for this one, but even if you don't get to see this one, the activity is good for any lunar eclipse! Save it for another day!

During a lunar eclipse, the full Moon passes through the Earth’s shadow, or umbra. The fainter outer shadow, or penumbra is often difficult to see with the naked eye because the Moon’s disk is so bright (-12 magnitude!). It is possible to capture it photographically however, by beginning your photographic sequence 30-45 minutes before the eclipse is scheduled to begin.

Although we ‘see’ the shadow passing across the face of the Moon; in reality, it is the Moon’s orbital motion which is carrying it bodily through the cone of the Earth’s shadow. Thus, when we see the shadow sweep across the lunar disk in one direction, the Moon is actually moving in the opposite direction! The drawing to the right should help you visualize this. You can see the Moon moving into the Earth’s shadow, and how the shadow passes across from first contact (the first time you can see the shadow touching the Lunar disk), to totality, where the Moon is no longer in direct sunlight.

You should be aware that the progress of the lunar eclipse is not at all like the progression of the lunar phases. The terminator is the shadow which separates the sunlit portion of the ‘daytime’ lunar surface from the dark of lunar night. This shadow makes the familiar phases of the crescent, quarter, and gibbous moon; it is a shadow which always stretches across 180 degrees of the lunar disk. In other words, the terminator always stretches from one lunar pole to the other. A lunar eclipse is not at all like this.

The Earth’s shadow or umbra is about three times the diameter of the lunar disk (~10,000 km wide at lunar orbital distance). As the Moon moves into this shadow, the curvature of the shadow across the Moon will not change very much! Likewise, Earth’s shadow is not anchored at the lunar poles, you should pay close attention to exactly where the edge of the umbra cuts the lunar disk in your observations!

Dan

Lab #17 - Lunar Eclipse - Dec 2011.doc