19 June 2010

The Path That Leads to the Earth's Shadow

We are in an "Eclipse Season," a five week period of time when the Moon's orbit around the Earth is aligned in a way that the Full or New Moon crosses the path of the Earth around the Sun. When that happens, we experience a Lunar or Solar Eclipse and right now we have one of each coming up: a partial Lunar Eclipse on Saturday 26th, and a total Solar Eclipse on July 11th. The Solar Eclipse will not be visible from North America so we don't get a chance to experience that, but the Lunar Eclipse will be. More on that next week.
During an Eclipse Season, the Moon's position in the sky is special, as it is moving just above or just below the plane of the planets (the "Ecliptic") in the interval from New Moon to Full Moon. A few days ago as the Moon emerged from the evening glare waxing each evening, you could see it move just below Venus, then below Regulus and Mars, and now it is below Saturn and Spica, all objects on or near the Ecliptic. As it nears Full Moon and the Lunar Eclipse on the 26th, it is moving closer and closer to the Ecliptic and will intersect the Earth's shadow on the 26th.
I like to imagine that there is a dark spot in the sky where the Earth's shadow projects out into space. Every month as the Moon nears Full, it sweeps close to that spot. But only during an Eclipse Season -- every six or twelve months -- does it slip into the shadow of the Earth and display to all of us the curved shadow of the Earth. That is something to look forward to next weekend.


Sidewalk Universe said...

Hi Paul hope you got to see this as I enjoyed this early sky show. What was really striking to me was just how dark earth's shadow was especially on the leading edge. Also seeing Earth's curvature was pretty impressive!

The Urban Astronomer said...

Hi Richard - unfortunately I was clouded out along the coast. That was disappointing, but the next big lunar eclipse is in prime time and should be awesome.

I am glad you saw it. Sounds dramatic.

Until the next eclipse season!