The Sun stood still and the Moon stayed — and hastened not to go down about a whole day! -Joshua 10:12-13
Did you have a good New Year’s Eve? Did you enjoy not just the fireworks and champagne, but also the Blue Moon (2nd full Moon of the month) that was out that night?
Were you in a fortunate enough part of the world that you not only saw the full Moon, but also managed to see the partial lunar eclipse that happened?
How this happened is pretty simple: the full Moon is always on the opposite side of the Earth from the Sun. When the side of the Moon that’s lit up faces Earth, we see it as being full.
When things are lined up juuust right, the Moon will actually pass through the Earth’s shadow, like it did on December 31st! This creates a lunar eclipse: it can be a total eclipse if the entire Moon gets covered by the Earth’s shadow, but — as was the case last week — a partial one if only a portion of the Moon passes through Earth’s shadow.
The Moon moves in a big ellipse around the Earth, taking a total of 29.5 days to go from full Moon to full Moon. So half a lunar cycle later (14-15 days), the Moon will be new again, which means the Moon will be in between the Earth and the Sun, and the half that’s lit up is not visible from Earth.
Could this create a Solar Eclipse? Could the Moon block out the Sun? Well, in principle, yes, it could. After all, the Sun will cause the Moon to cast a shadow, and so long as that shadow falls on the Earth, we’ll get a solar eclipse.
Here’s the thing: you’ll never have a total Lunar Eclipse and a total Solar Eclipse in the same Month! Why not? The Earth revolves around the Sun in a plane, but the Moon doesn’t revolve in that same plane; it’s inclined by about 5 degrees!
So when it’s 14-15 days after your eclipse, the Moon is no longer in the same relative position to the Sun and the Earth; the shadows fall on different areas of space.
But back on December 31st of 2009, the Moon just barely fell into the Earth’s shadow. 15 days later, the Moon will just happen to be perfectly aligned with the Sun and Earth, which will give us a total solar eclipse!
But wait just a second, it isn’t just any type of total solar eclipse; there are two different types! You see, the Sun and the Moon appear to change sizes in the sky over time, depending on how close they are to the Earth. The Sun varies between 0.524 and 0.542 degrees in the sky:
While the Moon varies between 0.498 and 0.565 degrees.
When the Sun appears to be smaller than the Moon, we get a total Solar Eclipse, where the Moon’s shadow falls directly on the Earth. But when the Moon appears to be the smaller, the Moon’s shadow never reaches the Earth, as the Moon never completely blocks out the Sun!
This creates a beautiful sight known as an annular eclipse, after the Latin word annulus, or ring!
On Friday, January 15, 2010, this is exactly what the Sun, Moon, and Earth will conspire to produce (with the Moon appearing slightly smaller than the Sun): an annular eclipse! This sight can be seen from anywhere highlighted on this map of the world:
And you can check out this interactive NASA site to see where you can actually view the full annulus! So get out and see this if you live in the right part of the world, and send me pictures!