Moon's giving up good librations
1/12/2014 10:17 PM
The Moon is tidally locked with Earth, meaning it only shows one "face" to us, no matter where it is in its orbit around Earth. However, from Earth, we can see more than 50% of the Moon's surface due to libration. See graphic at right for what libration looks like over the period of a month.
Here's the Wikipedia entry detailing why we see more than just 50% (in fact, up to 59%):
There are three types of lunar libration:
- Libration in longitude results from the eccentricity of the Moon's orbit around Earth; the Moon's rotation sometimes leads and sometimes lags its orbital position.
- Libration in latitude results from a slight inclination between the Moon's axis of rotation and the normal to the plane of its orbit around Earth. Its origin is analogous to how the seasons arise from Earth's revolution about the Sun.
- Diurnal libration is a small daily oscillation due to the Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—because the observer is on the surface of the Earth, not at its center.
So because our Moon does not follow the plane of the ecliptic, and because it's orbit is elliptical, we can "peek around the edges" - sometimes over the north when the Moon is below the ecliptic, sometimes to the south when the Moon is above the ecliptic, and sometimes around the edges depending on where the Moon is in it's orbit and if we can see the western or eastern edges better.
It's a fun thing to do, peeking around the side. There's a lot to see, and much of it can be seen with a simple, small telescope. The video here indicates some of what could be seen the week of January 13 through 19, 2014.