Back in December, when we had our last lunar eclipse, I went off on a rant about the phrase “dark side of the moon.” I pointed out the absurdity of this phrase given that the same side of the moon always faces us. My vocabularic expulsion was so ranty I even made the claim that the moon constantly has “exactly the same part of its surface facing the earth”. Eric quite rightly pointed out this was not quite correct and indicated that we can see about 59% of the moon.
At the time I accepted I was wrong without really digging into the associated learning opportunity too deeply.
Today I looked it up. I discovered a new word and some rad animation.It seems the premise under which I was operating was headed in the right direction but I had a substantial failure of imagination. The moon is indeed in a tidal lock with the earth, keeping one face pointing at the earth’s centre of mass. We still get to see a bit more than half of it though! This is due to the irregularities in the lunar orbit (it is neither perfectly circular nor perfectly in plane with its rotation) and due to our shift in viewing angle from moon rise to moon set. These geometric and angular irregularities cause an apparent motion in the moon called “libration”.
Excerpt from the wikipedia page:
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.