As the Moon orbits Earth, the same hemisphere always aims our way. But it’s like the face of a bobblehead doll — it bounces around a little bit. Over the course of the Moon’s month-long cycle of phases, this “bounciness” allows us to see a total of 59 percent of the lunar surface. And until spacecraft began flying around the Moon, that provided the only way to see any of the Moon’s hidden farside.
Several factors contribute to these motions, known as librations. The main ones are the Moon’s tilt on its axis and its changing orbital speed.
The Moon’s axis is tilted a bit with respect to its orbit around Earth. That causes first its north pole and then its south pole to tip toward us. That allows us to see over the poles a bit, revealing a smidgen of the Moon’s farside.
And the Moon’s distance from Earth changes, which causes the Moon to speed up and slow down as it goes around our planet. But the Moon spins on its axis at a near-constant speed. The difference in those two motions allows us to see around the Moon’s eastern and western limbs, adding tiny slivers of lunar territory to the view from Earth.
And the view will be especially pretty tomorrow, as the crescent Moon is in the eastern sky at first light. And it has a couple of bright companions. Venus, the “morning star,” is close to the upper left of the Moon. And Spica, the brightest star of Virgo, is a little farther to the lower left of the Moon.
Script by Damond Benningfield, Copyright 2012