If everything revolved in perfect circles and all planet and moon orbits were concentric, the solar system wouldn’t be nearly as much fun. Consider the moon. Orbiting in a circle rather than ellipse,its distance from Earth would never vary. There’d be no “super moons” or full moons at the time the moon is closest to the Earth.
The moon’s orbital speed would also be constant and never get out of sync with its rotation rate. Because the moon moves slower when farthest from the Earth (and faster than average when closest), we can peer around the east and west limbs of the moon for a few days each month to see craters and lunar seas that are otherwise hidden. This apparent rocking back-and-forth of the moon, called libration, exposes an extra 7.9 degrees of lunar longitude for our viewing pleasure.
Similarly, if the moon’s orbit were exactly concentric with Earth’s and the moon’s axis straight up and down, we’d never be able to peek over and under its north and south polar regions. We’re grateful that the combination of the 5.1 degree tilt of the moon’s orbit and the 1.5 degree inclination of its axis exposes an extra 6.8 degrees of latitude. As you might guess, this tippy business is called libration of latitude.
Add in 1 degree of diurnal libration caused by our changing perspective at moonrise vs. moonset, and altogether we’re able to see 59% of the moon. Pretty cool, eh?
You can see the effects of libration tonight through next week if you have a pair of 10x binoculars or small telescope.
Three lunar seas that normally are absent or appear as little more than skinny stripes along the extreme eastern edge of the moon are in good view this evening – Mare Humboldtianum (Sea of Humboldt), Mare Marginis (the Border Sea) and Mare Smythii, a sea named in honor of 19th century British astronomer Admiral Smyth.
Watch in the coming nights as the rock n’ rollin’ moon whisks them away.