Sunday, July 17, 2011

Movements of the Moon - Part IV

If you haven't already, read Part I, Part II, and Part III of this series first.

To date, we've now discussed phases of the Moon, the synodic month (a lunation or cycle of phases), and the sidereal month (an orbit of the Moon around the Earth).  Today, I'd like to discuss a couple of miscellaneous concepts.

The image at right shows the phases of the Moon from Waxing Crescent, 1st Quarter, Waxing Gibbous, to Full Moon and then back to Waning Gibbous, 3rd or Last Quarter, Waning Crescent, and New Moon (the blank space in the lower-right corner).

Note that the features you see on the face of the Moon never change.  In other words, the same side of the Moon is always facing the Earth no matter what the phase.  This is called synchronous rotation and is due, of course, to the strong gravitational attraction between the Earth and Moon.

The side always facing away from the Earth is called the far side and wasn't even seen by humans until imaged by the Russian Luna 3 probe in 1959.  As I mentioned in a previous post in this series, it's sometimes called the dark side but the word "dark" is used only in the sense of it being unknown (at the time), not an absence of light.

The so-called dark side of the Moon is fully lit by the Sun for two weeks each month.  A New Moon phase, when the Moon is between the Earth and the Sun, has the far side of the Moon in full sunlight while the near side that faces the Earth is in darkness (as illustrated by our old friend, the diagram below).

The interesting thing about the far side of the Moon, is that it's very different in appearance from the near side.  The near side has more mare (pronounced "mar-ey" - Latin for "seas") - large volcanic lava plains - and the far side is heavily cratered.  I'll discuss why another time.

Another interesting fact about this tidal locking of the Earth and the Moon is that the Moon "rocks" back and forth a little bit in its orbit allowing us to actually see a bit more than half (59%) of its surface from the Earth.  This is called lunar libration.  The image below illustrates this, both show photos of the waxing Moon at two different times.  The red dot denotes Mare Crisium ("Sea of Crisis") near the Moon's eastern edge.  See the slight difference in position from lunar libration?

Speaking of the size of the Moon, perhaps I should say a few words about the Moon illusion.  When a Full Moon is rising on the eastern horizon it often looks huge - much larger than it appears when higher in the sky.

It's an optical illusion.  You can photograph the rising Moon and establish that it's size really doesn't change - it just appears large near the horizon (the color change is due to blue wavelengths being scattered out near the horizon - leaving red wavelengths to get through - since you're looking though a thicker layer of the atmosphere).

The Moon is actually surprisingly small in the sky.  It covers 1/2° - for reference, the distance from the horizon to the zenith (the point directly over your head) is 90°.  That's means you hold your arm outstretched, extend your pinkie finger, and the fingernail on your pinkie will completely cover the Moon (even when it's on the horizon).  Don't believe me?  Go out at night and try it.

Why do we perceive the Moon larger on the horizon?  No one's quite sure (or, maybe I should say, lots of people think they're sure but they offer competing hypotheses).  Here's a summary of ideas from Dr. Donald Simanek, a retired physics professor.

Enough for today.  The topic for tomorrow, which I briefly mentioned yesterday, goes back to the sidereal month and the Moon moving through the constellations.


  1. The sunset moon picture is AWSOME!!

  2. Interesting site, good pics, good overviews. The pics of the moonrise are very cool.