When you look at the Moon, you get a static snapshot of a dynamic object. But there are many visible changes occurring over time. Most obvious is the phase: New, Crescent, Quarter, Gibbous, Full. Simultaneously, the Moon goes through three additional change cycles, none of which are synchronized. The combined motion is quite complex.
The time-lapse video above shows one year of these lunar cycles compressed into 2.5 minutes of time. That's a speed-up factor of about 200,000. The video was recorded by NASA's Lunar Reconnaissance Orbiter (LRO) over the full year of 2011. The phase cycle alone is fascinating in time-lapse — it clearly shows that the Moon is a spherical body illuminated from continuously changing directions. And if you watch closely, you'll see three other changes happening.
- Apparent Size: The apparent size of the Moon increases and decreases because its distance from Earth changes as it goes around its orbit. At closest approach, called perigee, it's 356,700 km distant (221,643 miles). When it's farthest from Earth, called apogee, it's distance is 406,300 km (252,463 miles). That 14% difference in its distance (and hence apparent size) is easily visible in this video.
- Libration: This is a "rocking" or "nodding" motion of the Moon, which rotates it slightly to the left and right, and simultaneously up and down. The same side of the Moon always faces Earth because of tidal locking. We call that half of the Moon the lunar nearside. But because of libration, we can see more than that 50% of the lunar surface. Over a full cycle of libration, some 59% of the surface can be seen.
- Nutation: This is the smallest effect, but it adds to the up-and-down motion of libration. This changing perspective is due to the Earth itself tilting from the pull of the Moon's gravity. It's a very gradual process, taking 18.6 years to complete one cycle, and difficult to visually separate from libration.
The clockwise and counter-clockwise "twisting" motion you see is an artifact of the LRO's orbit around the Moon. You see a similar effect from Earth, when a Quarter Moon looks like a "D" at sunset, and a "smile" at moonset.
Interestingly, the Earth as seen from the Moon would exhibit similar cycles — with a few big differences. Additional visual changes on the Earth include evolving weather patterns (clouds and hurricanes), natural disasters like volcanic eruptions and forest fires, seasonal changes in the color of ground cover and polar caps, and the display of lights turning on in major cities at nightfall. The physical surface of the Moon, by comparison, rarely changes.
When mankind colonizes the Moon, I expect Earth-gazing will be an extremely popular pastime. Further, the 0.17g gravity will also present some interesting possibilities for recreation and sports. And then there's the cloudless, crystal-clear sky which, unlike our terrestrial atmosphere, will never disappoint astronomers. Should be a fun place to live. Several nations have plans to establish permanent colonies by 2020-2030.
Next Week in Sky Lights ⇒ The Big Moon Photo Hoax