Moon Moves

a waning “C” crescent and a waxing “D” crescent

I was out last night with a friend who commented that there was a clear “crescent Moon.” People commonly use that term when a sliver of Moon is showing, but there are two versions of the crescent sliver.

The Moon is always waxing (growing in the lit area we see) and waning, and moving closer and farther away from us. It is surprising how many people have never really noticed that the Moon looks like a looks like a “C” crescent, and later looks like a “D” in its waxing phase.

The phases of the Moon as viewed looking southward from the Northern Hemisphere. Each phase would be rotated 180° if seen looking northward from the Southern Hemisphere. The upper part of the diagram is not to scale, as the Moon is much farther from Earth than shown here.

In Hinduism, every part of the cosmos is seen as an action of a god and time is the endless repetition of the same long cycle. In Hindu mythology, Soma represents the god of the Moon.

Soma rides a sky chariot drawn by white horses. Soma was also the name of the elixir of immortality that only the gods can drink. The elixir is stored on the Moon. When the gods drink soma, they draw away from the Moon and it becomes smaller. (I wrote about soma earlier in another context.)

Most people know that the Moon changes its distance from Earth continually because the orbit of the moon is not a perfect circle. It is more like an ellipse, so it will have a point of perigee (closest point to Earth) and apogee (farthest point) each month. Today, May 6, it is at apogee and it is 251,318 miles or 404,457 km away from us.

Back on April 20 perigee, it was  229,108 miles or 368,714 km away. In cosmic terms, a difference of 22,210 miles or 35,743 km is not that much and only astronomers take note of the diference. But occasionally the media will decide to write a story about the “biggest Full Moon of the year” or something similar.

There is a nice animation at time.unitarium.com/moon/ that shows the movement of the Moon in your area and illustrates nicely why we see a Full Moon and how it appears when waxing and waning.  You can set it to any date, so I know that on my next October birthday the Moon will be waxing gibbous and approaching full. Unfortunately, it doesn’t allow you to go back before 2000 or I would take a look at what the Moon was up to when I was born.