The moon has been making its way around the earth since the waxing crescent that was visible in the dawn hours on the first of the month (when we wrote about the presence of four planets near the spectacular crescent moon). It is now nearly opposite the earth from the sun, very close to a full moon (98% full). The full moon will occur on Tuesday (17th of May).

Half of the moon is always illuminated by the sun, but as it goes around the earth we see more or less of that illuminated half. When the moon is between the earth and the sun, the illuminated side is towards the sun and we cannot see any of it -- this is the new moon. It necessarily occurs during the daylight hours, since the moon at that time is between us and the sun and so we turn towards it during the day. When the earth is between the sun and the moon we see the full illuminated side and get a full moon -- it necessarily occurs in the middle of the night, since the moon at that time is on the far side of us from the sun and we turn towards it during the night.

Of course, the moon rises and sets like any other heavenly object, arcing across the sky from east to west as earth rotates on its axis and we move towards the east as the rotation moves us along. Its path is very close to the plane of the ecliptic we have discussed in previous posts (the "plane of the table" in our mental model of earth's movement around the sun discussed here and here). Its path is not exactly on the ecliptic but is tilted by about five degrees from the ecliptic -- if it were exactly on the ecliptic we would have a lunar and solar eclipse every month.

A lunar eclipse necessarily takes place only on a full moon, and a solar eclipse only on a new moon, and you can understand why if you think about the positions of the earth, moon, and sun described in the paragraphs above and shown in the diagram at top.

The other phases of the moon take place because we see more and more of the illuminated portion as the lunation proceeds from the new moon towards the full moon. The diagram below shows this process along with a diagram showing the moon's orbit around the earth.

The origin of the moon causes some problems for scientists. As Walt Brown explains in his book about the hydroplate theory, conventional explanations all contain significant flaws. He notes:
The Moon could not have spun off from Earth, because its orbital plane is too highly inclined. Nor could it have formed from the same material as Earth, because the relative abundances of its elements are too dissimilar from those of Earth. The Moon’s nearly circular orbit is also strong evidence that it was never torn from nor captured by Earth. If the Moon formed from particles orbiting Earth, other particles should be easily visible inside the Moon’s orbit; none are.
The influence of the moon on human behavior is well documented. Crime rate and delivery dates appear to be increase around the date of the full moon. Studies have also shown that human female menstruation is connected to moon cycles in a majority of women. The moon also influences animal behavior: hunters and fishermen have long observed the impact of the moon's phases on the best times for hunting and fishing.

Many ancient monuments around the world appear to relate to lunar observation. Taking the time to understand the cause of the moon's cycles and then observing its changes throughout the month is worthwhile and enjoyable.