The motion of the moon is fundamental to positional astronomy. As we've seen, most of the world's early cultures kept time by the moon and not by the sun. Lunar cycle is obvious to almost every position in the sky and we don't even need a dark sky to see it. The phases of the moon occur over a lunar cycle, but we can measure the cycle in two distinct ways. One is the sidereal period, 27.3 days, which is how long it takes the moon to complete one cycle of the earth and the second, a slightly longer time, is the synodic period, 29.5 days, which is how long between one full moon and the next. That was the basis of timekeeping. Since the synodic period is almost exactly 29.5 days, the first lunar calendars were based on alternating cycle of months with 30 days and 29 days, giving the average of 29.5. The synodic period, or time between full moon and next full moon is a day or so longer than the sidereal period of the moon because the moon is traveling around the Earth while the Earth travels around the Sun. The phases of the moon are caused entirely by the varying illumination of the moon as seen from the Earth. Remember that the moon is always a half illuminated by the sun, but we see fractions of that half illumination depending on the relative positions of the Earth, Sun and Moon. A time-lapse photograph shows what's familiar to everyone, the phases of the moon. We see the slight crescent moon occurs of course, when the moon is closest to the sun in the sky. The full moon occurs when the moon is opposite the sun in the sky. Because of this varying illumination, it's clear when you would look for a new and a full moon. You would always look for a full moon nearly overhead around midnight, while you would look for a new moon near sunrise or sunset and relatively close to the sun in the sky. When it's almost between us and the sun, the moon is essentially invisible. It's entire half-lit surface is pointing back towards the sun. Interestingly, the new moon is not entirely dark. It receives a small amount of light from the earth called earth shine and this has been detected. Another subtlety is added by the fact that the earth's orbit of the sun is tilted with respect to the moon's orbit of the earth by about four degrees. This means that when the moon passes between the sun and the earth around a new moon, it rarely directly intersects the line between the earth and the sun. It also means that a full moon, the sun is rarely on a line extending away from the sun in the opposite direction to the earth. The tilt of the moon-earth orbit, relative to the earth-sun orbit is what leads to the subtleties of eclipses. For a solar eclipse, the moon must pass directly between the earth and the sun, occulting the sun and damning its light and heat. For a lunar eclipse, the moon is on the opposite side of the earth to the sun and passes through the earth's shadow, darkening what would otherwise be a full moon. We can see instinctively and intuitively the solar eclipses must be rare than lunar eclipses. A solar eclipse will only occur when the sun is directly occulted by the moon and we happen to be on a position on the earth's surface to be in the shadow or umbra. However, a lunar eclipse occurs when the earth casts a shadow over the entire moon and anyone on the night side of the earth can see that. The simplest statement to make about eclipses is that if the earth-sun had no tilt relative to the earth-moon axis, solar and lunar eclipses should occur every month. They do not because of this four degree tilt. In fact, solar and lunar eclipses occur roughly every six months when the so-called nodes are lined up. Which means, the tilt of the earth-moon axis happens to line up with the earth-sun axis. The following diagrams are not exactly to scale, but they show why a lunar eclipse is going to be more easy to observe and more frequent than a solar eclipse. A lunar eclipse will occur lasting several hours, while the moon travels through the earth's shadow cast into space. Often the moon gets a reddish glow and is not completely dark and that's because of light refracting through the earth's atmosphere and being reddened by dust in the earth's atmosphere, a solar eclipse means that the shadow of the moon comes almost to a point at the earth's surface, sometimes spending only a few miles across. It travels across the earth's surface at the rate that the earth is spinning and it's almost very quickly. If you are at a fixed point in the earth's surface, it's rare for a solar eclipse to last more than a few minutes. The recent record was nearly seven minutes in 1991. Sometimes, because of the earth-sun distance varying by four percent and the earth-moon distance varying by 10 percent, there isn't a full apex of umbra or darkness and the eclipse is seen in penumbra or partial shadow. Depending on these earth-sun-moon distances, a solar eclipse may not be perfect where the angular size of the moon is exactly the same as the angular size of the sun. Occasionally, the moon can overfill the disk of the sun completely blotting it out and sometimes it under fills it, leading to an annular eclipse or a ring of light around the sun. The most spectacular solar eclipse occurs when the angular sizes are almost perfectly matched and we see a ring of thin light, a diamond B on a necklace around the edge of the moon. The blotting out of the sunlight also allows us to see rare atmospheric phenomena of the sun, like the chromosphere and the corona. This graph of recent and upcoming solar eclipses shows how rare they are. If we think back to ancient times before people could travel far, you can see that it would be rare in a human lifetime to ever catch a solar eclipse, whereas any human would probably see several lunar eclipses in their live. As a result of this, solar eclipses had enormous power in mythology. They exerted extreme psychic hold over ancient cultures. In fact in Greek myth, told in the tails of Hesiod and Homer, when a legendary battles between the Medes and the Lydians on the plane in Turkey near where Troy is situated, was decided by a solar eclipse. Unknown to the protagonists on the battlefield, a solar eclipse was occurring and passed a shadow across the battlefield. The warriors staggered and wandered dazed and confused from the battlefield and the tide of history turned because of a happenstance astronomical event. The moon is a prominent object in the sky and the phases of the moon are caused entirely by the varying amount of the always half-lit surface of the moon, based on the relative position of the earth, sun and moon. When the moon is almost between us and the sun, it's in its new phase. When it's almost opposite us from the sun in the sky, it's full moon and the varying cycle takes 29.5 days, the basis of lunar calendars. The moon's orbit of the earth is tilted by four degrees with respect to the earth's orbit of the sun, leading to the fact that eclipses of the moon and the sun do not occur every month but are relatively rare events. Solar eclipses are much rare than lunar eclipses because in a lunar eclipse, we're looking at the shadow of the larger earth which completely covers the moon and is visible from anywhere on the night side of the earth whereas in a solar eclipse, the moon shadow comes almost to a point on the earth's surface and is visible by a small number of people who happen to be at that location.
