The basic reason is that lunar phases are caused by the changing angles (relative locations) of the earth, moon, and sun as the moon circles the earth. The Babylonians devised a calendar based on lunar and solar cycles. They divided each year into 12 months, with each month being subdivided into 30 days.
Lunar phases occur because every full moon is at a slightly different angle relative to the earth. When the moon is completely covered by the earth's shadow, it disappears from view. But even though you can't see it, there is still sunlight bouncing off of the surface. And since the path that the moon takes around the earth is not exactly the same path that the earth takes around the sun, more than one day is spent in every month where the moon is completely inside the earth's shadow.
When this happens, the moon is said to be "full". Every part of its surface is visible, which explains why only one half of the moon is always turned toward the earth. The other half is always facing away from us. During a full moon, the center of mass of the moon is directly over any given point on Earth. So if Earth were a spinning top, the moon would be able to spin it back and forth like a toy top!
But even though half of the moon is in darkness, some parts are not.
When the moon is full, it is completely obscured by Earth's shadow; when it is new, it is directly opposite Earth in its orbit and receives no direct sunlight. As a result, there are no clouds to reflect light from the sun back to us during a new moon, and only half of the moon is illuminated during a full moon.
Lunar eclipses are when the moon passes through Earth's shadow. Because all direct sunlight is blocked from reaching the moon, only light from the sun after first hitting Earth reaches it. The part of Earth that blocks out the sun looks like a dark spot - this is why lunar eclipses are sometimes called "dark" or "night" eclipses. The rest of Earth appears normal daytime colors - blue-black for a total eclipse, red for a sunset/morning eclipse.
Lunar eclipses are visible on half of Earth. If you are somewhere where there is no night, such as inside a building or aboard a moving vehicle, you can see the eclipse!
The lunar calendar employs the phases of the moon to measure time, generally one month from new moon to new moon. One solar year is the amount of time it takes the Earth to circle around the sun. The period between the vernal equinoxes is commonly measured using the solar calendar. However, since the lunar orbit is closer to the Earth's surface than the solar one, the two calendars don't match up perfectly. They are based on different sets of rules and so can never be exactly concurrent.
The lunar calendar is based on the idea that the moon affects the ocean tides and this influence gets stronger as the moon grows larger. When the tide is high, we call this a full moon. When the tide is low, we call this a new moon. By measuring the distance in miles between high and low water, you can figure out how many days it takes the moon to go from full to new moon or vice versa. This is called an eclipse cycle and your local astronomy club will likely have data about these cycles for your area. A new moon on March 20th means that by the end of March there will be another full moon.
Lunar months vary in length from 29 to 30 days depending on when you start counting from either the first or the last new moon. These "leap months" occur every three years or so. If you start counting at the first new moon of January, February has 28 days and March has 31.
A summary of the lunar calendar. It's a calendar that is based on the moon's monthly phases. It is one of the world's oldest calendars, producing lunar months, often known as synodic months. As a result, it aids in determining where each month alternates between 29 and 30 days. The anachronistic name "lunar calendar" comes from the fact that it is based on observations of the tides and shadows cast by the moon. These were used to estimate when each moon had reached its full or new phase.
The first human beings probably developed their own systems for keeping track of time, but since they had no way of writing down what they knew, these systems were lost without a trace. It wasn't until about 3000 B.C. that someone came up with the idea of using sticks or stones to mark off periods of time, but even then there was no guarantee that they would be used again if found by someone else. It wasn't until much later that people began to use objects such as clocks and watches to measure time more accurately.
About 500 A.D. the Chinese invented our modern-day calendar, which is now used throughout the world. It is a calendar that is based on the cycles of the moon, but instead of counting back 10, 15 or 20 years to reach another lunar cycle, as many traditional solar calendars do, the Chinese calendar counts forward in time every 12 years or so when a new era begins.