A new moon occurs when the moon is between the Earth and the Sun, so that the illuminated side of the moon faces away from viewers on Earth. As a result, the moon appears as a thin crescent in the sky, or it vanishes entirely. In many lunar calendars, the new moon marks the first day of the month. The religious significance of the new moon is accompanied by an assortment of ethnic superstitions as well, as is often the case with astronomical phenomena.
The different appearances of the moon from new to full are sometimes referred to as lunar phases. The lunar phases have a cyclical and easily predictable pattern, and many calendars publish the projected dates of the new, full, and quarter moons for each month as a general reference. The phases of the moon are easily understood once one examines the relationship between the moon, the Earth, and the Sun.
The Earth is in a state of orbit around the Sun, and it takes around 365 days for the Earth to go all the way around the Sun. The moon, in turn, orbits the Earth, in a cycle which takes approximately 27 days. If one considers the new moon a starting point, the moon starts to appear in the sky as it moves behind the Earth, ultimately turning into a full moon when it is totally behind the Earth, and then shrinking in size until it disappears again. When the moon is getting smaller, this is known as waning; a growing moon is said to be “waxing.”
The side of the moon which we see from Earth is known as the near side of the moon. A trick of the relationship between the moon and the Earth ensures that we actually only ever see the near side of the moon. The other side of the moon is called the far side of the moon, or sometimes the dark side of the moon in a more poetic turn of phase. Many people are surprised to learn that the far side of the moon looks radically different from the near side, a twentieth century discovery enabled through the use of satellites and various lunar missions.
Occasionally, a new moon positions itself so perfectly between the Earth and the Sun that it actually cuts off or partially obscures the light of the Sun. This is known as a solar eclipse. Since the phases of the moon are well understood, along with the orbital patterns of the Earth, solar eclipses are very easy to predict. Scientists can figure out when an eclipse is going to occur, and they can find the path that it will take across the surface of the Earth so that curious people can watch a solar eclipse.