A new moon occurs when the moon is between the Earth and the Sun, so the illuminated side of the moon is away from Earth viewers. As a result, the moon appears as a thin crescent in the sky, or disappears altogether. In many lunar calendars, the new moon marks the first day of the month. The religious significance of the new moon also comes with an assortment of ethnic superstitions, as is often the case with astronomical phenomena.
The different appearances of the moon, from new to full, are sometimes called lunar phases. The lunar phases have a cyclical and easily predictable pattern, and many calendars publish the projected dates of the new, full, and fourth moon of each month as a general reference. The phases of the moon are easy to understand once one examines the relationship between the moon, the earth, and the sun.
The Earth orbits the Sun and the Earth takes about 365 days to revolve around the Sun. The moon, in turn, revolves around the Earth, in a cycle that lasts about 27 days. Considering the new moon as a starting point, the moon begins to appear in the sky as it moves behind the Earth, eventually becoming a full moon when it is completely behind the Earth, then decreasing in size until it disappears again. When the moon gets smaller, this is called waning; a crescent moon is said to “grow.”
The side of the moon 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 see the near side of the Moon. The other side of the moon is called the farthest side of the moon, or sometimes the dark side of the moon in a more poetic phase twist. Many people are surprised to learn that the far side of the Moon is radically different from the visible face, a twentieth-century discovery made possible by the use of satellites and various lunar missions.
Sometimes a new moon is positioned so perfectly between the Earth and the Sun that it partially cuts off or obscures sunlight. This is called a solar eclipse. Because the phases of the moon are well understood, as well as the orbital patterns of the Earth, solar eclipses are very easy to predict. Scientists can determine when an eclipse will occur and find the path it will take to the Earth’s surface so that onlookers can observe a solar eclipse.