MOON: What is a “New Moon”?

19 June 2014

The Short Answer (TSA)

The New Moon is that time in the lunar cycle when the Moon is not visible in the night sky.   The New Moon is sometimes called the “Dark Moon.”   In astronomy, the “New Moon” is that moment in the Moon’s orbit when it meets the Sun – the moment when the Sun and the Moon are in the same “orbital” position.

I say “orbital” position because, as seen from the Earth, only rarely are the Sun and Moon in exactly the same place in sky.  During most New Moons, the Sun and the Moon, as seen from the Earth, appear to “miss” each other with the Moon seeming to pass just above or below the Sun.

On those rare New Moons when the Moon passes directly in front of the Sun, there is a solar eclipse.  A solar eclipse happens when, as seen from the Earth, the Moon passes across and in front of the Sun visibly darkening the “face” of the Sun and reducing the amount of sunlight reaching the Earth.

The term “New Moon” also describes a formal “phase” of the Moon.  The “Phase of the Moon” is the shape of the sunlit part of the Moon as seen from Earth.   That shape changes, as the Moon goes through a full cycle of phases.  Each full cycle of the Moon’s phases takes about 29.6 days.

The Phases of the Moon you hear the most about are the “1st Quarter,” “Full Moon,” “3rd Quarter,” and “New Moon.”  By the way, the “quarters” refer to the cycle, not the sunlit part of the moon.  So, the “1st Quarter” is one quarter (or one fourth) through one complete cycle of phases.  But if you look at the 1st Quarter Moon, you’ll see that actually one half of the face of the Moon is sunlit.

Modern calendars use the symbol of a dark circle to refer to the new moon:

But there is another, older, meaning of the term “New Moon.”  Originally, the time when the Moon disappeared completely from the night sky was not called the “New Moon.”  Instead, the complete disappearance of the Moon was called the “Dark Moon.”  The Moon was only a “New Moon” when the first visible crescent of the Moon appeared in the night sky.

Outside of formal astronomy, the original definition of the New Moon, the first appearance of the visible crescent moon after the “Dark Moon,” is still a common one.  The first crescent still marks the beginning of the month in the Buddhist, Hebrew, Hindu, and Muslim calendars.