Tag Archives: precession

Making Friends with the Night Sky: The New Moon

A glimpse of the moon through the trees, as it phases from new moon to cresent.

A glimpse of the moon through the trees, as it phases from new moon to cresent.

Maui astronomer Harriet Witt describes the new moon, the start of the lunar cycle.

Listen here [3:26m]:

Download here [8.1 Mb]: ftp://space.mit.edu/pub/ajb/radiopio/astrofacts_090722_mfns-moonnew.mp3

What’s the facts:

“A month is a Moonth, a Moonth is a moon.”  For many traditional calendars – Hawaiian, Chinese, Hebrew, Islamic – this is still true, although our western calendar has been tweaked to fit the months into one year.  The new moon is the start of the lunar month, and the time when the moon lies between the Sun and the Earth (this is called syzygy, a great Scrabble word).  Imagine yourself floating out in space well above the Sun, Moon and Earth, and you will see these three bodies in a row, with the Sun-lit side of the Moon facing away from the Earth.  So where is the new Moon in the sky?  Directly in front the Sun!

Why doesn’t the Moon block, or eclipse, the Sun every time the new moon phase happens?  It’s because the plane of the Moon’s orbit is actually inclined by about 5° relative to the plane of the Earth’s orbit (the ecliptic plane).  So most of the time, the Sun, Moon and Earth are out of alignment during new moon.  However, the Moon’s orbit actually rotates, or precesses, every 27.2 days.  So perfect alignment at new moon occurs about once every 18 years, a period the Babylonians called a Saros cycle (a complete explanation can be found here).  In fact, there are several Saros eclipse cycles because the Sun-Moon-Earth alignment doesn’t have to be perfect, so we get about two eclipses at new moon every year.

New moon is a great time for star-gazing as the Sun and the Moon will have both set in the evening.  So be sure to enjoy the dark skies that accompany the new Moon!

Original air date 22 July 2009.