Understanding The Moon Phases
The Moon has phases because it orbits Earth, which causes the portion we see illuminated to change. The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days “catching up” because Earth travels about 45 million miles around the Sun during the time the Moon completes one orbit around Earth.
At the new Moon phase, the Moon is so close to the Sun in the sky that none of the side facing Earth is illuminated (position 1 in illustration). In other words, the Moon is between Earth and Sun. At first quarter, the half-lit Moon is highest in the sky at sunset, then sets about six hours later (3). At full Moon, the Moon is behind Earth in space with respect to the Sun. As the Sun sets, the Moon rises with the side that faces Earth fully exposed to sunlight (5).
|Moon phases calendar for 2013|
|Eastern Standard Time|
|New Moon||First Quarter||Full Moon||Last Quarter|
|-||-||-||Jan 4 22:58|
|Jan 11 14:44||Jan 18 18:45||Jan 26 23:38||Feb 3 08:56|
|Feb 10 02:20||Feb 17 15:31||Feb 25 15:26||Mar 4 16:53|
|Mar 11 14:51||Mar 19 12:27||Mar 27 04:27||Apr 2 23:37|
|Apr 10 04:35||Apr 18 07:31||Apr 25 14:57 p||May 2 06:14|
|May 9 19:29 A||May 17 23:35||May 24 23:25 n||May 31 13:58|
|Jun 8 10:56||Jun 16 12:24||Jun 23 06:32||Jun 29 23:54|
|Jul 8 02:14||Jul 15 22:18||Jul 22 13:15||Jul 29 12:43|
|Aug 6 16:51||Aug 14 05:56||Aug 20 20:45||Aug 28 04:35|
|Sep 5 06:36||Sep 12 12:08||Sep 19 06:13||Sep 26 22:56|
|Oct 4 19:35||Oct 11 18:02||Oct 18 18:38 n||Oct 26 18:41|
|Nov 3 07:50 H||Nov 10 00:57||Nov 17 10:16||Nov 25 14:28|
|Dec 2 19:22||Dec 9 10:12||Dec 17 04:28||Dec 25 08:48|
Why do we always see the same side of the Moon from Earth?
The Moon always shows us the same face because Earth’s gravity has slowed down the Moon’s rotational speed. The Moon takes as much time to rotate once on its axis as it takes to complete one orbit of Earth. (Both are about 27.3 Earth days.) In other words, the Moon rotates enough each day to compensate for the angle it sweeps out in its orbit around Earth. Gravitational forces between Earth and the Moon drain the pair of their rotational energy. We see the effect of the Moon in the ocean tides. Likewise, Earth’s gravity creates a detectable bulge — a 60-foot land tide — on the Moon. Eons from now, the same sides of Earth and Moon may forever face each other, as if dancing hand in hand, though the Sun may balloon into a red giant, destroying Earth and the Moon, before this happens.
The approximate age of the moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as January 1, 1900 or August 11, 1999) and reducing this modulo 29.530588853 (the length of a synodic month). The difference between two dates can be calculated by subtracting the Julian Day Number of one from that of the other, or there are simpler formulae giving (for instance) the number of days since December 31, 1899. However, this calculation assumes a perfectly circular orbit and therefore may be incorrect by several hours (it also becomes less accurate the larger the difference between the required date and the reference date); specialist usage taking account of lunar apogee and perigee requires a more elaborate calculation.