Virtual Reality Moon Phase Pictures

A Brief Explanation of the Moon Phases

The phases of the moon are caused by the relative positions of the earth, sun, and moon. The moon goes around the earth, on average, in 27 days 7 hours 43 minutes.

The sun always illuminates the half of the moon facing the sun (except during lunar eclipses, when the moon passes thru the earth's shadow). When the sun and moon are on opposite sides of the earth, the moon appears "full" to us, a bright, round disk. When the moon is between the earth and the sun, it appears dark, a "new" moon. In between, the moon's illuminated surface appears to grow (wax) to full, then decreases (wanes) to the next new moon.

The edge of the shadow (the terminator) is always curved, being an oblique view of a circle, giving the moon its familiar crescent shape. Because the "horns" of the moon at the ends of the crescent are always facing away from the setting or rising sun, they always point upward in the sky. It is fun to watch for paintings and pictures which show an "impossible moon" with the horns pointed downwards.

(above information courtesy of NASA)

New Moon

Waxing Crescent

First Quarter

Waxing Gibbous

Full Moon

Waning Gibbous

Last Quarter

Waning Crescent

Did you know that the moon's motion is harder to calculate than the earth, sun, or any of the planets? It's true. Our friendly moon follows a complex path as it circles the earth. Why? Because unlike the planets, the moon's motion is influenced by both the sun AND the earth.

The most basic moon phase programs use what is called the "synodic period number", which is 29.5305882. It basically means that the moon cycles completely around the earth in 29.5305882 days. The problem is, the moon's position (and phase) isn't perfectly "linear", so simple date arithmetic won't give accurate results.

It is possible to compensate for all that, making calculations things like the "mean anomaly", "orbit eccentricity", "parallax", "true orbital longitude" ... to churn out accurate numbers. This means that it is possible to get within a minute of the precise moon phase at any moment in time. Amazing!

Click on this link for more information on Sun and Moon Phenomena.

Click on this link to go to the Time Service Home Page

Another cool site is the StarDate Online | Moon Phase Calculator