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Lunar Phase Simulator – Background 1

Introduction to Moon Phases

The changing appearance of the moon is the most commonly observed astronomical phenomenon. The moon's cycle of phases has importance in almost all cultures, and even today we use periods of time — the week and month — which likely have their origin in lunar cycles. Even though almost everyone is aware that the moon goes through phases, it is challenging to explain the geometrical cause of phases.

A common assumption is that the earth's shadow falling on the moon causes its phases. Although the earth's shadow does fall on the moon occasionally this is a separate phenomenon called a lunar eclipse. Lunar eclipses occur only a couple of times a year and last for only a few hours. In fact, the dark part of the moon is the half of the moon that is on the opposite side from the sun — the nighttime half of the moon. While half of the moon is always in darkness, keep in mind that there is no permanent 'dark side' since the moon rotates.

However, since the moon rotates at the same rate it orbits around the earth it always keeps the same side toward us. In the pictures below features stay in the same relative place. The other side of the moon, the far side, can only be seen by astronauts and spacecraft.

pictures of the moon at different phases
New, first quarter, full, and last (or third) quarter are primary phases, while the other phases are intermediate. (The pictures were taken 3 to 4 days apart from the spacecraft Clementine with phases superimposed.)

The sequence of images above shows the phases of the moon in their relative order. During the first half of the cycle (new to full) the moon is illuminated on the right hand side, and during the second half of the cycle (full back to new) it is illuminated on the left hand side. Another way to remember this is to note that the edge of the shadow moves from right to left. (We assume our observer is based in the Northern Hemisphere).

To describe how much of the moon appears lit we can calculate the percent illuminated, or the fraction of the moon's disk lit as seen from earth. A new moon is 0% illuminated (completely dark) and a full moon is 100% illuminated (fully lit). Both first quarter and third quarter (also called last quarter) occur when the moon is 50% illuminated (half lit).

These phases — new, first quarter, full, and last quarter — occur at a precise moment in the moon's orbit around the earth and are called primary phases. Between these we define intermediate phases that get their names based on how much of the moon appears lit and how the moon's appearance is changing.

When the percent illumination is increasing we say that the moon is waxing, and when the percent illumination is decreasing we say that the moon is waning. When the moon is more than 50% illuminated it is gibbous, and when it is less than 50% illuminated it is a crescent.

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