LAB K: Astronomy                                                                                                    Back to main page
 


For any planetary body, the term “rotation” refers to the amount of time required to turn 360o on its axis which is called “one day” (see Figure 1).  For Earth, one day is 24 hours.  For the Moon, Earth’s largest natural satellite, it takes 27.3 days to make one rotation about its axis.  A “revolution” refers to the amount of time required for a moon to make one complete orbit about its planet (see Figure 2).  Earth’s moon orbits our planet in 27.3 days.  Thus the time it takes the Moon to make one rotation about its axis is the same amount of time it takes the Moon to travel once around Earth.  Since the length of a lunar day is identical to the length of its revolution around Earth, the same side of the Moon always faces Earth.  Therefore, we refer to the Moon as having a “near side” and a “far side.”

 

The plane that all planets revolve about the sun is called the “ecliptic”.

 

At the same time Earth is revolving about the sun in the course of a year, the Moon is revolving around our planet.  We can view the Earth-Moon-Sun dynamic from above the plane of the ecliptic.  Earth and its moon are both opaque objects, so only the side facing the sun of each is lit .

 

MOON PHASES

 

Your own experience with the Moon will confirm that the Moon has a different appearance from night to night during its orbit around Earth. Why is this? Since the Moon doesn't make any of its own light, the only light we see from it is reflected from the Sun toward Earth. At any moment during the Moon's 27.3 day orbit around Earth, half of the Moon is lit by the Sun and half is in darkness, just as half of Earth is lit by the Sun at any point during the day (see Figure 4). The only part of the Moon we can observe from Earth, however, is its "near side," which is the side of the Moon that faces Earth. The different appearances, or phases, of the Moon occur because, during its orbit, the relative positions of the Moon, Earth, and Sun change, causing the amount of illumination that can be seen from Earth to change.  Refer to Figure 5.  The side of the Moon facing Earth, known as the “near side” side, is highlighted in red.  This is the only side of the Moon that is visible to an observer on Earth at that position.

To help visualize the moon phases, refer to Figure 4.  During the new Moon phase, the Moon is between Earth and the Sun, and the entire far side of the Moon, which we can’t see, is illuminated by light from the Sun.  During these periods, the near side of the Moon is dark and the Moon is positioned over the illuminated side of the Earth. Thus, the dark side of the Moon appears in the sky during the daytime so we can’t see the Moon at all.  During a full Moon phase, Earth is between the Sun and the Moon.  The illuminated side of the Moon is the near side facing Earth so the entire disk of the Moon is visible.   At this position, the moon is positioned over the dark side of Earth, so it is visible at night.  Between the new moon and the full moon, some portion of the near side of the Moon is illuminated, and the remaining portion of the near side is dark.

 

Position 5:  The near side of the Moon is completely dark.  Therefore, the entire orb of the Moon is shaded in.

Position 6:  Less than half of the near side of the Moon is lit up.  The side that is lit is on the right as you look at it from Earth.  Using the illustrations in Diagram 6, the smallest portion of the Moon is lit on the right side.  The rest of the orb is shaded.

 

During each lunar orbit (a lunar month), we see the Moon's appearance change from not visibly illuminated through partially illuminated to fully illuminated, then back through partially illuminated to not illuminated again. Although this cycle is a continuous process, there are eight distinct, traditionally recognized stages, called phases. The phases designate both the degree to which the Moon is illuminated and the geometric appearance of the illuminated part. These phases of the Moon, in the sequence of their occurrence (starting from New Moon), are listed below.

 

New Moon - The Moon's un-illuminated side is facing the Earth. The Moon is not visible.
Waxing Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

First Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.

 

 

Waxing Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but not fully illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

Last Quarter - One-half of the Moon appears to be illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.

 

 

Waning Crescent - The Moon appears to be partly but less than one-half illuminated by direct sunlight. The fraction of the Moon's disk that is illuminated is decreasing.