PHASES OF THE MOON The most common incorrect

PHASES OF THE MOON

The most common incorrect reason given for the cause of the Moon's phases is that we are seeing the shadow of the Earth on the Moon! But this not correct – this only happens when the Moon is in partial umbral eclipse.

What is the dark side of the Moon? In spite of the phrase, there actually is no dark side of the Moon! Just like Earth, our Moon rotates on its own axis and experiences daylight and dark cycles. Our Moon's day and night cycles are a little longer than Earth's — the Moon spins on its axis once every 27. 3 days. Our Moon's period of rotation matches the time of revolution around Earth. In other words, it takes our Moon the same length of time to turn once on its axis as it takes it to go once completely around the Earth! This means that Earth observers always see the same side of the Moon (called the “nearside”). The side we do not see from Earth, called the “farside, ” has been mapped during lunar missions.

Nearside view of Earth's Moon as seen by the Galileo Spacecraft Farside view of Earth's Moon as seen by the Clementine Spacecraft

FYI The Moon doesn't generate any light itself; it just reflects the light of the Sun.

PHASES OF THE MOON

The Moon’s Orbit As our Moon orbits around Earth, the side facing the Sun is always illuminated, just like Earth's daylight side is illuminated by the Sun.

The Moon’s “Faces” Our Moon's shape doesn't really change — it only appears that way! The “amount” of Moon that we see as we look from Earth changes in a cycle that repeats about once a month (29. 5 days). The relative positions of our Sun, Earth, and Moon, cause these changes.

• Starting with the dark new Moon, we see the light part of the Moon “grow” from a sliver to a half to a full Moon — and then the illuminated part decreases, becoming thinner until there is no visible Moon in the sky and we are at the new Moon part of the cycle again. • We have a “new Moon” when our Moon's orbit around Earth moves it between Earth and the Sun. From Earth, the Moon's surface looks dark because the illuminated side is facing away from Earth. As our Moon continues its orbit counterclockwise around Earth (viewed from above the north pole), more and more of the illuminated part of the Moon becomes visible to us, until it reaches the “full Moon” stage. A full Moon occurs when the Moon has moved in its orbit so that Earth is “between” the Moon and the Sun. Change the phrase in blue to apply to the south pole.

Names of the Moon Phases When the illuminated part of the Moon • is precisely a half disk, it is called a quarter. • appears smaller than half the Moon disk, it is called a crescent • appears larger than the half Moon disk, it is called gibbous.

Waxing vs. Waning When the illuminated part of the Moon is growing it is waxing When the illuminated part of the moon is shrinking it is waning

Northern vs. Southern When you are in the Northern Hemisphere, you will see the Moon phases (and the constellations) the other way around than we see them here in the Southern Hemisphere. The Northern Hemisphere will see each phase rotated through 180°. This is because people on the Northern Hemisphere are "upside down" compared to us. Although they would probably prefer to say that we are "upside down", hence the term "down under". Anyway, there is no up and down in space, is there?

Moon Phases Northern Hemisphere Orientation

For example… • • • In the northern hemisphere, if the left side of the Moon is dark then the light part is growing, and the Moon is referred to as waxing (moving toward a full moon). If the right side of the Moon is dark then the light part is shrinking, and the Moon is referred to as waning (moving toward a new moon). Assuming that the viewer is in the northern hemisphere, the right portion of the Moon is the part that is always growing (i. e. , if the right side is dark, the Moon is growing darker; if the right side is lit, the Moon is growing lighter). In the southern hemisphere the Moon is observed from a perspective inverted to that of the northern hemisphere, so the opposite sides appear to grow (wax) and shrink (wane).

How long does it take our Moon to go around Earth? It takes 27 days, 7 hours, and 43 minutes for our Moon to complete one full orbit around Earth. This is called the sidereal month, and is measured by our Moon's position relative to distant “fixed” stars. However, it takes our Moon about 29. 5 days to complete one cycle of phases (from new Moon to new Moon). This is called the synodic month. The difference between the sidereal and synodic months occurs because as our Moon moves around Earth, the Earth also moves around our Sun. Our Moon must travel a little farther in its path to make up for the added distance and complete the phase cycle.

Copy & complete the following Hemisphere Direction of orbit Waxing side Waning side Diagram of the Sun, the Earth and the Moon’s positions and the corresponding Moon phases Northern Southern