The Phases of the Moon

The revolution of the Moon around the Earth makes the Moon appear as if it is changing shape in the sky. From Earth we see the Moon grow from a thin crescent to a full disk (or full moon) and then shrink back to a thin crescent again before vanishing for a few days.

The Moon phases are produced by the alignment of the Moon and the Sun in the sky. 

The lit part of the Moon always points the way to the Sun.

 The changing shape of the bright part of the Moon that we see is called its phase.


The moon is illuminated because it reflects the light from the sun. The part of the moon facing the sun is lit up. The part facing away from the sun is in darkness.

The phases of the Moon depend on its position in relation to the Sun and Earth. As the Moon makes its way around the Earth, we see the bright parts of the Moon's surface at different angles. These are called "phases" of the Moon.  

The phases of the moon work in a cycle starting with the new moon.

There are eight phases of the moon

The phases are named after how much of the moon we can see, and whether the amount visible is increasing, or decreasing each day.

Phases of the moon as seen in the Northern Hemisphere

 

 

Phases of the moon as seen in the Southern Hemisphere

 

It takes our Moon about 29.5 days to completely cycle through all eight phases. This is known as a Lunar month

The Moon orbits near the equator of the Earth. In the northern hemisphere, we're standing on the opposite side of the globe from countries in the southern hemisphere i.e. we are "upside down" from each other! We therefore see the Moon from a completely different vantage point from each other.

One of the biggest misconceptions about the Moon is that its phases are caused by the Earth's shadow. The Earth's shadow causes eclipses, but it has nothing to do with phases. Rather, the phases of the Moon are produced by the alignment of the Moon and the Sun in the sky.

The orientation of Sun and Moon determines the phase of the Moon. At new moon, the angular distance of the sun and moon is small, less than a few degrees. At first quarter, when the moon is half full, the Moon lies 90 degrees east of the Sun. As an example, if you were to point to the setting sun with one arm and to the moon with the other, the angle between your arms would be 90 degrees. At full, the moon is 180 degrees from the sun and at last quarter, it is 90 degrees west of the sun.

The new moon is essentially invisible because it is between the earth and sun and therefore lost in the glare of the sun. Even on day 2 or 3 it is tough to spot the moon because it is just the tiniest sliver and still very close to the sun. The waxing crescent moon grows till it is about half full in the phase we call first quarter. From there it grows to full moon and then begins to shrink back to third quarter, waning crescent and finally back to new moon. 

The first and last quarter moons mark the halfway points between the new moon and full moon. The first quarter moon is illuminated on the right hand side. The last quarter moon is illuminated on the left hand side. It seems conflicting to call a half full moon first of last quarter, but the quarter refers to the position of the moon in the sky, not it's phase. A quarter moon is one quarter of a full circle (90 degrees) away from the sun.

The lighted part of the Moon always points the way to the Sun. This means that a waxing crescent moon in the western sky at sunset has the lighted part on the right which is the direction of the Sun. The sequence of the lunar phases always proceeds with the lighted part of the Moon growing from right to left until the moon reaches full. After full moon, the light recedes from right to left until new moon.

The line boundary between dark and light on the Moon is called the terminator. It is rarely an even line because the surface of the Moon is not smooth. The terminator is closest to a straight line at first and last quarter. Librations affect exactly what portion of the face of the Moon is facing the Earth, during first and last quarter phase, the terminator's exact location may vary. This is illustrated when the terminator does not line up with the Moon's meridian at first and last quarter. In other words, you'd think that during first and last quarter when the Moon is half full, that the meridian (the imaginary line which runs from north to south on the Moon,) would be right on the terminator, but it isn't so. In fact the terminator can be almost 8 degrees off to either side of the meridian.

WORKSHEET 

The diagram below shows the Earth and Moon as viewed from space far above the north pole of the Earth.Write the name of the phrases.

 

Label the Moon Phases Diagram Read the definitions, then label the moon phases diagram below.

Definitions

Crescent Moon - when we can see only a sliver of the moon's disk (the side of the moon facing us) Full Moon - when the moon's disk is light because the Earth is between the sun and the moon Gibbous Moon - when we can see roughly three-quarters of the moon's disk Half Moon - (also called quarter moon) when we can see one half of the moon's disk (this is one-quarter of the entire moon's surface)

New Moon - when the moon's disk is dark (and invisible to us) because the moon is between the sun and the Earth Quarter Moon - (also called half moon) when we can see one half of the moon's disk (this is one-quarter of the entire moon's surface) Waning Moon - when the moon seems to be getting smaller, going from full to gibbous to half to crescent to new Waxing Moon - when the moon seems to be getting bigger, going from new to crescent to half to gibbous to full