Lunar Phases: The Changing Face of the Moon

The moon is constantly changing through a cycle of phases; what can you see during each phase and why are they important?

We only see the moon because it reflects sunlight so, if some magical force "turned off" the sun, the moon would disappear from view. Because the moon is a sphere, and because we only see it due to illumination from the sun, its appearance changes as the sun, moon and Earth's relative positions change.

The one phase of the moon that cannot be observed from Earth is the new moon. This occurs when the moon lies almost directly between the Earth and sun, allowing us to see the night time side of the moon -- which is, of course, dark and not visible.

In the days following the new moon, keep an eye on the western horizon just after sunset to spot a very 'young' moon appearing as a slender crescent hanging in the darkening sky. These very young phases of the moon stay close to the sun in the sky so are only visible low in the west as night falls. During the day, the moon appears to follow the sun across the sky.

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The term "waxing" relates to the fact that the illuminated portion of the moon is getting bigger and the crescent gives a clue as to its appearance before half of the lunar disk gets illuminated.

Look carefully at the dark portion of the moon as it hangs low in the western sky and if the conditions are right, you may see it gently illuminated. This is known as Earthshine and is the result of reflected sunlight from the Earth bouncing back to the moon.

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At this point of the lunar phase cycle, the moon has traveled around the first quarter of its orbit since the start of its cycle. It rises about noon when the sun lies south, so the two objects are 90 degrees apart in the sky and located high in the south when the sun sets.

Cast your gaze down the line of the terminator (where the light and dark halves of the moon meet) and you will be able to spot the Montes Alpes mountain range named after the Alps in Europe. The range runs around the north-east segment of Mare Imbrium within which are three prominent craters. The largest is known as Archimedes which measures 83km across. Depicting the south western portion of Mare Imbrium is the Montes Apenninus mountain range.

Along the southern portion of the terminator are countless craters some of which show very defined edges, others that are interlocking where impacts have taken place one after the other and some that have central peaks as a result of the initial impact temporarily melting the surface material.

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A waxing moon means it is getting larger and "gibbous" refers to the illuminated portion covering more than half of the lunar disk but not quite all of it. When the moon is at this phase it rises after noon, but some time before sunset, and appears high in the sky for most of the night.

The terminator is now dominated by the Goliath that is Mare Imbrium but about central along the terminator is one of the moon's most famous craters, Copernicus. It was named after the astronomer who helped to revolutionize our view of the solar system. The crater is not the largest (at 93 kilometers in diameter) but its appearance at this stage of the phase cycle is stunning.

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Perhaps the most spectacular of all the lunar phases is the full moon visible when the sun and moon are in opposite parts of the sky. Features on the moon stand out most along terminator but during full moon, the sun is high in the lunar sky and so shadows are almost non-existent causing features to look pretty flat.

The full moon is probably the best time to appreciate the large dark patches known as lunar mare. These were once thought to be seas but are now known to be large plains of solidified rock that were formed when lava seeped up from below the lunar surface filling in some of the largest craters. The most famous is Mare Tranquillitatis or the Sea of Tranquility on the eastern hemisphere as the dark patch in the center of the five, which somewhat resemble a man's head, body and two legs.

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A lunar phase preceded by the term "waning" means it is shrinking in size and "gibbous" refers to the illuminated portion of the moon being larger than half of the moon, but not quite full. The waning gibbous moon rises a few hours after sunset and as a result sets a few hours after sunrise so appears low in the daylight sky over in the west.

A couple of beautiful craters sit to the north-east of Mare Serenitatis and are nicely visible along the terminator at this time of the lunar phase cycle. To the north is Aristoteles which is 87 kilometers across and just to its south is Eudoxus, a little smaller at 67 kilometers.

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The moon has now traveled along three quarters of its orbit and the opposite half of it is illuminated when compared to the first quarter phase. It is interesting to compare the appearance of features along the terminator now compared to their appearance at first quarter because sunlight is striking them from the opposite side.

Lookout for the beautiful ray crater Tycho a few days after last quarter phase in the southern portion of the moon. The rays were produced from ejecta as the original impactor threw lunar material out in all directions.

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The waning crescent moon is often referred to as an "old moon" as it represents the last stages of the lunar cycle before the new moon and the start of the next series of phases. It can be seen in the east before dawn but because it is almost in a line with the Earth and sun, the majority of the illuminated portion is facing away from us, allowing us to see a thin slender crescent.

Kepler is a great little target at this lunar phase and is roughly half way along the terminator. It is only 32 kilometers across but appears as a very prominent but small crater during this phase.

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