THE MOON

Rachael O’DonnellImage credit: Marko Kolm

The moon is the Earth’s largest natural satellite, orbiting the Earth on a 27.3 day cycle at an average distance of 384,400km (Space Facts, 2012). The moon can be as close to Earth as 363,300km, or as far as 405,500km away (Choi, 2013).

Image credit: NASA

Apogee

Perigee

These two extremes of the moon's distance are called the Apogee when farthest away and the Perigee when closest (Timeanddate.com, n.d.).

The moon transitions between these two points every 12-16 days (Walker, 1997). A phenomenon called a super full moon occurs when the perigee coincides with a full moon (Timeanddate.com, n.d.). This is also known as lunar perigee-syzygy, and causes the moon to appear roughly 30% brighter than a regular moon at apogee (Nolle, 2011).

Picture credit: moonconnection.com

While it isn’t known for sure how the moon was formed, the leading explanation is the Giant Impact hypothesis. This theorises that a Mars-sized body (known at Theia) collided with Earth in the early days of the solar system when Earth had just been formed (Redd, 2013).

This sent chunks of the Earth’s young crust into space, where gravity pulled it together to form the moon we see Today (Redd, 2013).

The Giant Impact hypothesiswould explain why the moonis less dense than Earth, unlike another prominent theory, Co-formation theory, which would suggest that both bodieswere formed from the same materials (Redd, 2013).

Image credit: NASA/Goddard Space Flight Center

^ Average distance between the Earth and the Moon. Image credit: Nickshanks

Size comparison of Earth and moon. Image credit: NASA

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Any casual observer of the moon can see that the visible or illuminated

part of the moon grows and shrinks regularly, producing the different shapes we see, also known as the phases of the moon. These phases

range from the new moon to the full moon, waxing to full before waning

back to new.However, to understand the phases

of the moon we must first understand how the Earth, Moon and Sun

interact, as it is the rotation and orbit of the Earth and Moon that produce

what we see in the sky at night.

Image credit: NASA

The Moon rotates on its own axis at the same rate that it orbits the Earth, meaning the same side of the Moon is always facing Earth (Moonconnection.com, 2014). Similarly, half of the Moon is always illuminated by the Sun, as shown in the diagram (Moonconnection.com, 2014).

Image credit: Sswelm

These two facts explain the phases of the moon that we see, as we are simply seeing the illuminated half of the moon from different angles as it orbits the Earth. For example, the new moon occurs when the lit-up half of the moon is facing away from Earth. Likewise, the full moon occurs when the lit-up half is facing the Earth. The other phases occur in the transition between these phases.

Image credit: NASA

When the Sun, Moon and Earth line up perfectly, it creates a total lunar eclipse. This is when the Earth completely blocks the light of the Sun on its way to the Moon, casting a shadow over the Moon (Britt, 2014).

When they aren’t lined up perfectly it is not a total lunar eclipse, but is instead a partial or penumbral eclipse (Britt, 2014). A partial eclipse occurs when the Earth’s shadow is covering part of the Moon but not all of it, and a penumbral eclipse occurs when the Moon is in the Earth’s outer (penumbral) shadow (Britt, 2014).

Picture credit: David Paleino

The largest factor which influences the tides on earth is the gravitational pull of the moon (Enchantedlearning.com, 1999). This, combined with the centrifugal force created by the orbit of the Earth, produces the bulging of the water off the planet, or the high and low tides we observe (moonconnection.com, 2014). High and Low tides occur once a day in most places, though some places experience double tides or no tides at all due to the shape of the sea floor, currents or wind at that location (Astronomyknowhow.com, 2010).

Image credit: Sam Garza

The moon can also vary the size of the tides with where it is in its orbit. Spring tide occurs when the moon is in the new moon or full moon position, as the gravity of the moon pulls the water and the Earth itself towards it, creating a bulge on both ends due to centrifugal force (moonconnection.com, 2014). Spring tides are largest when the moon is in the new moon position, as the Sun also pulls the water and Earth towards it, and particularly when the Moon is in perigee (moonconnection, 2014).

Image credit: National University of Singapore

Alternatively, the Neap tide occurs when the Moon is in First Quarter or Third Quarter position, as the gravitational forces of the Sun and Moon are perpendicular to one another in relation to the Earth (moonconnection, 2014). With both of these forces working against each other, the high tides are lower and the low tides are higher, as opposed to the Spring tide when the opposite is true (moonconnection, 2014).

Reference List

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