• Luke

The Moon

Updated: Jan 17, 2019


The Moon during a lunar eclipse like the one occurring January 20th 2019

The Moon, Earth’s closest neighbor, easily captures the eye and imagination. You don’t need a telescope or any fancy equipment to view the Moon and is therefore the first thing that attracts astronomers of all walks of life to pause for a moment and take in the heavens, myself included.


Inspiring awe however is one of the less important responsibilities of the Moon. The Moon has many influences on life here on Earth, the most obvious of which I would argue is the tides. Tides are caused by the Moon and the Sun pulling on Earth. When the Sun and the Moon are in alignment (during new Moon and full Moon) we have spring tides (the largest tides) and when the Moon and the Sun are pulling perpendicular to one another (at first quarter and last quarter, when the Moon appears half illuminated) we have neap tides (the smaller tides).



Tides can be seen in this gif in the thin blue layer that stretches and smooths as the Sun and Moon pull on the ocean


At least that’s about all an astronomer will tell you about tides as that’s all we are taught. An oceanographer might point out that it’s more complicated for many reasons, one being the geography of a cove/shoreline will have an effect on tides, possibly causing outgoing tides to cancel out incoming tides (this is why some places have 2 or 4 tides a day and in some places tides are much more pronounced than others).



Differing tides along coastlines due to oceanographic effects


Doing most of my astronomy education and observing in landlocked Colorado I have just the basic understanding of the tides of the ocean. However Colorado experiences tides as well, earth Tides. Just like the tides of the ocean the solid Earth itself experiences tides which can displace the ground you are on right now to change up to 15 inches! You won’t notice earth tides as there is nothing to compare your movement to. This is like a boat out at sea during both high and low tide won’t be able to see the ocean rise and lower because it happens to all of the ocean within sight equally. I have seen some books on gardening that cite earth tides (and also the pull on the water table) as why you should plant your garden at different phases of the Moon depending on the plant.



Diagram showing different earth tides


The Moon is also pulling on you just as it does on the land and the ocean. I would like to market this as a new weight loss craze that if you wish to lose weight fast skipping the diet and exercise, just step on the scale at midnight on a full Moon! This as most crazes sounds great until you read the fine print: The Moon’s pull on you is one millionth the pull of Earth so when the full Moon is directly overhead you weigh one millionth what you would without the Moon. If a person weighs 150 lbs they would lose .0000001*150 lbs= .000015 lbs or 68.04 milligrams which is about the weight of a large grain of sand. You would probably do better just taking your shoes off when you step on a scale. If you go with something larger like your car, it’s probably around 4,500 lbs and would lose 2 grams or about the weight of a penny.


The force that causes the tides creates a sort of friction on the Earth (aptly called tidal friction). Just as the friction of the ground and air around a top eventually stops the top (or rather slows the top down to the speed of the ground), the tidal pull of the Moon is working to slow the Earth to the speed of which the Moon orbits us. This has been going on since the Earth and the Moon paired up and is why the Moon only shows us one side to this day. The tidal pull of the Earth on the Moon is 32.5 times that of the Moon's tidal pull on Earth. This paired with the fact that the Moon is smaller than the Earth and unable to hold as much rotational energy means the Earth has slowed the Moon down to a stop. This is why the Moon only shows Earth one face.


The Moon still rotates, it just rotates at the same speed which it rotates around us (the Earth). It is no coincidence that it takes the Moon about one month to orbit the Earth. The word month's root is Moon-th, the suffix -th indicates measurement (1/4th 100th or 10th place), so a month is a measurement of a Moon (phase to phase i.e. full Moon to next full Moon). The Moon takes 29.5 days to go around the Earth and one month is 30.4 days on average. The reason for the slight difference is our calendar is tuned to the rotation of the Sun more than the Moon giving us regular seasons. Our calendar, the Gregorian calendar, was created by Pope Gregory XIII in the 1500s when he noticed Easter was not occurring in the start of spring as the church had originally celebrated it.



Pope Gregory XIII


With yourself, two friends and a flashlight you can demonstrate the dance the Sun, Earth, and Moon do that causes the phases of the Moon seen on Earth. Each participant will take the role of either the Earth, the Moon or the Sun. The Sun stands back and shines the flashlight on the head of the person playing the Moon, the Moon will walk circles sideways around the person playing the Earth always looking at them.


From the person playing the Earth’s perspective they will see:


  1. the Moon’s face in darkness when they are standing in between the Sun and the Earth,

  2. the Moon’s face half in light when the Moon is ¼ the way around the circle (first quarter, half Moon),

  3. the Moon’s face will be fully illuminated when the Earth is between the Sun and the Moon (full Moon),

  4. the Moon’s face half in light when the Moon is 3/4 the way around the circle (last quarter, half Moon), and

  5. back to dark once they have made a full rotation (new Moon).

Doing this you will see from the Earth's perspective you only see one face of the Moon. However from the sun’s perspective the the Moon spins one full rotation and the flashlight shines on all sides of the Moon’s head.



This is the phases of the Moon shown in a static diagram. The gif I have to show tides shows the movement better.


There is no dark side of the Moon (unless you’re a Pink Floyd fan) as the Sun shines on all sides of the Moon. If you were living on the opposite side of the Moon from Earth you would still have a day. Anywhere on the Moon has days, they are just as long as a Moon-th (29.5 days). You can imagine living in the middle of the side of the Moon facing Earth and mid-day when the Sun is overhead would be full Moon and midnight would be new Moon making a full day midnight to midnight last 29.5 days. I think I’d prefer living on the Earth side as you would always have Earth in your sky meaning you would also get to see eclipses!