• 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!


A lunar eclipse is caused by an alignment of the the Sun, the Earth, and the Moon. Going back to the example of three friends doing the Sun, Earth, and Moon dance, an eclipse happens when the Moon is on the exact opposite side of the Earth from the Sun where the sun’s light is blocked from reaching the Moon by the Earth. During a lunar eclipse the Sun’s light is blocked from reaching the Moon by the Earth.



Lunar eclipse's are often called blood Moons due to the red hue.


If you were on the Moon a “lunar eclipse” looks like a solar eclipse where the Sun is blocked out by the Earth. On the Earth we see the fully illuminated full Moon go dark, usually a dark shade of red/orange. The red/orange color is caused by the Earth’s atmosphere bending light from the Sun much like a sunset. Switching back to the perspective from the Moon when you see the Sun tuck behind the Earth you are able to see a red/orange ring of sun around the Earth. This red/orange ring is the Earth’s atmosphere bending the light, so during a lunar eclipse an observer on the Moon would see all of the sunsets around the Earth at that moment. This is why lunar eclipses are different shades of reds and orange as what is going on in the Earth’s atmosphere changes the color. For example, a volcano in Chile made a lunar eclipse in 2015 extra dark because it released more particulates into the atmosphere.



What an eclipse seen from the Moon might look like


There will be a lunar eclipse next Sunday (January 20th, 2019) peaking just after 10 pm MST in Denver. Observing lunar eclipses is easy, requires no equipment and can be done anywhere that the sky is clear and you can see the Moon. Just check in on the Moon frequently on the night of the 20th and watch it. The Moon will be fully illuminated at the start of the night ~6 pm, you won’t even see a sign of an eclipse till after 8:34 pm when the eclipse starts. The Moon will get darker and darker and at 9:40 pm it will be completely shaded. The Moon will reach its darkest at 10:12 pm when it is in the middle of Earth’s shadow. Then as the Moon leaves Earth’s shadow it will get brighter and brighter getting its first direct Sun at 10:43 pm and going back to fully illuminated just after 11:50 pm.



This collage shows the transitional stages of a lunar eclipse.


Looking at the moon with a telescope or a simple pair of binoculars will yield amazing detail of the lunar landscape, and enhance watching Earth’s shadow progress across the Moon. The Moon is the second brightest thing in the sky (after the Sun) and can hurt your eyes if using too large of a telescope, so smaller might even be better. Looking in a telescope or binoculars you will be able to tell as the Moon gets dimmer as your eyes strain. You eyes will dilate looking at a bright full Moon around 6 pm, but at 10:12 pm you can look at it easily. Wearing sunglasses while looking in the telescope/binoculars will limit the strain on your eyes. You will also be able to see the effects of the Moon dimming when looking at other celestial objects such as Orion’s Nebula which will start the night more washed out by the Moon’s natural light pollution but will appear brighter while the Moon is eclipsed.



The location of Orion's Nebula is the middle star of Orion's sword which hangs below his belt.


If you are interested in seeing the lunar eclipse up close in my telescopes and have a more detailed explanation, I am holding a special lunar eclipse tour the night of January 20th. Please contact me for details as space is limited. An eclipse like this will not be visible in the Colorado skies again until 2025, do not miss it!




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