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Astronomy: The Moon, Earth, Sun, and Beyond

Analyzing why the moon has phases, why the Earth doesn’t crash into the Sun, and other fascinating astronomical phenomena.

I finished staking the tent down and looked at my watch. It read 5 pm, hours earlier than the typical day. But that was OK with me. A tingling ache permeated through my legs, and it felt like  scalding embers were scorching through the bottom of my feet.

I was in the middle of a long-distance backpacking trip. And I had time to kill. As I settled into my tent, my mind naturally began to wander and wonder, latching itself onto any natural phenomena I didn’t fully understand.

The sun began to set in the west, so I started to consider: what causes days and nights? And why does the sun set in the west and rise in the east?

As the sun dropped below the horizon, it lit up the sky into a beautiful masterpiece of orange and red paint strokes. Which naturally begged the question: why are sunsets red and orange?

I continued to lay there in my sleeping bag, as I observed more and more natural phenomena occur. The temperature started to drop considerably, since it was early fall. Which brought up the question to my mind: what causes seasons?

Not soon after, the sky opened up into a colossal snow globe of brightly shimmering stars. Not wanting to be left out, the moon soon joined the fray, and with it many questions to ponder.

What causes the phases of the moon?

What causes eclipses to occur, and what dictates the cadence they occur at?

Why does the lit part of the moon look different at different parts of the globe?

Staring at the moon, I also began to wonder why it even orbits the Earth? Why does it not crash into the Earth, or careen off into space? I realized the answer to this question is likely the same reason the Earth orbits the Sun.

I quickly took out my notebook and jotted down all of these questions. I would continue to ponder them that night and on subsequent backpacking days (this is what everyone does on backpacking trips right?). But I wanted to remember them all so I could research them and think through them once I returned home.

This post is the result of that research and thinking. It answers all of the random astronomical questions I started to analyze on that backpacking trip.

Some of the answers I had known before and simply forgotten. Others I already knew high-level but upon analysis learned details I hadn’t known previously. And some were new territory for me, so they were fascinating and challenging to think through and explore.

If you’re as curious about the natural world around you as I am, or if you want to impress your friends with your knowledge about why waxing gibbouses occur on the next long-distance backpacking trip you go on, then read on and your dreams will come true.

Questions

What causes days? Why does the sun rise in the east and set in the west?

Days are caused due to the rotation of the Earth. At any given time, roughly half of the Earth is facing the Sun. This half of the Earth is illuminated, experiences “daytime”, and sees the Sun in the sky. The other half of the Earth is faced away from the Sun and is “in the dark” so to speak. This side of the globe experiences “night time”.



As the Earth rotates, the portion of the globe that is illuminated changes. Parts of the globe that are entering the illuminated portion see the Sun rise in the sky from their perspective. Conversely, parts of the globe moving out of the illuminated side see the sun set as night begins for them.

So why does the Sun rise in the East and set in the west? Because, when viewed from atop the North pole, Earth rotates “counterclockwise”. We humans have assigned “East” to mean to the right if you’re looking at the Earth with the North pole at the top of your perspective.

This means that as you stand on the Earth as it rotates counterclockwise, the Sun will come into view “eastward” from your perspective, and appear to set “westward” as the Earth rotates farther and you move back into the darkness.

What causes seasons?

The Earth revolves around the Sun in an elliptical orbit, which means there are times that the Earth is farther away from the Sun, and other times when it’s closer. My initial instinct was that seasons are caused by this variance in distance. This, however, is wrong.

The Earth’s distance from the Sun varies by about 3.2 million miles. This sounds like a lot. However, it accounts for a variance of only around 3.4 percent when you consider that the Earth is on average 92 million miles away from the Sun. This difference is not enough to cause seasons.

Furthermore, the Southern Hemisphere experiences summer while the Northern Hemisphere experiences winter, and vice versa. This would not be the case if seasons were caused by Earth’s distance from the Sun.

The Earth takes roughly 365 days to revolve fully around the Sun, a time period we know as one year.  We know that each year we experience a winter season and a summer season. Winter is generally accompanied by shorter days and colder temperatures, while summer has longer days and warmer temperatures. What is going on astronomically to cause this?

It turns out seasons are caused because Earth’s axis is tilted! Specifically, it is tilted 23.5 degrees relative to its orbital path around the Sun, as measured from the North Pole. Why does this cause seasons?

Because Earth’s axis is tilted, there are certain times during Earth’s revolution where sunlight is hitting the Northern Hemisphere at a more direct angle, and the Southern Hemisphere at a less direct angle.

The angle of sunlight causes the Northern Hemisphere to experience warmer temperatures, while the Southern Hemisphere experiences colder temperatures. The further North or South you are, the more extreme these seasonal effects are.

Additionally, days are longer in the Northern Hemisphere when the Northern Hemisphere is facing more directly towards the Sun.

This is because a given point in the Northern half of the globe is in the illuminated part of the Earth for more than half of its rotation. If the Earth was not tilted on an axis, any point on the Earth would be in the illuminated portion of the Earth for roughly half of the Earth’s rotation.



If you’re having trouble imagining why this is the case, think of it this way. If you take a horizontal cross section (aka a flat washer) of the Earth at a specific latitude in the Northern Hemisphere during summer, more than half of that washer is in sunlight at any given time. If you took a horizontal cross section of the Southern Hemisphere, less than half of that washer is in sunlight at any given time.

Conversely, when the Earth is a full 180 degrees farther along in its orbital path, the situation is the exact reverse of what was just described. The Northern Hemisphere experiences everything the Southern Hemisphere experienced roughly 6 months ago: colder temperatures, and shorter days: aka winter.

Another thing to note. The larger the degree of axis rotation, the more extreme these effects would be. So if Earth was angled at 40 degrees instead of 23.5 degrees, summers and winters would be more extreme.

What causes the phases of the moon?

Roughly half of the moon is illuminated by the sun at all times. How much of that illuminated half of the moon is facing Earth is what determines the phases of the moon.

The moon revolves around the Earth counterclockwise. As the moon orbits the Earth, how much of the illuminated half of the moon that is facing Earth changes.

At first, the entire illuminated half of the moon faces away from the Earth. This is known as a “new moon”. With the exception of eclipses (see next question), the moon is not generally even visible from Earth because none of the side of it that is facing Earth is illuminated.



As the moon revolves counterclockwise, a tiny portion of its illuminated part becomes visible to Earth. This portion of the moon that is lit up shows up on the “right” side of the moon if you’re in the Northern Hemisphere. This is because the Sun is to the right of the moon as seen from the Northern Hemisphere of the Earth.

As the moon continues in its orbital path, more and more of its illuminated side becomes visible. Thus, the moon is said to be “waxing”, which just means growing. Because less than half of its illuminated side is visible to the Earth, the lit portion makes a crescent shape, so this phase of the moon is known as a “waxing crescent”.



Eventually, the moon gets to a point where over half of its illuminated portion is visible to the Earth. Thus, it enters what is known as the “waxing gibbous” phase.



As it continues to orbit, the moon finally gets to a point where its entire illuminated portion is facing Earth. This is known as a “full moon”.



Continuing on from there, less and less of the illuminated portion of the moon is facing the Earth. From Earth’s perspective, it appears that the light is gradually receding across the moon to the left.

Thus, the next two phases are known as “waning gibbous”, which is when more than half of the illuminated portion is still facing Earth, and “waning crescent”, when less than half of the illuminated portion is facing Earth.






This continues all the way until the entire illuminated portion of the moon is facing away from the Earth again (new moon), and the next moon cycle begins.

How long does all of this take? One revolution of the moon around the Earth takes around 27.3 days. And yet, the moon cycle is 29.5 days. How can this be? What causes this discrepancy?

This is because while the moon was revolving counterclockwise around the Earth, the Earth was also revolving counterclockwise around the Sun. So in the 27.3 days the moon spent completing its revolution, the Earth moved a small amount (roughly 1/12th) the way around the Sun.

So the point in which the entire illuminated section of the moon is facing away from the Earth (new moon) has shifted counterclockwise around the Earth. Essentially, the moon needs to revolve about 13/12th the way around the Earth to get from new moon to new moon.

This means the moon spends an extra 2.2 days “catching up” to get to the next new moon point, making the total moon cycle take 29.5 days instead of the 27.3 days.



Why does the lit part of the moon look different at different parts of the globe?

If you’ve ever been to the Southern Hemisphere, you may have observed that instead of the moon appearing to “wax” right to left, the opposite occurs. The sunlight appears to spread across the moon left to right as the moon phase transitions from waxing crescent to waxing gibbous to full moon. Why is this?

Imagine you go from the North Pole to the South Pole. Essentially, you would be turning yourself upside down. With this altered perspective, the Earth would appear to rotate clockwise, and revolve clockwise around the Sun. Additionally, the moon would appear to revolve clockwise around the Earth. 

Consequently, the moon’s waxing phase would occur “left to right” from your new perspective. To imagine why this is the case, refer back to the diagrams above and picture the moon revolving clockwise around the Earth to convince yourself that the directions of waxing and waning would be reversed.

To visualize this, an analogy can help. If you and I were standing on the ground, we’d both agree which direction left and right is. However, if I were standing on the ceiling facing the same direction as you, I would think that your right is my left, and vice versa.

Or consider turning a screw in a wall counterclockwise (lefty-loosey). If I were standing on the other side of the wall and could see you turning the screw, the screw would be appearing to turn clockwise for me (or righty-tighty).

What causes eclipses to occur, and what dictates the cadence they occur at?

First of all, what are eclipses? Well, there are two types.

A lunar eclipse is where the Earth gets between the sun and the moon. When this happens, Earth’s shadow blocks sunlight that would normally hit the moon. The moon is “eclipsed”, as Earth’s shadow gets projected onto the moon. From certain points of view, this makes the entire moon appear dark.



A solar eclipse is when the moon gets in between the Earth and the Sun. The moon blocks a certain amount of sunlight from hitting Earth. The moon’s shadow gets projected onto Earth, but because the moon is smaller than Earth, this shadow only shows up along a certain path across Earth.



After our discussion about the moon’s phases, two things likely jump out to you about these diagrams:

  • A lunar eclipse can only occur during a full moon
  • A solar eclipse can only occur during a new moon

But this begs the question. Why don’t eclipses occur on every new and full moon? Shouldn’t the Earth block light from hitting the moon once per moon revolution? And shouldn’t the moon block light hitting the Earth once per moon revolution?

The answer is that the moon’s path around the Earth is tilted relative to Earth’s path around the Sun by about 5 degrees.

This means that throughout the moon’s revolution around Earth, there are only 2 points where the moon is on the same “horizontal plane” as the Earth and Sun. At all other times, it is either “above” the plane formed between the Earth and the Sun, or below that plane.

The diagram below shows this. The dotted line is the moon’s orbital path around the Earth. The X’s mark the only two points in this orbital path where the moon intersects with the plane between the Earth and the Sun.



Why does this matter? Because the Earth is also revolving around the Sun. This means that most of the time a full or new moon occurs, the moon will be below or above the plane between the Earth and the Sun.



In order for an eclipse to occur, two things must occur simultaneously:

  1. A full or new moon must occur, AND
  2. The Earth must be at a location of its orbit around the Sun where the positions of the full or new moon place the moon on the horizontal plane between the Earth and the Sun.

When these two things occur simultaneously, a straight line is formed between the Sun, Earth, and Moon (or Sun, Moon and Earth), and an eclipse occurs.



It turns out there are two positions of the Earth in its orbit around the Sun (e.g two times per year) where the Earth is in a position that makes the eclipses possible. These are known as “eclipse seasons”, and are indicated by the above diagram.

During eclipse seasons, both lunar and solar eclipses are possible. Luckily, eclipse seasons actually last about 34.5 days. This is because the Earth and moon don’t have to be perfectly aligned for one to obscure the path of sunlight onto the other.

Even if they’re off by a certain amount, eclipses can still occur. And it turns out, the range of time where they are “close enough” to being in line with each other lasts about 34.5 days.

Remember that the moon’s entire cycle lasts only 29.5 days. This means that during an eclipse season, at least one solar eclipse and one lunar eclipse occur. There may even be 2 of one or the other depending on where the moon is in its cycle when the Earth enters eclipse season.

Because there are 2 eclipse seasons each year, that means there are at least 2 solar and lunar eclipses per year (at least 1 per eclipse season), and no more than 4 of each type.

How do we know whether the eclipse will be a total or a partial eclipse?

This is determined by how close the Earth and moon are to being directly in line with each other when the full or new moon occurs during eclipse season. If they are perfectly in line with each other, the eclipse is a total eclipse. Otherwise, it’s a partial eclipse.

Lunar and solar eclipses occur at a similar frequency. Nonetheless, lunar eclipses are more common to see at a given point in the Earth, because the Earth projects a wider shadow over the moon than the moon does on the Earth. So you have to be at just the right locations on Earth to observe solar eclipses when they occur.

Why does the moon not crash into the Earth or careen off into space?

We will discuss the moon orbiting around the Earth here, but everything we discuss more generally applies to any body orbiting around any other body, including the Earth’s orbit around the Sun.

Any two objects have a gravitational force between them that is proportional to the masses of the objects. Earth is very massive, and thus has a strong gravitational force that pulls other objects towards it. This is why you can’t jump more than a few feet off the ground (unless you’re Air Jordan, in which case you can fly).

Earth pulls down on the Moon with this gravitational force.

OK, so then why doesn’t the moon crash down into the Earth?

The answer is because it has inertial velocity. This just means that it is moving relative to the Earth. There is a physics principle called conservation of momentum. This just means that objects in motion tend to stay in motion at that same speed and direction, unless some other force acts upon them.

So essentially, Earth is trying to pull the moon towards it. But at the same time, conservation of momentum is trying to force the moon to continue in its path and careen off into space.

These two physical forces are constantly fighting each other. Their interaction and strength relative to each other dictates what happens to the moon.

There are a few different potential cases to consider.

Case #1: Moon going too slowly 

If the moon was not going fast enough, the force of gravity would be too strong relative to the Moon’s velocity, and Earth would pull the moon into it and the moon would come crashing down like in Majora’s Mask. The velocity the Moon needs to achieve for this not to happen and for it to stay in orbit is called the orbital velocity. 

Case #2: Moon going too fast 

However, if the moon was going too fast, the force of gravity from Earth would not be able to pull the moon into it strongly enough. The moon would simply jet off into space due to conservation of momentum, getting farther and farther from Earth.

This would occur because the strength of gravity is inversely proportional to the distance between two objects, meaning the farther apart the objects are, the weaker gravity is. Thus, if the moon was going fast enough, it would continue to get farther and farther from Earth, which would be exerting less and less gravitational force as this happened.

Eventually, the gravitational force from Earth would be miniscule and inconsequential to the now-free-flying moon. The velocity the Moon needs to achieve to escape Earth’s gravitational pull is called escape velocity, and is higher than the orbital velocity.

Case #3: Moon’s speed is just right

If the moon were going at exactly the orbital velocity, then conservation of momentum would be perfectly balanced with the strength of gravity. The moon would stay in a perfectly circular orbit around the Earth, with the Earth at the center of that circle.

The radius of the orbital circle, which would be the distance from Earth to the Moon, would remain constant.

But why does the moon just happen to have this magical velocity that exactly balances out the force of gravity from the Earth? It turns out it doesn’t, it’s actually going faster than this magical velocity. 

So what happens in this case?

Case #4: Moon going faster than orbital velocity but slower than escape velocity

So the Moon is going fast enough to not crash into Earth, but too slowly to fully escape Earth’s gravitational pull. Does this mean that it stays in orbit? Yes.

This means there’s actually not one magical velocity, but a range of velocities that keep one body in orbit around another. The moon’s velocity is greater than orbital velocity, but less than escape velocity.

In this case, the Moon still orbits the Earth, but not in a perfect circle. Its orbit around the Earth forms an ellipse, which is essentially a stretched out circle. Its distance from the Earth varies depending on where it’s at in the path of its elliptical orbit.

As it turns out, all of the planets (including Earth) also fall under this same case, and orbit around the Sun in an elliptical pattern.

But why do these objects stay in orbit when their velocities are between orbital and escape velocity? And why is their orbital path an ellipse?

Let’s consider the case of the Moon orbiting around Earth to find out why.

Explanation

Suppose the Moon starts out a certain distance d away from Earth traveling at a velocity greater than orbital velocity moving in a straight line away from Earth. Because the strength of conservation of momentum is greater than the pull of the force of gravity, the moon initially gets farther away from Earth.



As this happens, the force of gravity gets weaker and weaker. However, gravity is still exerting force on the Moon, trying to pull it closer. This alters the Moon’s path, so that the moon starts traveling at an angle.

Additionally, the Moon’s velocity gets smaller. This is because as it continues along its path, more of its velocity is pointing in the opposite direction of the force of gravity, which has the effect of slowing the Moon down.



Eventually, Earth’s gravitational pull has been able to pull the Moon’s velocity vector so that the Moon is now “moving” perpendicular to the path it had originally been on, with less velocity than it initially had. The Moon is farther from Earth at this point than when it started.



Moving forwards in time, gravity continues to pull the Moon and exert enough force that changes the angle the Moon is traveling at. As this happens, now the force of gravity and the velocity of the Moon are working together because they start to get closer to pointing in the same direction.

This causes the Moon’s velocity to get faster and faster. As this happens, the Moon actually starts to get closer to the Earth as its angle of velocity changes more and more. These effects compound since the force of gravity gets stronger the closer the Moon gets to Earth.



Eventually the Moon has sped up enough that it has the same velocity v it started out with. It just so happens that at this time, the Moon is the same distance d away from the Earth as it was when it started out, but on the opposite side of Earth.



The cycle then repeats itself, and everything we just discussed occurs going the opposite direction, until the Moon gets back to its initial position with its initial velocity. Then the entire cycle repeats itself over and over again. And this is why the Moon is in a constant elliptical orbit around the Earth.



This tells us a lot about how the Solar System was formed. Essentially, the Solar System started as a spinning disk of cloud and dust around a star (the Sun). Anything that was not going fast enough (velocity < orbital velocity) collided/merged with the Sun and became a part of it.

Any object going too fast (velocity > escape velocity) escaped the gravitational pull of the Sun and rocketed out into space. Everything else stayed in orbit (velocity > orbital velocity, but < escape velocity) and these objects are what we know now as the planets of the solar system.

Why are sunsets red and orange?

The Sun emits light waves of all different frequencies. This means that sunlight contains all light on the visible light spectrum, from the lowest frequency visible light waves (red) all the way to the highest frequency visible light waves (blue).

This sunlight enters our atmosphere. Why then, do sunsets tend to look red and orange?

This is because the molecules in Earth’s atmosphere actually reflect and scatter higher frequency light, while allowing lower frequency colors to pass through. So the lower frequency light waves (red, orange, yellow) can pass through the atmosphere, whereas the higher frequency light waves (violet, blue, green) reflect off the atmosphere and bounce away.

This is the same reason the sky appears blue! Now you have an answer when someone asks you why the sky is blue!

Conclusion

It’s easy to take for granted a lot of natural phenomena that we observe here on Earth, like the phases of the Moon. But when you stop and analyze the astronomical physics behind these phenomena, you realize how amazing the universe really is!

I hope that you learned something from this post and enjoyed it. Let me know in the comments below what natural phenomena you find yourself pondering when you’re outdoors or reflecting!

Sources

1: https://spaceplace.nasa.gov/eclipses/en/

2: https://stardate.org/nightsky/moon

3: https://www.timeanddate.com/eclipse/how-often-solar-eclipse.html

4: https://earthobservatory.nasa.gov/

Published by Analytical Aspergian

I am an Aspergian who loves logically analyzing the world around me. On this blog, I analyze anything that interests me, from economic design to electromagnetism to sports nutrition and recovery.

5 thoughts on “Astronomy: The Moon, Earth, Sun, and Beyond

  1. Nicely done! I love your mind. I appreciate that you were pondering these questions while being outside in nature!

    Wonderful examples of why the moon looks different at different parts of the globe.

    I know there have been studies on lunar phases affecting sleep and mood. Also how women’s menstrual cycles follow the phases of the moon.

    Thanks for sharing!

    Carol

    On Saturday, November 13, 2021, Analytical Aspergian wrote:

    > Analytical Aspergian posted: ” Analyzing why the moon has phases, why the > Earth doesn’t crash into the Sun, and other fascinating astronomical > phenomena. I finished staking the tent down and looked at my watch. It read > 5 pm, hours earlier than the typical day. But that was OK with ” >

    Like

    1. Thank you for the compliments 🙂 It is suspicious to me that the menstrual cycle length is so close to the Moon cycle length. Could be coincidence, but considering the impact the Moon has on water in the ocean with tides and the fact that the human body is mostly water, it’s definitely feasible that the moon cycle length and menstrual cycle lengths are somehow correlated.

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  2. Wow fascinating read, thanks! I learned a lot about planetary and moon orbits and why they are in lockstep with bigger bodies. Also interesting about eclipses and how that all works. Great stuff! One thing I’m still a bit cloudy on though is the color of the sunset. If I look straight up in the sky it appears blue for the reason that you said about higher frequency reflecting back up. But when I look out to the horizon towards the sun, I am seeing the lower frequency light coming through like yellows and oranges. I would think I would see the same exact thing at Noon when I look directly up, I shouldn’t see blue, I should see yellows and oranges coming through, right? I think I’m still missing something here.

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    1. Thank you! So to hopefully answer your question about the Sun: my understanding is that the path the Sun’s light rays take through our atmosphere is longer at sunset than it is at noon, when the sun is directly overhead. This extra distance the Sun takes through the atmosphere gives more opportunity for the atmosphere to scatter the blue and violet light rays. Whereas at noon, the light rays path through the atmosphere is shorter, so the blue and violet rays don’t get scattered as much as they do at sunset.

      Like

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