The Earth is like a Merry-Go-Round
The Earth is like a merry-go-round showing us seasonal constellations
That iconic childhood ride. Round and round each day we go, round and round each year we go, where we stop nobody knows! When we look out from the edge of the ride we can see the space beyond. Sometimes the Sun occupies that space, and sometimes that space is the night sky filled with stars.
The Sun is like a campfire
The central blaze, the outdoor hearth, the gathering place. It draws us like moths to the flame. We circle it, center our attention on it, we turn away from it to warm our backs, then we turn to face it again. It teaches us, it warms us, its memory runs deep in our collective mind.
On our path around the Sun we keep a good distance away because it is hot, but we always stay nearby because if we move outside its warmth we will freeze in deep space.
The stars are like points of light placed on a giant spherical dome
That dome of stars encloses the Sun and the Earth. They surround us completely as if we are floating at the center of a gigantic sphere. The stars do not move*.
Riding a merry-go-round that turns once a day
Imagine riding on a merry-go-round (it moves once per day in a counter-clockwise direction). Imagine that the merry-go-round is next to a campfire. You can look out from the edge of the merry-go-round and see the campfire some of the time, but you can’t see the campfire all the time because you cannot look across the middle of it.
Each time the merry-go-round turns you see the campfire get closer and then you pass it and eventually the fire is no longer visible as you get carried away by the merry-go-round and turn away from the campfire. While you are turned away from the campfire, you do not see any light from the fire. You are looking away from the campfire into the night – you see the stars beyond.
A merry-go-round near a campfire under a dome of stars.
These three parts of the seasonal constellation model can help us understand why we see different stars on winter nights than we do on summer nights.
Why do we see different constellations at night during different seasons?
We see seasonal constellations because the night sky that is visible from the Earth changes over time. It shows us a slightly different view of the stars each day. As you turn away from the Sun (campfire), you cannot see the Sun (campfire) anymore, but you can see the space beyond – and the stars and constellations that fill that space.
Models of Earth and Sun
To help us understand, we will build a physical model of the Sun as a campfire and a physical model of the Earth as a merry-go-round circling that campfire. In our Physical Astronomy model the campfire is the Sun and our bodies are the merry-go-round Earth. We will spin day after day on the daily merry-go-round.
We started with one merry-go-round to model the day, but the Sun itself is a second, bigger merry-go-round that carries the Earth around once a year. These two merry-go-rounds work together to present the ever-changing seasonal constellations. We show this idea below in an image.
The campfire is the Sun, we are the Earth
In this physical model of our solar system, the campfire will be the Sun. Our bodies will be the Earth. Our movements will be like the merry-go-round. We can face the campfire or we can turn away from it.
Looking “up” means looking “out” – into space
When we turn to face the campfire and look at the campfire (the Sun) our faces are like the people on the day side of the Earth. People on Earth look “up” during the day and see the Sun.
When we turn away from the campfire our faces are like the people on the night side of the Earth. We look “up” and see the stars out in space.
Day = face toward the Sun, Night = face away from the Sun
The Earth has two sides – the day side and the night side. Half of the Earth is always in daytime and half of it is in night. Round and round the Earth rotates each day like a merry-go-round. This merry-go-round never stops. Half of the time we are facing the Sun and half of the time we are not.
We call the side that faces the Sun (the campfire) the day. All the people there can see the Sun “up” in their sky.
As the Earth spins it carries people from the night side into the day side and they experience sunrise.
We call the side that faces away from the Sun (the campfire) the night. All the people there can see the stars “up” in their sky.
As the Earth spins it carries people from day into the night side and they experience sunset.
Spinning and spinning each day, the Sun and other sky objects above us seem to move into view and then disappear out of view. This is because the merry-go-round Earth carries us around each day and we see a new view of the space around the Earth.
Two motions, Two Merry-Go-Rounds – Daily and Yearly
The Earth moves in two ways as it circles the Sun: daily and yearly.
You can think of these two motions like a smaller merry-go-round (daily) being carried along on a bigger merry-go-round (yearly).
Daily – the smaller merry-go-round
As we learned earlier, our Earth’s daily motion is to spin, to rotate. The Earth carries us once-a-day on a merry-go-round ride. As we work, play, eat, and sleep on the surface of the Earth, it carries us under and then away from the Sun.
During the day we face the Sun (from dawn to day’s end), then we are carried away from the Sun (from sunset to night’s end). As you think about this idea you begin to realize that night is where you are more than what time it is.
Yearly – the bigger merry-go-round
Our yearly motion is to orbit, to revolve, to follow a path around the Sun. It takes the Earth one year to complete this path. It takes 365 days, 365 small merry-go-round turns to travel the entire yearly orbit of Earth around the Sun.
As we learned earlier, one side of the Earth is always facing the Sun, this side of the Earth is always day.
Throughout the entire year, one side of the Earth is always day, and one side is always night. There is always daytime somewhere. Fun Fact: If you could fly in a plane for a year at the same speed the Earth is rotating, you could stay on that daytime side and experience continuous daytime!
Seasonal constellations – the night sky changes during the year
As the Earth follows its yearly path in orbit around the Sun (the campfire), the stars in the night sky change. We see winter, spring, summer, and fall constellations in yearly rhythm.
Why does this change of seasonal constellations happen?
Let’s go back to the two merry-go-rounds. Our small merry-go-round is turning quickly like the day. The bigger merry-go-round is turning slowly as the year. The day merry-go-round spins 365 times a year. So, every year, the small merry-go-round gives us 365 views of the night sky!
Every day on our small-merry-go-round we see the daytime sky and every night we see the night time sky. At night (facing away from the Sun) we are able to see the stars.
This image shows how we are able to see different views of the star dome (in this case the zodiacal constellations) as the Earth goes through its orbit. In January, Gemini is visible at night. In May, Libra and Scorpio are visible at night.
We can’t see stars during the day, but they are there!
However, during the day there are other stars in the sky but they are not visible because the Sun is too bright. We cannot see those seasonal stars and seasonal constellations in the night sky until the bigger merry-go-round carries us further along the yearly path. The Earth moves along the yearly path and each night stars seem to move higher and higher in the eastern sky.
As we move on this bigger merry-go-round our view of the night sky changes slightly. Each day as we look “up” into space at night, we are looking away from the Sun. And each day the Earth moves a little bit further along on its yearly path.
Eventually, as the bigger merry-go-round carries us around in half its yearly motion, the night sky will show us a completely different set of stars. 6 months from now, the stars will be completely changed in the night sky! We will see a different set of seasonal constellations.
Summary
The merry-go-round is a good model that can help us understand the daily and yearly motions of the Earth. We used the campfire as a model of the Sun.
We learned how to imagine the stars painted on a giant sphere with the Earth at the center. This idea of a star sphere is a simplification, but it helps us to understand how seasonal constellations change over the year.
As part of the model, we use our bodies to become the Earth and experience day and night by spinning and traveling the yearly Earth orbit path.
We also learned that when we are looking “up” into the sky, we are actually looking “out” into space.
Science background and terms
Daily Earth motion is called rotation on an axis.
Yearly Earth motion is called orbit.
* Stars do move, but only very slowly – it is called “proper” motion.
Seasonal constellation changes are caused by “sidereal” motion.
Nice analogy. One thing that kind of blew my mind when I realized it is that we don’t just see half the starry sky each night (ie not just the parts behind an extension of the day/night line on the earth in your blue zodiac circle), but actually we see all but about a quarter of the sky-sphere (since the horizon is a tangent from earth, with stars visible an hour or so on the dark side of each terminator). Of course, for most people, it’s what’s up at mid-evening that matters….and that sure does wheel around month to month.
Yes! We get a generous tour of the sky every night… if we are willing to stay up late. But, as you observed, it’s really the stars that are out in the early evening that make the most impact.
I like that: “the horizon is a tangent from the earth.” I’ve got an article in draft discussing “how far we can see” under many conditions. I love showing kids the Andromeda galaxy! It’s so far away and the light is unimaginably old. It makes a huge impression.
Jim, I’ll be giving this presentation this Saturday March 3rd, 2018 in Croton-On-Hudson, NY. Thanks for the observations on the write up!