Tell Me Why #8: Why is the Earth, Sun, Moon and all planets round in shape?
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Today we are answering a very interesting question asked by class 10 student Vivek Kumar, from Hyderabad. He wants to know:
Why are the Earth and other planets round?
Wait, what? The Earth is actually flat, haven’t you heard?
Kidding! We’ve actually known that the Earth is round for at least 2,000 years. In fact, the vast majority of objects in our solar system are spherical, from the Sun to planets to moons.
But before we answer that question, we must actually ask ourselves how do we know for sure that our home is a globe and not a flat flying disk.
How do we know the Earth isn’t flat? Prove it!
Fair enough! As students of science, we must not accept any truth without asking for proof.
The most optimal way to determine whether our planet is a thick disk or a huge sphere is to view it from a more distant perspective. One such picture would be enough to falsify one hundred claims about the Earth being not a globe.
Satellites and astronauts have provided us with many pictures of Earth’s blue curvaceous body, but apparently, these pictures are not enough. Those who believe that the Earth is flat could claim that they all are fake pictures. The technology to make such realistic fake images does exist and can be quite easily afforded by multi-billion-dollar government-funded organizations like NASA. So, fair enough.
Simple yet undeniable evidence that proves a curved Earth was gathered more than 2,000 years ago. Around 250 B.C., a few Greeks realized it with just two sticks. They fixed two sticks on the Earth’s surface 500 miles apart from each other in two different cities.
If the Earth was flat, the Greeks would have observed that both sticks cast no shadow when the Sun was directly above them. However, what they observed was that the stick located 500 miles towards the north did cast a shadow while the other one did not!
It’s a no-brainer that this is possible if and only if the Earth’s surface is curved such that the other stick is “inclined”.
Two sticks, that’s all it takes!
Here is another proof. Greek philosopher Aristotle observed that the shadow Earth casts on the moon during a lunar eclipse is shaped like an arc. It is then Clark-Kent-is-Superman kind of obvious that such a shape can only be witnessed when the illuminated object is curved, more precisely, a sphere.
If the Earth is really round, why can’t we see it?
This is a reasonable question to ask: if the earth is round, why don’t I “feel” or “see” this curvature. The surface does appear flat when viewed from the top.
Actually, people can’t “see” the curvature because they simply haven’t climbed high enough. For a six-foot-tall person, the curvature is approximately 3 miles away. Studies place the threshold altitude for seeing Earth’s curvature at about 35,000 feet. Even at this height, it is difficult to discern the curve as the observer requires a wide angleview.
That’s the same reason why it is impossible to see the curvature of the Earth even from an aeroplane that flies up to 28,000 feet above sea level.
If you really want to see the curvature of the Earth, you just have to climb up high enough. Or take a trip to space like Jeff Bezos did!
Now that we know the answer to how do we know that the Earth is a sphere, we must ask the ‘why’ question.
Why are Earth and all the planets spherical in shape?
Seriously, why are planets spherical? Why don’t we live on a cylindrical or cube-shaped planet?
The main culprit is gravity. Gravity is a force, which means that it pulls on things. But the Earth isn’t the only thing which has gravity. In fact, everything in the universe, big or little, has its own pull because of gravity – even you.
Why does everything have gravity? Because everything has mass. Mass is a measure of how much material there is in anything. It could be in the form of rocks, water, metal, gases, people – anything..
Isaac Newton was one of the first scientists to figure out the rules of how gravity behaves. And one important rule of gravity is that it always pulls toward the center of mass.
The Earth and all planets are round because when the planets formed, they were composed of molten material – essentially very hot liquid. Since gravity always points toward the center of a mass, it squeezed the stuff the Earth is made of equally in all directions and formed a ball. When the Earth cooled down and became a solid, it was a round ball.
And voila! We have our round abode! What if Earth stops rotating all of a sudden? What would that lead to?
We hope we have answered all the questions. And thank heavens that the planets are spheres! If they were tetrahedrons or toruses or pentagonal prisms, it would be much trickier to make a solar system model.
Read more Tell Me Why stories like these here!