20 Reasons We Know the Earth Is Spherical

Jun 13, 2018 · 10 min read
Photo by AJ Colores on Unsplash

In their attempts to cover all the topics in the given curriculum, teachers often focus more on outlining results and theorems; as opposed to taking us on a journey to discover how the science was derived through pristine ideas, experiments and often-elegant mathematics. One problem this leads to is that we end up knowing several facts and formulas without remembering how they were discovered or why they hold true.

Knowledge that has existed for several years is especially susceptible to being taken for granted. One such example is the knowledge that the Earth is spherical as opposed to flat. In this article, the word ‘spherical’ is used lightly; the exact shape of the Earth is an oblate spheroid, a type of ellipsoid.

Our size relative to the Earth is too small to notice a curve. To a tiny creature that has just learned to walk around a small area of the Earth, there would be no immediate indication that the Earth is round. However, our ancestors gradually collected evidence that contradicted the primitive notion that the Earth is flat. This article is a journey through all the ideas and observations that suggest that the Earth is spherical.

1. Ships and the horizon

The horizon is the line at which the Earth’s surface and the sky appear to meet. When ships sailing away disappear into the horizon, they do so bottom-first. The top vanishes later, which creates the illusion that the ship is sinking. Similarly, when ships appear from the horizon, the top appears first, and then the rest of the ship.

2. We cannot see very far away

Let’s say that you are standing on the West coast of North America on a clear day. While you are able to see the sun and moon in the sky, which are quite far away, you cannot see Japan if you look to the west. The reason you cannot see that far away on Earth is that light travels in a straight line and hence cannot follow Earth’s curve.

3. Visibility and elevated areas

Many sailors are aware of the fact that elevated areas of land are visible at greater distances than those less elevated. Moreover, if one is standing on an elevated area, they are able to see farther away into the distance compared to if they were standing at a lower altitude. The curvature of the Earth is responsible for these observations.

4. Other planets are spherical

Mercury, Venus, Mars, Jupiter and Saturn can all be seen with the naked eye. In 1781, William Hershel used his telescope to observe Uranus’ motion and discovered that it was a planet, not a star as was previously thought. Based on small perturbations in Uranus’ orbit, it was then predicted that a more distant planet also existed. In 1846, Neptune was discovered to be that planet (it was also previously thought to be a star). If the other planets in our solar system can be observed to be spherical, why should ours be any different?

5. Most things are spherical

Not only are planets spherical, but so are stars and moons. In fact, the forces of nature are such that objects tend to form into spheres, whether they are celestial bodies or mere soap bubbles. In the case of soap bubbles, surface tension, which wants to make the bubble smaller in all directions, causes the spherical shape. In the case of cosmic objects, it is gravity that tries to collapse matter in all directions since all the atoms are pulled toward a common center of gravity.

If a spherical object is rotating, the rotation flattens out the middle, making the sphere wider across the equator and narrower across the poles. This is because, in the case of rapid spinning, the centripetal force overcomes the gravitational attraction trying to create a spherical shape. Earth is an example of this, hence the oblate spheroid shape. Jupiter is the fastest spinning planet in our solar system and therefore is more flattened than Earth. The sun rotates slowly, but there are other stars that spin rapidly and also have flattened shapes. Rapid spinning is also the reason black hole accretion disks, solar systems, and galaxies exhibit flattened disk shapes.

6. Temperature variation from equator to poles

The Earth is tilted 23.5 degrees relative to the sun. The northern hemisphere is tilted toward the sun for 6 months, while the southern hemisphere is tilted away, and vice versa. Consequently, while equatorial areas receive direct sunlight all year round, polar areas spend half of the year pointed away from the sun. This difference in sunlight exposure results in higher temperatures closer to the equator.

The Earth’s tilt also explains the extreme length of day and night at polar locations. While the length of the day at the equator is almost exactly always 12 hours due to the equator always receiving direct sunlight, the length of day and night at the poles is influenced by the Earth’s position relative to the sun.

The Earth’s tilt is also the reason that there are four seasons, and that as we approach the equator, the intensity of the seasons decreases until they are completely nonexistent right at the equator.

7. The Earth revolves around the sun

The idea that the Earth revolves around the sun was first proposed in the 3rd century BC by Aristarchus of Samos. By then, the ancient Greeks had already figured out that the Earth is round, and had even calculated the size of the Earth as well as its distance from the sun and moon. However, the fact that the Earth orbits the sun allows us to conclude in several ways that the Earth must be spherical. For instance, the sun rises and sets. Since the sun does not move much relative to the Earth, the Earth itself must be spinning around its axis in order for the day and night cycle to be possible. For the Earth to be spinning in this manner, it must be round.

8. Shadows of sticks

Sticks placed vertically in the ground at distant locations have shadows of different lengths. The ancient Greeks were the first to compare the shadows of sticks at different locations. For instance, they found out that when the sun was directly overhead in one place, the stick there cast almost no shadow. At the same time, in another city, the stick there did cast a shadow. If the Earth were flat, both sticks would show the same shadow because they would be at the same angle toward the sun. Not only did the ancient Greeks conclude that the Earth must be round, they also used the shadow measurements to calculate the Earth’s circumference with decent accuracy.

9. Tides

The moon’s gravitational attraction causes the oceans to bulge out in the moon’s direction. The bulge occurs on both the side facing the moon and the opposite side. While it may not be immediately obvious why there is a bulge on the side opposite to the moon, the reason is that the Earth itself is also being pulled toward the moon, and hence away from the water on the far side. Since this occurs as the Earth is rotating, tides are generated. In particular, this leads to two (high) tides each day. Of course, this could only occur if the Earth were spherical.

10. Coriolis effect

The Earth rotates faster at the equator than it does at the poles. This is because the Earth is wider at the equator, so a point on the equator has to travel farther in a given period of time compared to a point spinning at a pole.

Let’s say that you have set up a gun at the North Pole which is aimed at a target somewhere on the equator. If we assume that the gun is perfectly precise, generates enough power for the bullet to reach the equator, that there are no obstacles in the way, and that there is no wind, will the bullet hit the target? Probably not. Because the target is on the equator, it is moving faster than the gun, and the bullet will probably land to the side of the intended target. This apparent deflection is the Coriolis effect.

The wind is like the bullet. It appears to bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Therefore, in the Northern Hemisphere, hurricanes and other storms swirl counter-clockwise, while they swirl clockwise in the Southern Hemisphere.

Pilots are aware of the Coriolis effect and take it into account when charting long-distance flights. This means that most planes are not flown in straight lines from origin to destination.

The Coriolis effect also plays a role in the existence of the Earth’s magnetic field. Magnetic fields produced due to the flow of liquid iron in the Earth’s core are roughly aligned in the same direction due to the Coriolis effect, which leads to the production of one vast magnetic field permeating the Earth.

Therefore, the Earth’s magnetic field and other consequences of the Coriolis effect such as the direction of flow of wind systems in the Northern and Southern Hemisphere are all testaments to the Earth’s spherical shape.

11. Gravity

If the Earth were a flat plane, its center of mass would be the center of the plane and the force of gravity will pull anything on the surface in that direction. This means that if you stand near the edge of the plane, gravity will be pulling you sideways toward the middle of the plane.

12. Variations in the Earth’s gravitational field

Earth’s gravity is slightly weaker at the equator than at the poles. There are two reasons for this. First, since a point at the equator spins faster than a point at a pole, the outward centripetal force at latitudes near the equator is greater and counteracts the Earth’s gravity more. The second reason is that the Earth’s equatorial bulge (itself also caused by the centripetal force) causes objects at the equator to be farther away from the Earth’s center than objects at the pole, and the gravitational pull between two objects is inversely proportional to the square of the distance between them.

The variations in Earth’s gravitational attraction can be measured and provide concrete evidence of the Earth’s shape.

13. The Earth’s shadow

During a lunar eclipse, the sun, Earth, and the moon are aligned such that the Earth’s shadow falls onto the moon. The Earth’s shadow has been observed to be curved as the planet is.

14. Different constellations at different latitudes

At any given point on Earth at a given time, about half of the possible sky will be visible. If you are exactly on the North or South Pole, the sky will appear to rotate around you and you don’t get to see new stars as time goes on. For any other points on Earth, the visible constellations change as the Earth rotates. However, constellations that are too far north or south cannot be seen from the opposite hemisphere because they would always be below the horizon. Constellations that can be seen from both above and below the equator, such as Orion, appear to be upside-down when you cross from one side of the equator to the other.

The first person to observe this difference in visible constellations and use it to conclude that the Earth must be round was Aristotle (384–322 BC).

15. Double sunset

It is possible to witness the sun set twice on the same day. One way this could be done is to lay down in an open field, watch the sun set, and then quickly rise and you would notice that it had not set completely from this higher altitude. You could also bring along a friend. One of you lies down and the other stands and both try to time when the sun sets. The person standing would have clocked a slightly later time.

For a more dramatic effect, you could go to the base of a tall tower, such as the Burj Khalifa in Dubai. Observe the sunset, then quickly take the elevator to the highest possible floor open for tourists (the elevators travel at 10m/s). You should be able to watch the sun set again.

This double sunset, of course, would only be possible if the Earth were spherical. The reverse experiment can also be conducted at sunrise.

16. Maps are 2D projections

It is impossible to flatten an orange peel without distorting it in some way (tearing, stretching etc.). Similarly, it is impossible to produce a 2D map of the Earth without introducing distortions in terms of shape, distance, direction or land area. This is the reason there are several map projections of the Earth such as the Mercator, Gall-Peters and Robinson projections. If the Earth were flat, producing a world map would have been a lot more straightforward.

17. We can travel around the world

The first global circumnavigation in history was an expedition by the Portuguese explorer Magellan and his crew, completed in 1522. While 223 out of the initial 241 men or so that set out on the journey had died, including Magellan himself, the few remaining crew members successfully made it back to Spain after a trip around the world.

Today, the fastest military aircraft are capable of circumnavigating the globe in less than 10 hours.

18. We have photographic evidence

The first pictures of Earth from space were taken at an altitude higher than 160km in 1947, using captured German V-2 rockets from World War II. In 2018, NASA released a picture of the Earth and moon taken from a distance of over 63 million kilometers away, showing the Earth and moon as isolated bright dots.

19. The testimony is reliable

We can rely on the fact that no cartographer, geographer or physicist ever pauses to think that the Earth might be flat. In fact, Vogel notes that, since the 8th century, “no cosmographer worthy of note has called into question the sphericity of the Earth.”

20. Physics rules everything around me

Flights, GPS, satellites and other modern technologies work due to our understanding of the shape and size of the Earth to extraordinary precision. If we were wrong about even minute details in our measurements of the Earth, we would have found out.

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