How Do We Know The Earth Is Round? [Astronomy Basics]

How do we know the Earth is round? Unfortunately, probably due to the decreasing educational standard in North America, this question has — since it was demonstrated scientifically beyond doubt thousands of years ago — once again risen to prominance.

The fact is that there are a number of observations that anybody can make that clearly demonstrate the Earth to be round. These only require a basic level of knowledge in Astronomy (which you will already possess if you have followed the Astronomy textbook up until this point).

The following list explains succinctly ancient and modern observational proofs that the Earth is round:

Stars Orbiting The Celestial Poles

Watch the stars orbit Polaris in the Northern Hemisphere or orbit a region in space in the Southern Hemisphere — if you watch the circumpolar stars near the celestial poles at night, you can watch them orbit a point (either Polaris in the north or a region of space in the southern sky). If you live on the equator you are best situated to observe this phenomenon.

This is because the Earth is turning on its axis — just like the model globes people are familiar with. The round Earth revolves on its axis, and its axis points in these north-and-south directions, which is why the stars appear to orbit these points as the Earth revolves.

Changing Stars At Zenith

This is my favourite proof, because it is the one that has no alternative explanation.

From wherever you are on Earth, look up at the sky at night — literally, look straight up towards zenith. You can literally pick any star that you see that is as close to straight up as possible. Ideally, you might want to pick 4 stars (each 90° apart from one another) so that you have a star in each quadrant of the sky so that you have a reference point in any season.

The next time you travel anywhere north or south from your home location (ideally a few hundred miles/kilometers away so the distance is noticeable) make a point to observe the stars during the night. Know your four stars that you have chosen, and you will notice that they are not at zenith anymore — even when you wait for them to be (nearly) directly overhead.

This is because you have travelled north (or south) along the curvature of the Earth, so from your new position, zenith is literally pointing in a different direction.

If you want to go further, and get more technical, you can actually measure the offset in degrees of your chosen star, which will perfectly match the new latitude of your location.

Indeed, a cool experiment that you can do on your own is calculate the latitude of any place you visit using the 4 stars you have chosen as your reference point. This is how ancient navigators (land merchants, sailors, explorers, voyagers, vikings) all determined how far north or south they needed to go. Landmarks served as reference points for East-West distances (because longitude was a little bit more tricky to calculate) but latitude could be reliably and easily computed with the stars.

Changing Elevation of the North Star Star With Longitudinal Travel

This proof is the same as the above, but using Polaris in the Northern Hemisphere, or the circumpolar stars in the Southern Hemisphere as reference. If you live in the Northern Hemisphere, measure the angle above the horizon of Polaris in the sky. (You can do the same but in reverse, with the South Celestial Pole, in the southern sky.) This angular measure is your reference point.

Next time you travel north or south — as far south to the equator, or all the way to the north pole is fine — and you will see Polaris get closer to the horizon, or move towards zenith. This is another way that you can calculate latitude.

Again, this occurs because you are travelling along the curvature of the Earth towards (or away from) the Earth’s axis of rotation, proving that the Earth is round.

The Earth’s Shadow On The Moon

During a lunar eclipse, you can see the shadow of the Earth on the Moon. (This is what a lunar eclipse is.)

Every single time you watch a lunar eclipse, the shadow of the Earth is always round. You might argue that this could be true of the Earth was a flat disc. However, if the Sun and Moon are moving around the flat-disc-earth, then sometimes the Sun would be in alignment with the plane of a disc and cast a shadow that is a line or oval.

This never happens — ever — because the Earth is a sphere not a disc.

When You Travel South You See New Stars

If you live in the Northern Hemisphere, every single time you travel southwards, new stars become visible over the southern horizon. Stars that you will have literally never seen before.

This is because you are moving along the curvature of the Earth, and literally seeing a brand new part of the sky that is literally hidden by the horizon (the body of the Earth globe) from your normal location.

This is also true in the southern hemisphere, but when you travel north.

Make Friends In Other Time Zones

If you live in North America, and make a friend in Europe, India, and Japan, if you were to call them all at the same time and date — say dawn in your location when the Sun is just rising in the East — they would all report the Sun to be in different locations.

In some locations relative to you, while the Sun is just over the eastern horizon where you live, in another location it will be high in the sky towards zenith (noon). In another location it will be setting in the west. On the other side of the world, it will be night and thus below the horizon, invisible.

This can only be explained by a round Earth. If we all lived on a flat disc, the Sun would be at the same position for all people at the same time.

Observe The International Space Station

You can see the international space station circling the Earth once every 90 minutes or so as it orbits the Earth.

Now, this is not direct proof the Earth is round, since (leaving the science of gravitation and other physics aside) some will argue that things can orbit a planet of any shape — sphere, cube, disc, or turtle.

However, the images taken from the ISS and other satellites clearly show the Earth to be a sphere from every angle. From pictures taken from space, we can literally see the Earth revolve on its axis. Pictures taken at different times show different landmasses beneath as the orb revolves.

Watch Ships Sail Over The Horizon

If you live near the ocean, you can watch ships leave port and sail out to see. On a clear day, you can see very far indeed and — even with a telescope — you can watch the body of the ship slowly fall beneath the horizon leaving only its sails, mast, or top parts visible until it disappears entirely below the hoirizon.

In Conclusion — Earth is Round

In conclusion, the Earth is round. there is no shortage of proofs (including even more than those I listed here) that undeniably show the Earth to be round.

All you have to do is take a little time to learn the basic theory, and perform the observations yourself.

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