How do we know that the Earth is spherical?

Hiya, Sean Lally Physics Guy here. Today on “How do we know that?” we are looking at the spherical Earth. So, how DO we know that the Earth is spherical (or close to it)?

So, you live on Earth – at least I presume that you do. That seems pretty reasonable. You’ve known that the Earth is a sphere (or close to it) for as long as you can remember. However, this isn’t really intuitive, is it? Look around you – it’s probably pretty flat where you are. Drive through Ohio or Iowa – yeesh, that’s some flat Earth! How do we know that the Earth actually isn’t flat?

Think about this for a moment – talk with a friend. If you had to prove to someone that the Earth was spherical, someone not from Earth perhaps (?!?), how could you convince him or her? What would be sufficient evidence?

Write down ways that would convince you that the Earth is spherical.

Now look at your list again, but back up 2500 years. Which of these would likely have been plausible then?

Find a partner and explain your choice(s) to him or her. Listen to your partner’s choices. Are these convincing arguments? Is your partner convinced by your reasoning?

Let’s see how your list compares to some of the classic reasoning by the leading thinker of 2350 years ago – Aristotle.

Aristotle, like most ancient Greek philosophers, believed that there were only 4 elements – earth, air, fire and water. Knowing now that there are well over 100 chemical elements, this seems a little kooky. However, the discovery of the chemical elements is a relatively recent thing – chemistry, as we know it, is less than 200 years old. It’s not even as old as the United States! It seemed reasonable to the ancients that all things could be categorized as one of the “big 4”, or some combination of these “elements.” Actually, if you think about if for a moment, you could probably convince yourself that this is pretty smart – or at least not totally ridiculous. Mind you, Aristotle certainly didn’t create the idea of a spherical Earth – his teacher Plato had the notion of spheres within spheres, and others (particularly the Pythagoreans) before both of these gentlemen had spherical theories.

Ancient Greeks (the so-called natural philosophers – predecessors of today’s scientists) believed that these elements would (if given the chance) arrange themselves according to their weights. In an absolute sense, Earth is heavier than water, which is heavier than air, which is heavier than fire. So naturally, things made of earth (which include most things within your reach right now) would tend to collect below the others. Since things can move from all directions, it was natural to imagine that things should collect at a center – that is, arrange themselves in a spherical orientation. (Actually, as we understand gravitation today, that’s not too far removed from how we believe stars and planets are formed.)

Aristotle went even further – he imagined the ideal scenario in which there was a sphere of earth, surrounded by a sphere of water and then spheres of air and fire. Think about – it’s not totally crazy. In fact, it’s somewhat intuitive. After all, if you put something heavy into a swimming pool, it usually sinks, right? (Of course, you can probably come up with objections to Aristotle’s logic, but that’s ok – we don’t pay much attention to Aristotle’s science anymore.)

Are there other reasons to convince someone that the Earth is spherical? Yes, and these are easier to understand and accept.

Imagine that you live near water – like so many early civilizations. Imagine watching boats go away from the shore. What would you see as the boat got further away from you?

Imagine climbing a mountain and staring off at the far away horizon. What do you think you would notice?

Finally, consider the phenomenon of eclipses. Maybe you have not yet seen a lunar or solar eclipse, but when you finally do – wow, they are awesome things to behold. Try to find some pictures of solar eclipses – that is when the Moon gets in the way of the Sun. This happens every year and a half or so, and is usually only visible from a very small portion of the Earth, so it’s likely that you, your family or friends may have never seen one before. Draw what you would imagine this would resemble.

Ask your family or teachers if they have ever seen a solar eclipse. Find some images online. The first thing you notice is the Moon is a dark circle across the Sun (with the Sun’s corona extending around the Moon). This isn’t too surprising – you see the Moon all the time and it certainly looks round, at least during the full Moon phases.

But now look at images of lunar eclipses – that is when the Earth comes between the Sun and the Moon. The shadow of the Earth creeps across the face of the Moon. You may have never seen one directly, but draw what you might expect to see.

Another experiment you could try would be to look at the shadow that a ball casts on a wall when placed in the path of a bright lamp.

Now find some images of lunar eclipses online. Try to find images that show the Earth’s shadow creeping across the surface of the Moon, rather than the time that the shadow completely covers the Moon (though this is interesting, too). What do you notice about the shadow?

What would a lunar eclipse resemble if the Earth were flat? How about if the Earth were a flat disc (as Samuel Rowbotham argued in the 1800s)?

Is this convincing evidence that the Earth is spherical? Can you imagine other arguments that might have worked 2500 years ago?

In any case, Aristotle’s writings convinced nearly everyone (if they could read, of course) and the Earth as a sphere became part of the accepted knowledge base that virtually all people had. Often, Christopher Columbus is given credit for having proven that the Earth is spherical – this is simply not the case. That idea is a myth of more recent origin.

One more thing – I’ve been using the word spherical pretty loosely. The Earth is, in fact, not spherical – it is bulged a bit (oblate) at the equator. The equatorial diameter is about 40 km (27 miles) longer than the polar diameter. Consider how that compares to a billiard ball.

According to the Billiard Congress of America,

16.16. Balls and Ball Rack
All balls must be composed of cast phenolic resin plastic and measure 2 ¼ (+.005) inches [5.715 cm (+ .127 mm)] in diameter.

40 km may seem like a large difference in diameter. However, by percent of diameter, the Earth is closer to a perfect sphere than the average billiard ball.

Wow! Anyway, that’s all for now – see ya soon, see ya on the Moon!

Further Reading

The Beginnings of Western Science
David C. Lindberg

This is about the best text on early science that you will ever find.

Flat Earth?
The wikipedia entry on the Flat Earth Society is certainly worth your time, as is a book by Christine Garwood, titled Flat Earth.

More on the Earth, spherical or flat:

Aristotle - On the Heavens
R. Dicks – Early Greek Astronomy to Aristotle
David Lindberg - Beginnings of Western Science
http://www.bca-pool.com/cgi/site/framegate.cgi?url=http://www.bca-pool.com/play/tournaments/rules/rls_gen.shtml&cat=p

ALL TEXT COPYRIGHT SEAN LALLY 2009