There’s been a recent internet craze over rapper B.o.B. who claims that the Earth is actually flat. He posts lots of “evidence” (I couldn’t put enough quotation marks around that word if I tried) on his twitter account, coming up with rather comical phrases such as ‘Once you go flat, you never go back’. It has escalated to the point where Neil Degrasse Tyson felt it necessary to have a bit of a rant, which includes a sharp tangential comment at Donald Trump. Tyson is one of my favourite science popularisers and writers (read Death by black hole, it’s great), and he’s spot on. You go and demonstrate how gravity works, Neil!
However, Tyson intends to fight fire with fire, which may not always be the best approach, although, it’s hilarious to see. I’d prefer to settle the question with a bit of armchair philosophy because, as it turns out, one can easily reach the conclusion that the Earth isn’t flat while doing next to no scientific observations. Personally I believe that B.o.B. has started this media craze to get worldwide attention with the oldest trick in the book: shock people. Hearing someone proclaim that they believe the Earth is flat is shocking to us, because it seems so obvious that it isn’t. And it seems so obvious because it is actually quite obvious that the Earth isn’t flat.
Let us recline and put on our thinking caps and try to discover the truth with (quite literally) day-to-day observations. The only other thing we need to consider is time. Time zones, to be precise. Time zones exist for a very simple reason, the sun doesn’t come up everywhere at the same time. In fact, we know that it may be night in one place and dark in another. Many people will have experienced time zones first-hand when travelling.
Even in the United States alone, there are four different time zones. In Europe, there’s three. Flying any significant distance from the west to east, if you were to bring a watch on your trip, you might notice that sunset is actually shifted a couple of hours.
If the Earth were flat, there’d be no such thing. A flat Earth would have only one single time zone, the sun would come up and set at exactly the same time anywhere on Earth. In fact, crudely speaking, that’s how time zones are defined, a strip of land where the sunrise and sunset appears to be at the same time (it’s actually slightly more complicated and involves measuring on a special date called the equinox).
The fact that we on Earth feel the need to have time zones tells us the Earth is not flat in the east-west direction. We haven’t yet reasoned the Earth to be a sphere, but it’s certainly not flat. As far as B.o.B. is concerned, we’re done. And we didn’t even need to get up out of our chair.
Incidentally, we could also do the same thing, imagining travelling north or south and discovering that the time of sunset changes. For instance, travelling north on the northern hemisphere during the winter would result in fewer daylight hours. On the poles, the Sun doesn’t even come up during winter time (just go see March of the Penguins). The same argument applies as before and we are forced to conclude that the earth is also curved in the north-south direction.
Besides from the fact that a flat Earth is pretty much in contradiction with almost every known scientific fact, it is also very easily disproved. It’s so deep in our collective consciousness that it’s actually shocking to hear some people believe the Earth is flat. Even some five centuries ago, Galileo Galilei thought it was quite obvious, citing tides as his proof. It’s basically the same argument as I have presented, but using the Moon instead of the Sun and gravity instead of light. The gravity of the Moon causes tides, with high tide and low tide occurring at different times at different places.
The argument I’ve put forward is generally valid for any roughly spherical celestial body. Consider for instance the Moon, look long enough and you’d discover it goes through phases. As a celestial body moves with respect to the Sun, the light shining on that body causes shadows to form. However, from our perspective on Earth, the shadow the Moon casts on itself changes shape, as we see different configurations of the Moon lit by the Sun.
From the phases, we can directly observe the Moon being a sphere, as no other shape would cast such shadows on itself. The human eye is marvellous at inferring the shape of an object based on its shadow, and any five year old will tell you the Moon is a sphere.
My earlier argument about the shape of the Earth was exactly the same, except for the small difference that we look at the celestial body we are on ourselves. The shadow cast by the celestial body you happen to be on is called night and edge of that shadow sunset. The time of sunset differing from place to place is a measure for where the shadow runs that the Earth casts on itself as it’s being lit by the Sun. We might also turn to picture evidence from, for instance, the Apollo program. Looking at several different pictures of Earth from the Moon reveals those exact same phases, but then of Earth. Although, I guess it’s not too much of a stretch for B.o.B. to deny the Moon landings as well.
So far, we’ve been able to conclude that the Earth and Moon are roughly spherical. We didn’t need any fancy science equipment or anything, just a watch and a pair of eyes, but you could conclude the same with even less equipment. It has literally been known for thousands of years that the Earth is round. The ancient Greeks already knew, with Pythagoras sometimes cited as the first to state it. Aristotle also pointed out that constellations seem to rise higher in the night sky as you go south on the northern hemisphere, as well as the fact that the shadow of the Earth on the Moon during a Solar eclipse is always round. According to him, and anyone with half a mind, two very convincing observations.
Hence, it’s been widely known that the Earth is round for over two millennia. However, the first person to compute the radius of the Earth probably was Eratosthenes around 240 BC. And he didn’t do it using a watch, as back in the day there was no reliable way to tell time. He measured the length of the shadow cast by a vertical stick in two different places on Earth on the same date. And he wasn’t that for off either, we don’t know exactly how the units he used translate into modern day units, but he might have been only 2% off, which is completely amazing!
Since the Earth was known to be round, it seems reasonable to suppose that the other planets are round as well. Galileo Galilei proved this notion after he constructed his own telescope and used it to observe the phases of the planets. However, Galilei’s observations weren’t meant to show that all the planets are round, everyone kinda knew that already. They were important for another reason: the Heliocentric solar system. Sure enough, today it is also common knowledge that the Earth revolves around the Sun, as presumably even B.o.B. would admit.
So unfortunately for B.o.B., the Earth is round, and so are all the planets, moons and the Sun. As a matter of fact, any celestial body of sufficiently large mass is round, due to gravity. But perhaps we should instead say fortunately for B.o.B. as he managed to shock millions and thereby become a huge internet phenomenon. I guess we’ll never know if he really believes it or if he’s just trying to increase his fame by any means necessary.