It’s Professor Dave, let’s take some measurements. We just talked about a few of the observations
of the night sky that must have happened in every civilization throughout the world at
the dawn of human history. But science is much more than just observation. It is about what we do with these observations. We look for explanations. We make models, and predictions. We take measurements, and see how they hold
up to our predictions. If they don’t match, we revise the model
and try again. After many centuries of pure observation,
our approach to astronomy became more mathematical. Some of the earliest known scientific calculations
happened during the classical period of astronomy, in Ancient Greece and other contemporaneous
civilizations. What were some of these calculations, and
what did they tell us? First came the realization that the earth
is round. This idea first cropped up around the time
of Pythagoras, although at that time it was not really based on logic but rather on the
aesthetic beauty and perfection of the sphere, so it wasn’t very scientific. But just a bit later, with Aristotle, a more
logical approach arose. He noticed that during a lunar eclipse, the
shadow cast on the moon by the earth has a curved edge. This is representative of earth’s spherical
shape. It was also realized that the stars that are
visible in the night sky depended entirely on one’s location in the world. Moving north to south, a completely new set
of stars becomes visible, and all the familiar ones vanish from sight. This is easily explained with a round earth,
as the other half of space that surrounds the earth is only visible to the other half
of the earth. Once it was determined that the earth is spherical,
the next logical step was to attempt to measure the dimensions of the sphere. Eratosthenes was the first to do this, and
with impressive accuracy. He reasoned that when the sun is directly
overhead one object, it must cast a shadow on some other object sufficiently far away. He used a well in one part of Egypt, and an
obelisk in another part of Egypt to take some measurements. At noon on the summer solstice, the sun shone
directly down the well, illuminating the very bottom. Simultaneously, the sun cast a shadow on the
obelisk in Alexandria revealing that the sun was seven degrees off the vertical. Now let’s draw a line from the bottom of
the well to the center of the earth, and another one back up to the base of the obelisk. By simple geometry, we can see that the angle
of this sector is seven degrees, which means that this distance represents a little less
than a fiftieth of the way around the world. The distance between these locations was known
to be five thousand stadia, so we can use a simple ratio to deduce that the circumference
is around 250 thousand stadia, which is around 25 thousand miles. Given that in ancient times, the only tool
available was the naked eye, these are all demonstrations that you can reproduce yourself,
in case you’re curious to try. Now that we are all set with the earth, what
about the distances to other objects in the sky? And how big are those? Incredibly, we were able to deduce some of
these quantities as well. Take the moon for example. Around the time of Eratosthenes, another Greek
named Aristarchus did some similar work. He looked at the shadow of the earth on the
moon during a lunar eclipse and by comparing the curvature of the shadow and the moon itself,
he deduced that the moon must have a diameter around one third that of earth. He also made estimates regarding the relative
distances to the moon and sun, and although those were not as correct as his other work,
he was the first to suggest that the sun is much larger than the earth, and even proposed
that the earth goes around the sun. There was not sufficient evidence for this
idea at the time, so the geocentric model with the rotating celestial sphere reigned
supreme for many more centuries. Eventually, we did correct this oversight,
and as this was one of the defining paradigm shifts in the history of astronomy, let’s
move forward and talk about that next.
