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.