How do you know what you know?
What is the shape of the Earth?
Earth's shadow is always circular...
Ship's masts are the first to be seen and the last to disappear...
All other planets and moons are spherical...
Then there was a man named Eratosthenes who noticed that different parts of Earth had different shadows at the same time...
This was that man. He not only knew that the world was spherical, but exactly how large the world was thanks to a simple ratio that we will use as well when we recreate his experiment.
We have traveled around the Earth completely without falling off...
We have seen it from outer space...
...so I have seen enough evidence to convince me.
With more precise measurements, we get this:
An oblate spheroid
How big is the Earth?
Let's do what Eratosthenes did...
What is the earth made of?
What is this ball of rock, water, and nitrogen doing?
Does it sit there in space?
Rotation is spinning on our axis. It takes 24 hours to do it.
Revolution is traveling around the sun. It takes roughly 365.25 days to do it.
But how do we know?
This is called the Coriolus Effect and proves that the Earth is rotating.
Night and day can be explained away, but not the Coriolus Effect.
Coriolus Effect - the deflection of loosely held fluids on a spinning surface.
Ours deflects clockwise in the Northern hemisphere and counterclockwise in the Southern due to the direction of rotation.
Time zones are further proof of rotation. As is the movement of every object in space across the sky at the exact same rate (15 degrees per hour). The simplest explaination is that they are relatively stationary and we are spinning at that rate.
Paralax explains the apparent movement of stars in space as viewed at different times of the year.
That's called paralax and is the result of viewing from two different points, in other words the Earth must move around the sun in space.
Paralax - the apparent movement of near objects relative to distant objects due to the objects being viewed from two different locations.
The seasons are caused by the uneven heating of the Earth as a result of our tilt as we revolve around the sun.
Make note of the solstices and equinoxes. Our tilt is 23 and a half degrees. That number shows up again and again (tropics of Cancer and Capricorn, Arctic and Antarctic circles).
Seasons could be explained away, but are further evidence of revolution.
So Earth is a big spheroid object made of rocks and water and air rotating and revolving in space, but why?
How did it get this way?
Let's look at how planets are formed to illuminate this:
Once all that gas and dust settled into a spheroid, early earth would have looked something like this:
With no atmosphere to burn up meteoroids and so many flying around, we were riddled with craters.
But all that bumping of molecules generated a lot of heat and so beneath that calm, rocky surface, rocks were melting and getting hot.
As you know heat rises:
Volcanoes cropped up everywhere.
You will notice in there that there is water vapor. This condensed into clouds and rained down in thunderstorms that lasted thousands of years.
There was no ozone layer for absorbing harmful, mutating UV rays from the sun.
It was an energetic place.
But over time the Earth cooled off and the meteors died down (fewer particles left flying through space as well as an atmosphere to burn up in), organisms developed that produced oxygen and soon an ozone layer was present; things calmed down to where they are today, but it took a long time to get here.
How long, you ask?
Here is a quick glimpse: