Looking at nature with open eyes, observing the motion of the Sun across the sky will reveal the nature of things very quickly.
When the Sun rises or sets the furthest to the south, it is winter solstice. If it's furthest to the north, it's summer solstice.
Measuring the difference between the maximum altitude of the Sun reached on the days of the solstices gives twice the tilt of the Earth's equator, and hence the tilt of the Earth's axis of rotation, and so forth.
You don't need satellites or space travel for that. So it's no surprise at all that this was known very early to humans. --Martin Gotz - Physics Department, Brown University, Providence, RI