winter solstice

Figuring out the changing seasons, the tilt of the Earth's axis, the date of the solstice, etc. is _very_ simple. E.g. look at how the rising (or setting) point of the Sun changes during a year. 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.

Looking at nature with open eyes, observing the motion of the Sun across the sky will reveal these things very quickly. So it's no surprise at all that this was known very early to humans.

Martin Gotz - Physics Department
Brown University, Providence, RI