The Earth spins on an axis, which makes our days. We also rotate around the sun, which makes years. The axis is tilted, which makes our seasons.

As we rotate, the axis tilts one way or another in relation to the sun. Winter is when the north axis is pointed away from the sun, and summer is when it's tilted toward the sun.

But equinox occurs when it's pointed neither toward nor away from the sun. Our equator then lines up with the center of the sun for a day of balance.

As we change seasons, we get more sunlight until the summer solstice, the day of the year with the most daylight. Remarkably, centuries before telescopes or devices to measure this rotation or tilt, people knew something was up.

You can see this in person if you visit the Spiro Mounds Archeological Center in eastern Oklahoma. It's the only pre-historic American Indian site in Oklahoma that's open to the public.

Thursday and Friday, visitors can take a guided tour of the mounds and see a sunset that lines up with the earthen mounds created by the people who lived there as a permanent settlement starting around 800 A.D.