It's easy to figure out the rough length of the year. You can count, and you know roughly when it gets cold, and roughly when it gets hot. You notice that the Moon goes through phases, about once every 30 days. And hey, after about 12 cycles of the Moon, it goes from hot to cold and then back to hot. So how did they figure out exactly 365 days?

Let's say you build a house. You can walk outside and look at your cool house, and you notice that the sun casts a shadow on the ground. The top of your roof makes a shadow that goes right there. You're an inquisitive kind of guy. So let's say that you kind of keep an eye on how far out that shadow goes into your yard. Some days when it's too hot to do anything, you can just watch the shadow move across the grass.

So then, being the inquisitive kind of guy that you are, you notice that depending on the time of year, the shadow reaches a different part of your yard. It doesn't always go out as far. So one day you go out when the shadow is at its maximum distance, and you go poke a stick in the ground. You mark the farthest point that the shadow reached that day. The next day, the shadow doesn't reach quite as far. Almost, but not exactly.

Keep in mind that if you're lucky, you might not even have to poke a stick in the ground. Maybe on the day of the year when the shadow is at its very longest, it just happens to touch the base of a tree, or your fence line, or your barn.

So anyway, you just start counting the days until it touches that stick again. On the longest shadow day of the year, you find that it's 365 days from when it touches that stick, to the next day the shadow touches the stick. And that repeats, basically every single year.