A place I've been gives up a secret

In Ohio, there's a mound built by ancient Native Americans, in the form of a serpent. Years ago in my distant youth, my family and I visited there.

Here's a view of it from up in the sky.

It's not a burial mound, so it's purpose is not to commemorate members of the ancient tribes. Rather, it's an agricultural indicator, with the head of the serpent and the orb in its mouth aligned to indicate the occurrence of the summer solstice -- the alignment points toward the setting sun on the day of the solstice.  According to numerous sources, for a successful corn crop, plant on the day of the solstice.

So what secret did the Science Channel find?  You'll have to watch the video for that revelation.

What is the purpose of this Serpent Mound?