There is a myth that the autumnal and vernal equinoxes have a special property which enables eggs and brooms to balance in a unique way. Today might be an opportunity to demonstrate how to debunk a myth. Students might also like to consider measuring a shadow (perpendicular to the ground), erecting a sun dial, or marking the point on the horizon where the sun rises in the morning and sets in the evening - something which can also be done using an app like "The Photographer's Ephemeris," or viewing a simulation on a website.
However, it was on a summer soltice that Eratosthenes calculated the circumference of the earth using some "simple" geometry (n.b. geo = earth and metry = measure). Thinking ahead a few months . . . I wonder if some classical Lutheran schools around the country (or the world?) might re-enact Eratosthenes' experiment on the next December solstice -- or perhaps the following June -- and communicate their findings via Skype . . .