The autumnal equinox is an instant in time — late yesterday afternoon — when the angle of the earth’s poles lines up exactly square to the sun so that exactly half the earth is in light and the other half in dark. It’s also the day when the advancing night catches up with the retreating day, so that the night and day split the 24 hours evenly, more or less. From now on, in the northern hemisphere, the night races ahead, and sunset comes earlier each day. It’s the conventional end of summer in the north. It’s the beginning of spring and the promise of summer in the southern hemisphere. All this and more formed the material for Tory Brady’s talk at the Cesar Chavez Memorial Solar Calendar in the park shortly before sunset yesterday. Using a big globe with a toothpick to mark our location, Brady — an amateur astronomer and retired San Francisco Exploratorium teacher — demonstrated the apparent movement of the sun from its low incline during winter to its high post in midsummer, and showed how the tilt of the earth in its revolution around the sun produced the various seasons experienced at different latitudes of our blue globe. The site of her talk, attended by a select group of aficionados, had roots in farmworker leader Cesar Chavez’ deep ties to agriculture and the rhythms of the earth. The site, curated by Santiago Casal, hosts celebrations at each of the equinoxes and solstices.