The equinox, when the sun appears directly over the equator so that night and day is approximately equal all over the planet, happened Thursday. Today comes the consequence to the earth continuing in its orbit as the south pole appears to point farther away from the sun, as it's done since the last solstice.
In just about two hours, at 17:09 UTC, the sun sets on the Amundsen-Scott South Pole Station, and doesn't rise again until around 01:18 UTC on September 20th.
Good luck to the over-winter crew at Amundsen-Scott. Enjoy the long night.