JACKSON, Wyo. — The sun will set on the summer season one last time, Friday, Sept. 22.
The autumnal equinox is at 12:50 a.m. on Sept. 23, which marks the first astronomical day of fall in the northern hemisphere and the first day of spring in the southern hemisphere.
According to the Old Farmer’s Almanac, an equinox describes when the sun’s center crosses the celestial equator—earth’s imaginary extension of the equator line into space. When the sun crosses the equator going from north to south, this marks the autumnal equinox in the northern hemisphere; when it crosses from south to north, this marks the vernal equinox.
Across the world, this happens at the exact same time but the hour and day differ depending on the time zone, so the equinox falls on Sept. 22 or Sept. 23.
What this all means is that after the equinox, days become shorter than nights because the sun continues to rise later and nightfall arrives earlier, until the winter equinox. The arc of the sun will slowly creep south, the leaves will begin to change and the temperatures will begin to dip.
While astronomically fall begins tomorrow, snow has already dusted the Tetons, and more is on the way. According to Buckrail Meteorologist Alan Smith, Areas above 9,000 feet could see anywhere from a dusting to a few inches of snow from Friday evening through Saturday morning, with light dustings possible as low as 8,000 feet. Moving up in elevation, several inches of snow will be possible above 10,000 feet. A dry warm pattern is expected to settle in next week, with highs in the low 70s on Monday.