I thought tomorrow was the first day of official fall. However, a quick web search turned up the information that the autumn equinox happens in the first couple of hours of Sept. 23. Thereafter, nights will be longer than days until some three weeks into March.
Translation: it happens on the 23rd at 8:20 UTC (Zulu time to the military). UTC runs 6 hours later than here in Mountain Daylight Time. Their 8:20 is our 2:20 a.m.
