The curious case of the start of fall is one that might take a second to understand. Residents of the Lower 48 are celebrating all things fall Monday, Sept. 23 but Alaskans enjoyed the first day of fall a day earlier. 

Fall officially started at 11:50 p.m. Alaska Daylight Time (AKDT) Sunday night, making it the first day of fall for the 49th State. But since the change of seasons happens at the same moment worldwide, what was Sunday night for Alaska was Monday for just about the rest of the world. 

Confused yet? Let's break it down.

Fall officially starts when the sun passes directly over the equator; this is called the equinox. The event happens twice a year — once to mark the start of spring and another time in the beginning of fall. Because the start date and time depend on when the sun passes over the equator, the exact time and date of the season change varies from year to year. 

Not only can the date change annually, the start date can vary around the globe depending on when the sun crosses the equator. For example, 11:50 p.m. in Alaska is 3:50 a.m. the next day on the East Coast of the U.S. and 7:50 a.m. in Greenwich Mean Time. 

The exact start time of fall this year meant it started late Sunday in Hawaii and Alaska and early Monday morning in the Lower 48. 

Since the sun crossed the equator at 11:50 p.m. AKDT Sunday, the season change occurred early Monday morning at corresponding times in the Lower 48. The Pacific time zone welcomed fall at 12:50 a.m. Monday morning and those in the Central time zone saw fall at 2:50 in the morning. 

So when all of your friends start posting on social media about the changing season, remind them — it started earlier in the Last Frontier.

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