It’s Summertime Already

as of 4:51 p.m. EDT

Revelers gather at the ancient stone circle of Stonehenge to celebrate the summer solstice near Salisbury, England, on June 21, 2023. (Kin Cheung/AP)

First day of summer: June 20 solstice marks the longest day of the year – The Washington Post

Summer is arriving a bit earlier than usual this year, and not just because a major heat wave is baking the eastern United States and Canada this week. Thursday’s summer solstice — the longest day and shortest night of the year in the Northern Hemisphere — is the earliest in 228 years, since 1796, when George Washington was president.

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The main reason for the early solstice is that human calendars aren’t perfect. While a normal year (or non-leap year) has 365 days, Earth’s orbit around the sun each year is not exactly that long. On average, it takes about 365 days, 5 hours and 49 minutes — or 365.24219 days, according to timeanddate.com. To account for that extra quarter-day, the Gregorian calendar we adopted in the late 16th century adds one extra day (Feb. 29) to the calendar every four years. This is why we have leap years that are 366 days long.

Over time, this means that the solstices and equinoxes drift earlier by about 45 minutes every four years, or about three-quarters of a day per century. By 2096, the solstice will occur at its earliest time of this century.

To correct this discrepancy, the Gregorian calendar introduced a special rule: Any year divisible by four is a leap year, except for century years, which can only be leap years if they are divisible by 400. Therefore, 1600 and 2000 were leap years, while 1700, 1800, 1900 and 2100 are not.

Skipping three leap years every 400 years means the average length of the Gregorian calendar we use is 365.2425 days instead of 365.25 days. Those extra decimal places may not sound like a lot, but they keep our calendar in sync with the actual time it takes Earth to orbit the sun (365.24219 days).

Without this calibration, the solstices would move earlier indefinitely. Instead, the solstices (and equinoxes) will arrive earlier with each passing leap year until the end of the 21st century. However, the cycle will eventually reset. Because 2100 is not a leap year, the time of the solstice will start to drift later again.

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