Answer Man

Thanks to an odd orbit and weird time zones, the first day of summer isn’t always the same

Chicago Tribune Service

Q: Next year, my husband and I will celebrate our 58th wedding anniversary. For 57 years, we always have celebrated our anniversary on June 21, the first day of summer and the longest day of the year. I have kept calendars for the last 25 years or more, and on all of these calendars, June 21 is listed as the first day of summer. So why do all of my new 2016 calendars list the first day of summer as June 20? Whose bright idea was it to change this date after all these years? The first day of spring, fall and winter are all the same.

Clifford and Judy Gruner, of Highland

A: Dare I risk ruining a half-century of marital contentment by suggesting that you two lovebirds have been laboring under the misconception that June 21 always heralds the start of summer?

Well, hoping you agree that ignorance is seldom bliss, let me put it to you straight: Because of our less than perfect calendar, summer can begin not only on June 21 but June 20 and even June 22. During your own marriage, summer in St. Louis arrived on June 20 seven times. But because of the system of time zones we use on our planet, you simply didn’t realize it. Let me try to explain:

The biggest problem may be that you think the earth makes a complete orbit around the sun every 365 days. So, you figure, if the planet is at a certain point in that orbit today, it will be at that same point at the same time on Dec. 26, 2016. Therefore, the summer solstice should be at the exact same time each year. What could be simpler than that?

In reality, it’s not that simple at all. It actually takes the Earth about 365 1/4 days to orbit the sun — or, as “Star Trek’s” Mr. Spock would tell you, 365 days, 5 hours, 49 minutes and 48 seconds to be exact. So it requires nearly an additional six hours to arrive at the same point in our orbit from year to year. As a result, the summer solstice gets pushed back about six hours every year, too.

Of course, Pope Gregory XIII tried to correct for this with his new calendar in 1582 by adding one extra leap day almost every four years. By doing this, we at least keep the timing of the summer solstice within a reasonably narrow time frame rather than watching it drift to a date earlier and earlier in June as it had under the old calendar introduced by Roman emperor Julius Caesar. Still, even our calendar is poor at taking our odd orbital time into account. Therefore, because of this constant fudging to make the calendar agree with Mother Nature, the start of summer still can fall within a three-day window.

Solstice, by the way, combines the Latin words “sol” for “sun” and “sistere,” which means “to come to a stop or stand still.” At the time of the summer solstice, the sun reaches its northernmost position as seen from the Earth. At that moment, it does not move north or south as it would on almost every other day of the year but stands still before it starts to decline to the south again. The opposite occurs at the winter solstice.

As you note, this usually happens on June 21. In 2013, St. Louisans greeted the arrival of summer at just past midnight — 12:05 a.m. In 2014, it was 5:52 a.m. — or nearly six hours later, as I pointed out earlier. This year, it was later again by about six hours —11:39 a.m.

Following this pattern, you’d expect the solstice in 2016 to occur about 6 p.m. on June 21. But guess what? It’s a leap year, so they’re throwing an extra 24 hours into the year. In return, the solstice gets shoved back an entire day — to 5:35 p.m. on June 20. Even in 2017, summer will start here at 11:25 p.m. on June 20 before moving back to June 21 for 2018 and 2019.

The solstice occurring on June 20 has not been as rare during your marriage as you have come to believe. Since 1988, it has occurred on June 20 during every leap year — seven times in all.

So why have all of your calendars told you it was June 21? Simple — it’s because of the difference in time zones. Remember that the summer solstice occurs at the exact same moment for everyone on Earth whether it’s midday where they are or the dead of night. But the calendar you hang on the wall can’t take every local time zone into account, so it uses UTC (the coordinated universal time), which is similar to Greenwich (England) Mean Time, as its standard.

As it turns out, UTC is five hours ahead of Central Daylight Time in St. Louis, so if the summer solstice occurs here anytime after about 7 p.m. on June 20, it’s already June 21 in the UTC region — and that’s the day you’ll find on your calendar. But I’ll bet if you check your 2012 calendar, you’ll find that summer started on June 20 because that was the first time since 1896 that the solstice occurred on June 20 in the UTC zone. Starting next year, you can expect it to happen much more regularly — 12 times over the next 34 years, in fact. Conversely, there hasn’t been a summer solstice UTC on June 22 since 1975 nor will there be another until 2203. St. Louis hasn’t seen one since 1951.

Not surprisingly, the spring and fall equinoxes as well as the winter solstice have similar two- and three-day windows. In 1931, for example, autumn did not start until after midnight on Sept. 24 UTC. You can find all the dates and times for all the seasons in all time zones from Year 1 to 2099 at www.timeanddate.com/calendar/seasons.html.

Today’s trivia

True or false: The Earth is actually at its most distant point from the sun during our torrid summers.

Answer to Friday’s trivia: In 1974, Canadian postal workers noticed that letters addressed to Santa were being classified as undeliverable, so, rather than disappoint the young writers, the workers began answering the letters themselves. Volume soon increased exponentially, prompting Canada Post to set up an official Santa Claus letter-response program in 1983. Now, children are encouraged to write Santa in care of the North Pole, Canada. St. Nick has even been given his own personal postal code — H0H 0H0, of course.

Roger Schlueter: 618-239-2465, @RogerAnswer

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