Friday, June 22 , 2018, 12:13 pm | Fog/Mist 61º


Dennis Mammana: Summer Is Coming, All at Once on Different Days

View the seasonal sun paths after dark this week. Click to view larger
View the seasonal sun paths after dark this week. ( illustration)

It’s hard to believe it’s mid-June once again. That, of course, means a change of seasons is about to arrive in just a few days.

Do you know which day marks the beginning of summer this year? If you said June 21, you’re absolutely right. If you said June 20, well, you’re equally correct.

“Now, wait just a minute!” I can hear you saying. “How can that be? How can the onset of summer take place twice?”

It doesn’t. It occurs only once but on two separate days.

Before you suspect that I’ve either taken leave of my senses or I’m running for public office — or both — let me explain.

As the Earth orbits the sun, our tilted axis causes our sun to appear to drift northward and southward in our midday sky throughout the year. The official arrival of summer is marked by an astronomical event known as summer solstice, the exact moment when the sun reaches its northernmost point over our planet.

On this day, early morning sky watchers will notice that the sun rises farthest to the northeast, arching to its highest point around noontime and setting farthest to the northwest in the evening.

During this season, when the sun rides high across the daytime sky, it can be above the horizon for 16 or more hours, giving rise to much higher temperatures than at other times of the year.

The exact moment of the solstice, however, can occur only once. And it happens this year at 4:24 a.m. June 21. Problem solved, right?

Well, not really. The confusion comes because we live on a planet with 24 different time zones, and, unless I cite the time zone to which I’m referring, you have no idea when the event will occur where you live.

To keep things relatively simple, astronomers measure time by what we call Universal Time, or UT. That’s just another way of saying Greenwich mean time, or GMT. In other words, the time of the solstice I cited above is for Greenwich, England, where the Earth’s prime meridian is located.

Since Eastern Daylight Time, or EDT, is four hours behind UT, summer begins this year on the East Coast of the United States at 12:24 a.m. June 21. But on the western side of the continent, people using Pacific Standard Time, or PST, are seven hours behind UT, so, for them summer officially begins at 9:24 p.m. June 20.

It can be particularly confusing because some calendar makers — and even TV meteorologists — can sometimes cite dates of astronomical events without compensating for time zone differences. So, before believing what you read or hear, you might like to check these things out for yourself.

Click here to find the official dates and times for seasonal changes on the U.S. Naval Observatory.

Once you’ve got it all straight, consider that in the Earth’s Southern Hemisphere, the upcoming solstice marks the first day of winter. From the other side of the equator, viewers now see the sun moving quite low across their daytime sky.

For them, the onset of winter occurs on June 21. Or June 20.

Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at [email protected] and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.

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