Summer Solstice Around the World
A visitor scans the skyline and harbour of Sydney from a viewing platform at the Sydney Tower Eye in Sydney, Australia. (Eugene Tan/Hausmann Communications via Getty Images)
The longest day of the year will stretch over two days this year.
The summer solstice – the day on which the sun reaches its highest point in the sky and provides the year's longest daylight – is set to occur late Thursday night across much of the western U.S., followed by an early Friday arrival on the East Coast.
(To be exact, the summer solstice is set to occur at 10:04 PM Pacific Time and 11:04 PM Mountain Time on Thursday, and 12:04 AM Central Time and 1:04 AM Eastern Time on Friday.)
In the interview below, weather.com meteorologist Nick Wiltgen explains why:
Q: Why does the solstice fall at 1:04 AM? Why not midnight, or 6:00 AM or sunrise?
Over the course of the year, the sun appears directly overhead at midday at varying latitudes. In the spring and summer, those latitudes are north of the Equator, and in the fall and winter, those latitudes are south of the Equator.
The location where the sun is shining directly overhead is known as the "subsolar point." The June solstice occurs when the subsolar point reaches its northernmost position, at the Tropic of Cancer (just north of 23 degrees North latitude).
Q: Why is the solstice split into separate days for different time zones in the US?
The solstice is split into separate days for different time zones this time around simply because the solstice happens to occur so close to midnight. That puts the solstice in the wee hours of Friday morning for the Central and Eastern time zones, and in the late evening hours of Thursday for the Mountain and Pacific zones.
Q: How often does this split happen? When is the next time it will happen?
This split happens more often than you might think – for the Lower 48 alone, it happened 11 times from 2000 through 2012 (52 solstices and equinoxes). On three other occasions Hawaii's solstice or equinox happened on a different day from the U.S. mainland.
The next "split solstice" will not come until June 2017, when the summer solstice happens in the U.S. at 12:24 a.m. EST on the 21st. However, there are "split equinoxes" before then – autumn 2015 (Hawaii Sept. 22, the other 49 states Sept. 23) and spring 2016 (March 20 Eastern Time, and March 19 from the Central zone westward).
This all assumes, of course, that Daylight Saving Time rules won't change in the coming years.
Q: Who/what determines when the solstice will occur?
Anyone with the correct mathematical formula can calculate the solstice, but the U.S. Naval Observatory publishes a list of solstices and equinoxes for reference.
Q: Will this happen at the next (winter) solstice?
No. The next winter solstice (in the northern hemisphere) comes at 11 minutes past noon Eastern Standard Time on December 21, 2013.
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