June 21. This day is know it as the longest day of the year. But how exactly does that work?
On this day, the earth’s “circle of illumination” will be from the Arctic Circle on the far side of the earth (in relation to the sun) to the Antarctic Circle on the near side of the earth. The equator receives twelve hours of daylight, there’s 24 hours of daylight at the North Pole and areas north of 66°30′ N, and there’s 24 hours of darkness at the South Pole and areas south of 66°30′ S.
June 20-21 is start of summer in the Northern Hemisphere but simultaneously the start of winter in the Southern Hemisphere. It’s also the longest day of sunlight for places in the Northern Hemisphere and the shortest day for cities south of the equator.
However, June 20-21 is not the day when the sun rises earliest in the morning nor when it sets latest at night. As we’ll see – the date of earliest sunrise or sunset varies from location to location.
We’ll start our tour of the solstice in the north, with Anchorage, Alaska and head south in the U.S. and then move on to international cities. It’s interesting to compare the difference in sunrise and sunset in various locales around the globe.
Here’s the science, put simply:
Start by imagining the Earth circling around the sun. Now picture a vertical line running through the planet from the North Pole to the South Pole. That’s the Earth’s axis, the line we rotate around once each day.
The official solstice, when the sun is directly above the Tropic of Cancer, will take place at 9:39 a.m. on the West Coast of the U.S. and 12:39 p.m. on the East Coast. But all you really need to think about is how you’re going to spend the longest day of the year.