Today, June 20, 2016 marks the summer solstice, or the longest day of the year in the northern hemisphere. The sun’s rays are directly over the Tropic of Cancer, right at 23.5° N, at 6:34 pm ET.
Depending on where you are, there could be about 12 hours between sunrise and sunset closer to the equator or up to 24 hours in the Arctic Circle.
This extra daylight results from the fact that our planet spins on an axis of about 23.5°. During the year as the Earth travels around the sun, a larger portion of the planet is tilted closer to or farther away from the sun, which is why we have the four seasons.
The southern hemisphere experiences the opposite seasons. Today, countries below the equator will receive the least amount of sunlight (it’s a long night on the South Pole). Conversely, around December 20 this year at winter solstice, the northern hemisphere will receive the least amount of sunlight, when the southern gets the most. This change coincides with when the sun’s rays hit directly over the Tropic of Capricorn, which is at 23.5° South. (The area between 23.5°N and 23.5°S is, not surprisingly, called the tropics.)
Interestingly, the summer solstice doesn’t coincide with the earliest sunrise or the latest sunset. According to the Washington Post, this is because the sun’s position in the sky, called a solar day, doesn’t always match up with a 24-hour cycle. A solar day measures the time it takes for the sun to reach its highest point in the sky one day to the next, which can vary as much can be up to 30 seconds off from 24 hours; depending on where you are, sunrise and sunsets can vary even more. So today, although the difference between the sunrise and the sunset is the greatest, it doesn’t mean that tonight’s sunset is the latest.
Nor does is correspond with the hottest day of the summer, though it is regarded as the start of the season. In the northern hemisphere, we can expect those days to hit once the oceans have finally warmed up after winter (around July in the continental US).
Pagans celebrate the day as Litha in a number of ways, including a massive celebration at Stonehenge, where the sun is aligned with the massive rock construction. This year, the summer solstice also falls in the middle of Ramadan, a Muslim holiday, marked by a month of fasting from early morning twilight to sunset, and is calculated by the lunar calendar. Because of the timing, fasts this year lasts a really long time; today in the mid-Atlantic region of the US it goes for almost 17 hours. (Muslims living in the northern most parts of the world without a proper sunset can use this system to calculate when they should eat.)
After today, days in the northern hemisphere will continue to get shorter and shorter through the winter solstice in December. Fortunately, the changes in daylight occur gradually; there are plenty of long days and good weather ahead.