The solstice marks the point in the year when the Earth’s north pole is tilted furthest towards the sun—at an angle of around 23.4 degrees.
On the June solstice, the sun will be farthest north in the northern hemisphere sky, appearing directly over the Tropic of Cancer, and will take the longest route between rising and setting. This means that the day will have the most hours of sunlight and the shortest night of 2020 in the northern hemisphere.
In the southern hemisphere, on the other hand, the June solstice will be the shortest day of the day, with the longest night.
While the term “summer solstice” is often used to refer to the longest day of the year, technically, this astronomical phenomenon occurs at a specific moment in time when the sun is directly above the Tropic of Cancer. In 2020, this will occur on June 20 at 9:43 p.m. UTC, or 5:43 p.m. EDT.
In astrological terms, the June 2020 solstice marks the beginning of summer in the northern hemisphere, which will last until the September 22 equinox. Conversely, it marks the beginning of astronomical winter in the southern hemisphere.
However, in meteorological terms summer in the northern hemisphere actually started on June 1 and ends on August 31. This is because meteorologists divide up the seasons based on annual temperature cycles instead of the orbit of the Earth around the sun.
Solstices occur because the Earth is titled on its axis by around 23.4 degrees relive to the plane of its orbit. As a result, both hemispheres spend six months of the year tilted towards the sun, and the other half tilted away.
“We live on a planet with a rotational axis that is tilted. And what that means is that it’s pointing to more or less the same place in the sky all year—not far from the pole star in the northern hemisphere,” Robert Massey, Deputy Executive Director of the Royal Astronomical Society in the U.K., told Newsweek.
This results in the experience of seasons on Earth. When the northern hemisphere is tilted toward the sun, for example, it receives radiation at a more direct angle, leading to warmer temperatures and the summer season. In winter, the reverse is true.
This effect becomes less and less noticeable as you move closer to the equator. At the equator, the sun’s rays are relatively constant throughout the year, hence why there is no true summer or winter in these regions.
On the other hand, the effect of the solstice is most extreme in the polar regions. On June 20, the Arctic Circle will experience 24 hours of total sunlight, while the Antarctic circle will experience 24 hours of darkness.
Depending on the year, the summer solstice can fall anywhere between June 20 and 22, although a June 22 solstice is rare, with the last one occurring in 1975, and the next scheduled for 2203.
The reason that the solstice does not always occur on the same day comes down to a slight discrepancy between the Gregorian calendar—the most widely used calendar system in the world—and the “solar year.”
The solar year is the length of time it takes for the Earth to complete one revolution around the sun, which is almost exactly 365 days, 5 hours, 48 minutes and 45 seconds. The Gregorian calendar, meanwhile, simply defines the year as 365 days long, meaning it is slightly out of sync with the solar year.
An extra day is added to the year every four years, what’s known as a leap year, to make sure that the calendar is synchronized with the seasons.
