This year, summer solstice-- the longest day of the year in the Northern Hemisphere, falls on June 21, 2017. The day marks the change of season from spring to summer in the Northern Hemisphere and from autumn to winter in the Southern Hemisphere.
The first day of summer officially kicks off on June 20th or 21st each year. It is not fixed because of the difference between the human calendar year of 365 days—and the astronomical year- of 365.25 days.
Did you know solstice is derived from two Latin roots – ‘sol’ for sun and ‘sistere’ to stand still? The Vernal Equinox is the start of spring, when day and night are roughly equal. From there on, the noon day sun rises higher and higher in the sky each successive day until summer solstice, when it appears to almost stand still.
Summer solstice is a result of the Earth's tilted axis (at 23.5 degrees) relative to the sun. As the Earth revolves around the sun, the North Pole is tipped closer to the sun on this day than on any other day in the year. This bathes the Northern Hemisphere with sunlight (as shown in the picture), causing summer.
On Summer Solstice different parts of the earth receive different amounts of daylight. North Pole and the Arctic circle receive 24 hours daylight, the Sun is directly over the Tropic of Cancer at noon and the South Pole does not have a shred of light for 24 hours.
Is this the hottest day of summer? No. Earth's oceans and atmosphere act like heat sinks, absorbing and radiating sun's rays over time. Like an oven which takes a long while to heat up and a while to cool, the sunlight absorbed on summer solstice takes several weeks to be released. Hottest days of summer usually occur in July or August.
It is a time for celebration in the Northern Hemisphere. During ancient times, the beginning of summer was always a joyous occasion signifying a new beginning. With snow thawed, and leaves green, food was easier to find and crops could be planted. Hundreds of groups celebrated it with religious festivities.
Ancient Swedes decorated trees, Egyptians marked the beginning of the New Year coinciding with the flooding of the Nile, Celtics and Latvians lit bonfires, North American Indians perform Sundance and other rituals to honor rain and fertility and the Chinese honored the Goddess of Light - Li. The pagans marked solstice as midsummer to note the sun’s decline from that point on through winter. Christians mark this time as St. John’s Eve.
Even today, the biggest celebration of summer solstice happens each year at Stonehenge on the outskirts of London. Thousands of New Age followers gather to dance, play music and watch the sun rise. Every year unfailingly, on summer solstice, the sun rises over the Heel Stone, which stands just outside Stonehenge's stone circles.
For the folks in the Southern Hemisphere it marks the shortest day of the year. Now, you can measure the Earth's tilt yourself -- check out the fascinating video below!