The Longest Day, which is incidentally also a classic 1962 film on World War II, is a literal description of how the day will turn out today — sunlight will seem to last forever and nighttime will be shorter than ever if you live north of Earth's equator. It is the summer solstice, traditionally considered the first day of summer — although daily temperatures suggest summer began months ago.
The day is a big deal in many places around the world. In the UK, thousands watched the sun glint over the horizon at Stonehenge early on Thursday, celebrating the summer solstice at the Neolithic stone circle. The sun rose behind the Heel Stone, which traditionally marks the spot on the horizon for the sunrise, at 4.52 am.
Crowds cheered and raised mobile phones for images as the rays flooded through the monument and announced the longest day of the year in the Northern Hemisphere. English Heritage, which cares for historic sites, tweeted that it was the "perfect morning for the #SummerSolstice sunrise at Stonehenge."
— English Heritage (@EnglishHeritage) June 21, 2018
In Mumbai, the sun rose at 6.02 am and will set at 7.19 pm on Thursday. At the other end of the country, the sun rose today at 4.23 am in Kohima and will set shortly at 6.08 pm, offering over 13 hours of sunlight.
But why do we have a summer solstice? The simplest answer, perhaps, is because the Earth rotates on a tilted axis.
21 June (the date shifts to 20 or 22 June on some years as well) marks the exact day when the Sun’s rays reach as far north as is possible, appearing straight overhead along the Tropic of Cancer, which is at 23.5 degrees north latitude. The Northern Hemisphere, hence, sees the sun take its highest and longest path through the sky.
By contrast, the Southern Hemisphere observes peak sunlight on 21, 22, or 23 December and the north hits peak darkness — which is known as the winter solstice.
However, just because the north is observing the summer solstice today, it does not mean the entire hemisphere will witness the latest sunset. That may happen within a span of a few days.