The summer solstice of this year is in front of us: the 20th and 21st of June will be the longest days of this year for anyone that is living north of the equator. If pagan rituals are your thing, this will probably be a big moment for you. If not, the solstice is still pretty neat.
1. Anyway, why do we have a summer solstice?
A lot of people know this one. The Earth orbits around the sun on a tilted axis. It could be a result of a collision with some other massive object billions of years ago during the creation of the earth. So, between March and September, the Northern Hemisphere of our planet gets more exposure to direct sunlight over the course of the day. The rest days of the year, the Southern Hemisphere gets more. It is the reason for the four seasons:
So between March and September, the Northern Hemisphere of the Earth will get more exposure to direct sunlight during one day. The rest of the year, the Southern Hemisphere receives more. It’s the reason for the seasons: spring, summer, autumn, and winter. In the Northern Hemisphere, the “peak” sunlight often happens on the 20th, 21st, or 22nd of June of any year. That is the summer solstice. On the contrary, the Southern Hemisphere reaches peak sunlight on the 21st, 22nd, or 23rd of December, and the north hits peak darkness, that is our winter solstice.
2. How many hours of sunlight will we get on that day?
It depends on where we live, the further north you are the more sunlight you will see during the solstice. The Alaska-based climatologist Brian Brettschneider created a traffic guide shown below. If you live somewhere around the Arctic Circle, the sun never really sets down at a time of the solstice. Opposite of this, during the winter solstice, Fairbanks only gets about three hours of sunlight. Also, you should notice that the solstice gives us the longest twilight of the year, often 1 to 1.5 extra hours after sunset.
3. Will the solstice be the latest sunset of the year?
Not necessarily. Just because the 20th of June will be the longest day of the year for the Northern Hemisphere, it does not mean that every location has its earliest sunrise or latest sunset on that day. For instance, if you live in Washington DC, you can see the earliest sunrise, as it will occur on the 13th of June. Also, you can catch the latest sunset on the 27th of June. If you are sleep-lover, that means that day will probably be the most exciting one of the summer.
4. What is the connection with Stonehenge?
No one still knows why men built Stonehenge some 5,000 years ago. But, there is one possibility that it was used to mark solstices and equinox. That is why during the summer solstice, the sun rises just over the structure’s Heel Stone and hits the Altar Stone dead center. Nowadays, people still gather to pay homage to the summer solstice at Stonehenge; they are just using modern technology.
5. Is this the longest day in the entire history of our planet?
It is probably not, but it is close. This is also the reason why it is quite impressive. Joseph Stromberg did a fantastic deep dive into this topic for Vox several years ago. Ever since our planet has had liquid oceans and a moon, its rotation has been gradually slowing over time because of the tidal friction. That actually means that the days have been getting steadily longer. About 350 million years ago, it took only 23 hours, but today, of course, it takes 24. And the days are going to get longer gradually.
But, the tidal friction is not the only thing that affects the rotation of Earth – there is also the melting of glacial ice, which has been happening since the end of the last ice age which was 12,000 years ago. It actually speeds up the rotation of the planet, shortens the days by a few fractions of a millisecond.
When we put all these factors together, it appears that the longest day in Earth’s history likely happened in 1912. In that year, the summer solstice was the longest period of daylight in the Northern Hemisphere has ever seen.