StÃ©phane Schneider, University of Massachusetts Amherst
The summer solstice marks the official start of summer. It brings the longest day and the shortest night of the year for the 88% of the earth’s population that lives in the northern hemisphere. All over the world, people traditionally observe the change of seasons with bonfires, festivals and Music Day celebrations.
Astronomers can calculate an exact time for the solstice, when the Earth reaches the point in its orbit where the North Pole is at the closest angle to the sun. That moment will take place at 5:44 p.m. EST on June 20 of this year. From Earth, the sun will appear farthest north of the stars. People living in the Tropic of Cancer, 23.5 degrees north of the equator, will see the sun pass directly overhead at noon.
In six months, the sun will reach its southernmost point and pass over the inhabitants of the Tropic of Capricorn, and the northerners will experience their shortest days of the year, at the winter solstice.
The angle of the sun to the earth’s equator changes so gradually as the solstices approach that, without instruments, the offset is difficult to perceive for about 10 days. This is the origin of the word solstice, which means “solar stop”.
This slow change means that June 20 is only about 1 second longer than June 19 in northern mid-latitudes. It will take about a week before there is more than a minute of change in the calculated amount of daylight. Even this is an approximation – the Earth’s atmosphere bends the light on the horizon by different amounts depending on the weather, which can introduce changes of more than a minute in the times of sunrise and sunset.
The monuments of Stonehenge in England, Karnak in Egypt, and Chankillo in Peru reveal that people around the world have been taking note of the sun’s north and south shifts for over 5,000 years. From the standing stone circle of Stonehenge, the sun will rise directly over an ancient avenue leading northeast at the solstice. We know little about the people who built Stonehenge, or why they put so much effort into building it – moving stones weighing several tons from rocky outcrops up to 140 miles.
All this to mark the place on the horizon where the sun returns each year to rest for a while before heading south again. Perhaps they, like us, celebrated this signal of the coming season change.
This is an updated version of an article originally published on June 18, 2018.
Stephen Schneider, professor of astronomy, University of Massachusetts Amherst
This article is republished from The Conversation under a Creative Commons license. Read the original article.