Today, June 21, is the longest day of the year for anyone living north of the equator. It is called the Summer Solstice --- when the sun is directly over the Tropic of Cancer, or 23.5 degrees north latitude. For those living in the South of the Equator, it is unofficially the beginning of winter. Solstices occur at the same time around the world, but their local times vary with time zones.
When do we have Summer Solstice?
From March to September, the Northern Hemisphere is tilted more toward the sun, driving its spring and summer. The sun appears directly over the Tropic of Cancer, the latitude line at 23.5 degrees North. From September to March, the Northern Hemisphere is tilted away, so it feels autumn and winter. The Southern Hemisphere's seasons are reversed. During the Northern Hemisphere’s winter solstice, the sun appears directly over the Tropic of Capricorn, the Tropic of Cancer’s southern mirror image.
In the Northern Hemisphere, peak sunlight usually occurs on June 20, 21, or 22 of any given year. That’s the summer solstice. By contrast, the Southern Hemisphere reaches peak sunlight on December 21, 22, or 23 and the Northern hits peak darkness — that’s our winter solstice.
What is Stonehenge’s connection with Solstice?
One theory suggests that the pre-historic English Stonehenge was used to mark solstices and equinoxes. That’s because during the summer solstice, the sun rises just over the structure’s Heel Stone and hits the Altar Stone dead center.
NASA explains Stonehenge's connection with Summer Solistice. (Photo: NASA)
Do other planets also have solstice?
Yes, any planet with a tilted rotational axis would see them, too.