The 2019 North American summer solstice happens on June 21, 2019 at 11:54 AM EDT.
That’s the very moment when, essentially, the sun stands still at its northernmost point as seen from Earth. Its zenith doesn’t yearn north or south, but waits patiently at the Tropic of Cancer before switching directions and heading south again. This is where the word solstice comes from; the Latin solstitium, from sol (sun) and stitium (to stop).
Why Doesn’t the Summer Solstice Fall on the Same Date Each Year?
The summer solstice in the Northern Hemisphere ranges in date from June 20 to 22. This occurs in part because of the difference between the Gregorian calendar system, which normally has 365 days, and the tropical year (how long it takes Earth to orbit the Sun once), which has about 365.242199 days. To compensate for the missing fraction of days, the Gregorian calendar adds a leap day about every 4 years, which makes the date for summer jump backward. However, the date also changes because of other influences, such as the gravitational pull from the Moon and planets, as well as the slight wobble in Earth’s rotation.
Did You Know?
Question: Why isn’t the summer solstice—the longest day of the year—also the hottest day of the year?
Answer: Earth’s atmosphere, land, and oceans absorb part of the incoming energy from the Sun and store it, releasing it back as heat at various rates. Water is slower to heat (or cool) than air or land. At the summer solstice, the Northern Hemisphere receives the most energy (highest intensity) from the Sun due to the angle of sunlight and day length. However, the land and oceans are still relatively cool, due to spring’s temperatures, so the maximum heating effect on air temperature is not felt just yet. Eventually, the land and, especially, oceans will release stored heat from the summer solstice back into the atmosphere. This usually results in the year’s hottest temperatures appearing in late July, August, or later, depending on latitude and other factors. This effect is called seasonal temperature lag.
7 things to know about the longest day of the year“, Vox, June 20, 2017, “