Astronomical summer begins with the summer solstice, the moment when the Northern Hemisphere is tilted most directly toward the sun. It’s the longest day of the year in terms of daylight and a key turning point in Earth’s seasonal cycle around the sun.
The solstice typically occurs on June 20 or 21, though the exact date and time vary slightly from year to year. At that precise moment, the sun reaches its northernmost position in the sky relative to Earth’s equator. The word “solstice” comes from the Latin solstitium, meaning “sun standing still.” On the June solstice, the sun’s apparent northward movement in the sky pauses before reversing direction.
Why it’s the longest day
Because Earth is tilted about 23.5 degrees on its axis, the Northern Hemisphere receives its most direct sunlight of the year on the summer solstice. The sun traces its highest and longest arc across the sky, rises at its earliest time of the year (within a few days of the solstice) and sets at its latest time.
In D.C., the solstice brings more than 14½ hours of daylight, with twilight stretching usable light even longer. The midday sun also reaches its highest elevation of the year, nearly overhead compared with the low winter sun that hugs the southern sky.
Although the solstice has the longest period of daylight, it is not necessarily the hottest day of the year. Summer temperatures usually peak in mid-July or early August, because of what’s known as seasonal lag. Even after daylight begins to slowly decrease following the solstice, land and oceans in the Northern Hemisphere continue absorbing and storing solar energy, allowing temperatures to climb deeper into summer.
Why the date shifts
The summer solstice doesn’t fall on the exact same calendar date every year because Earth’s orbit around the sun doesn’t take precisely 365 days. Leap years and subtle orbital variations shift the timing by several hours each year, occasionally nudging the date between June 20 and June 21.
Astronomical vs. meteorological summer
There are two ways to define the start of summer:
- Astronomical summer begins at the solstice – the precise moment when the sun appears at its northernmost point directly over the Tropic of Cancer, 23.5 degrees north of the equator.
- Meteorological summer runs from June 1 through August 31, a convention used by meteorologists and climate scientists for consistent record-keeping.
Meteorological seasons align better with temperature patterns, while astronomical seasons follow the geometry of Earth and sun.
A turning point, not a peak
Although the solstice marks maximum daylight, it also signals a subtle shift. After this date, daylight begins to shrink, at first almost imperceptibly. Sunrise inches later and sunset edges earlier. The changes are small in late June and early July but accelerate by late summer.
Still, the weeks surrounding the solstice are among the brightest of the year. In DC, dawn begins around 5 a.m. and dusk extends past 9 p.m. On the solstice, sunrise and sunset are at 5:43 a.m. and 8:37 p.m., respectively.
What you’ll notice around the solstice
In the days leading up to and just after the solstice:
- The sun rises and sets at nearly the same times for several days.
- Sunrise and sunset occur at their northernmost points along the horizon.[JG5]
- Shadows at midday are at their shortest of the year.
- Ultraviolet radiation is near its annual peak, increasing sunburn risk.
Daylight lingers deep into the evening, and twilight lasts longer than any other time of year, especially at northern latitudes.
