Every June, the Northern Hemisphere reaches a celestial milestone: the summer solstice. It’s the longest day of the year and the official start of astronomical summer.
But what causes this seasonal shift, and why does the sun behave so differently on this day? Keep reading for all you need to know about the summer solstice, why it happens, and summer solstice events happening in the Bay Area where you can mark the occasion.
The solstice — and our seasons — owe it all to the way Earth tilts, in comparison to its planetary siblings.
Our planet rotates on a tilted axis, about 23.5 degrees off vertical — while “normally, planets should orbit with their axis straight, so they rotate in the same way that they revolve around the Sun.” Andrew Fraknoi, instructor at the Fromm Institute of the University of San Francisco, told KQED by email.
So what happened to the Earth that explains this difference? “Astronomers now think that our planet got hit by another world, very early in the history of the solar system,” wrote Fraknoi. “And, like many accident victims, it couldn’t straighten out again.”
Due to this tilt, as Earth orbits the Sun, different parts of the globe receive varying amounts of sunlight throughout the year. This tilt is the reason for our seasons.
On the summer solstice, the Northern Hemisphere leans most directly toward the sun. That means more direct sunlight and the longest stretch of daylight hours — up to 15 hours overall — for the year. Near the North Pole, the sun doesn’t set at all — a phenomenon known as the midnight sun.
Meanwhile, in the Southern Hemisphere in regions inside the Antarctic Circle, it’s the exact opposite. At this time, they experience their winter solstice, during which the South Pole experiences a polar night, when the sun does not rise above the horizon for at least 24 consecutive hours.
This year in the Bay Area, the summer solstice starts on Friday, June 20, 2025, at 7:42 p.m. P.S.T.
(Check where the summer solstice will begin in your region.)
You might notice the solstice doesn’t fall on the same day each year. That’s because our calendar system doesn’t perfectly match Earth’s orbit.
The Gregorian calendar has 365 days, but the Earth actually takes about 365.25 days to orbit the Sun. That extra quarter-day is why we have leap years — and why the solstice shifts between June 20 and June 21.
It depends on how you define “summer.” There are two main ways to mark the seasons:
Because Earth moves in an elliptical orbit and not a perfect circle, seasons aren’t all the same length around the world. According to timeanddate.com, Northern Hemisphere summer averages 93.6 days, while Southern Hemisphere summer lasts about 89 days.
During the Northern Hemisphere’s summer, Earth is actually at its furthest from the sun. This point, known as aphelion, occurs a couple of weeks after the solstice.
This year, aphelion will fall on July 3, when Earth will be about 94.5 million miles from the sun, according to timeanddate.com. The opposite — perihelion — happens in early January, when Earth is closest to the sun.
However, the difference in distance doesn’t significantly affect seasons. It’s the tilt, not the distance, that drives our weather changes.
No: even though the solstice brings the most daylight, it’s not usually the hottest day. That’s due to seasonal lag — the time it takes for Earth’s land and oceans to absorb and release heat, according to the Royal Meteorological Society.
Our planet’s water-covered surface (about 70%) soaks up energy slowly, meaning peak summer temperatures typically arrive in July or August. Last year, the hottest day globally was July 22.
The summer solstice isn’t just a marker for the longest day of the year — it’s an ecological cue and a reminder that our climate is shifting.
Historically, researchers believed that these extended hours of daylight helped plants schedule growth. A new study has shown that while the day of the summer solstice may serve as a key cue for plant growth and reproduction, climate change may disrupt this timing.
Lawrence Hall of Science, Berkeley
Summer Solstice Celebration: Saturday, June 21, 10:30 a.m.–2 p.m.
Chabot Space & Science Center, Oakland
Paint & Sip: Solstice Stars: June 19, 6:30–8:30 p.m.
Hike & Sip: Solstice Stargazing: June 21, 6–9:30 p.m.
California Academy of Sciences, San Francisco
Solstice Sea Star Search: Happening throughout June.
East Bay Regional Parks
Solstice Sunrise Hike at Coyote Hills Regional Park: June 21, 8–10 a.m.
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