Astronomers believe that Earth’s unique axial tilt, which leads to the variation in sunlight received around the globe, is the result of a collision with another celestial body early in the solar system’s formation. This tilt is responsible for the changing seasons across the planet.
On the summer solstice, which occurs when the Northern Hemisphere is tilted closest to the Sun, regions experience the longest daylight hours of the year, with up to 15 hours of sunshine in areas like the Bay Area, where this year’s summer solstice will start on Friday, June 20, 2025, at 7:42 p.m. P.S.T. Conversely, the Southern Hemisphere experiences winter during this time, resulting in the phenomenon known as polar night, particularly over regions within the Antarctic Circle.
The date of the solstice shifts yearly due to the Gregorian calendar’s mismatch with the Earth’s orbit around the Sun, which takes approximately 365.25 days. This discrepancy leads to the implementation of leap years and causes the solstice to fluctuate between June 20 and June 21.
Summer length varies based on definitions, with astronomical summer beginning at the solstice and lasting until the fall equinox around September 22 or 23. In contrast, meteorological summer, marked by a calendar-based system, spans from June 1 to August 31. Notably, Northern Hemisphere summer averages about 93.6 days, while Southern Hemisphere summer lasts around 89 days.
Interestingly, during the Northern Hemisphere’s warmest months, Earth is actually at its furthest point from the Sun, a phase called aphelion, which will occur on July 3 this year when Earth is about 94.5 million miles away. The distance from the Sun, however, does not significantly impact seasonal weather; the axial tilt is the primary factor.
Despite the summer solstice offering the longest day of the year, it doesn’t typically coincide with the hottest day. This phenomenon is attributed to a delay in the release of heat from Earth’s land and oceans, causing peak temperatures to usually occur in July or August.
As we look forward to the summer solstice, it’s also essential to recognize the ecological significance of this time. While historically, the extended daylight has guided plant growth cycles, recent studies indicate that climate change may disrupt these natural cues.
For those interested in celebrating the summer solstice in the Bay Area, several events are planned, including a Summer Solstice Celebration at the Lawrence Hall of Science and various stargazing activities at the Chabot Space & Science Center.
The summer solstice brings a wonderful opportunity to connect with nature and reflect on the changing seasons, while reminding us of the ongoing challenges posed by climate change.