Summer Solstice Hits at 4:24 a.m. EDT, Bringing Boston 15 Hours of Daylight

Q: If today is the year's longest day, why do we have to wait weeks for summer's hottest weather?

The hottest weather usually arrives weeks after the summer solstice because Earth’s surface and oceans take time to warm up, a delay known as seasonal lag. The solstice brings the most daylight and the strongest overall solar input of the year in the Northern Hemisphere. But maximum sunlight does not mean maximum temperature on the same day. Land, and especially water, absorb heat gradually. For a while after the solstice, the planet is still taking in more energy from the sun than it is sending back into space, so temperatures keep rising. This is similar to the daily cycle: the sun is highest around noon, but the warmest part of the day often comes later in the afternoon. On a seasonal scale, the same physics plays out over weeks rather than hours. Oceans are a major reason the lag is so noticeable. Water has a high heat capacity, meaning it warms slowly and releases heat slowly. That stored heat helps keep the atmosphere warming into late July or August in many places, even as daylight has already begun to shorten after the solstice. The reverse happens in winter. The shortest day comes at the winter solstice, but the coldest weather often follows later because the ground and oceans continue losing more heat than they gain for some time afterward. So the solstice marks peak daylight, not peak heat.

Q: The solstice sun once rose in Cancer but is now in Taurus. Where will it appear for our distant descendants?

For our distant descendants, the June solstice sun will next appear in **Aries**. That shift happens because of **Earth’s axial precession** — the slow wobble of Earth’s spin axis over roughly **26,000 years**. Precession gradually moves the solstice and equinox points westward against the background of the zodiac constellations. So the June solstice point does not stay fixed in Cancer, even though the term **Tropic of Cancer** remains. Historically, the June solstice sun was in **Cancer**. As precession carried the solstice point backward through the zodiac, it moved through **Gemini** and is now seen in **Taurus**. If that motion continues, the next constellation in the sequence is **Aries**. Over much longer timescales, the cycle keeps going. After Aries, the solstice point will continue into **Pisces**, then onward through the zodiac, eventually returning to **Cancer** again many thousands of years later. This is an astronomical effect, not a change in the seasons themselves. The solstice still marks the Northern Hemisphere’s longest day because that depends on **Earth’s tilt relative to the sun**, while the constellation behind the sun changes only because Earth’s axis slowly shifts its orientation in space.

3 articles · Updated · The Boston Globe · Jun 20

4:24 a.m. EDT on Sunday marks the exact summer solstice, when the sun reaches its northernmost point and astronomical summer officially begins.
Boston will get 15 hours, 17 minutes and 3 seconds between sunrise and sunset, the year's widest daylight span, though the earliest sunrise came around June 14 and the latest sunset is still about five days away.
Earth's axial tilt and elliptical orbit create that mismatch, even as the solstice marks the Northern Hemisphere's maximum lean toward the sun before daylight begins shrinking almost imperceptibly.
By July 4, Boston will have lost only about five minutes of light, while average temperatures typically keep rising into the third week of July because oceans warm more slowly than land.