The 2024 summer solstice will occur on June 20, marking the earliest solstice in 228 years. This trend will continue, with the solstice happening progressively earlier each leap year until 2100. But why is this happening, and what causes the variation in day lengths throughout the year?
The term “solstice” comes from the Latin for “sun stands still.” In the Northern Hemisphere, the summer solstice is when the sun rises and sets at its most northeasterly and northwesterly points. This phenomenon is due to Earth’s 23.5-degree axial tilt. As Earth orbits the sun, one hemisphere tilts toward the sun while the other tilts away.
On the June solstice, the Northern Hemisphere experiences the longest day of the year, while the Southern Hemisphere has its shortest, marking their winter solstice. This year’s solstice is the earliest since 1796, the year George Washington gave his farewell address. The reason for this variation lies in the Gregorian calendar.
The Gregorian calendar divides the year into 365 days, but Earth’s actual orbit around the sun takes 365 days, 5 hours, 48 minutes, and 45 seconds. To account for this discrepancy, we add a leap year every four years. However, this adjustment overshoots by 45 minutes every four years. To correct this, we skip a leap day every four centuries, with the next skip in 2100. Until then, each leap year will push the solstice 18 hours earlier than the previous year and 45 minutes earlier than the previous leap year.
The origins of Earth’s tilt trace back over 4 billion years, to a time when Earth experienced massive collisions with other celestial bodies. The last of these, with a Mars-sized planet called Theia, resulted in Earth’s tilt and the formation of the moon. This tilt is responsible for the seasons and the ecological cycles that have shaped life on our planet.
On June 21, the day after this year’s solstice, skywatchers can enjoy the Strawberry Moon, the lowest full moon of the year.
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