2024 marks the occurrence of the summer solstice on June 20, the earliest such event since 1796, during George Washington's presidency. Typically, the summer solstice falls on June 21 when the northern hemisphere's tilt is at its peak towards the Sun. The Earth undergoes a cyclical orientation in space that includes:

• Maximum tilt towards the Sun (summer solstice),
• Alignment with Earth’s orbit (autumnal equinox),
• Maximum tilt away from the Sun (winter solstice),
• Anti-alignment with Earth’s orbit (spring equinox).

These events pertain to the northern hemisphere; in the southern hemisphere, the seasons are inverted.

This year, the summer solstice occurs at 20:51 UTC (4:51 PM EDT/1:51 PM PDT). This phenomenon is noteworthy, as it hasn't happened since 1796, a fact that holds scientific significance for the remainder of the century.

The Earth’s journey around the Sun involves multiple activities:

• An elliptical orbit completed in approximately 365 days, 6 hours, 9 minutes, and 10 seconds,
• Daily rotation about its axis, taking 23 hours, 56 minutes, and 4.09 seconds,
• Precession of its axis, requiring over 20,000 years for a full cycle relative to the Sun.

Humans mark time using days and years, with each day consisting of 24 hours and a year defined as either 365 or 366 days, depending on leap years. The difference between Earth’s movements and human timekeeping is crucial in determining the exact moment of the solstice.

The discrepancy arises because a day (24 hours) does not equate to a complete rotation (23 hours, 56 minutes, and 4.09 seconds). As Earth orbits the Sun at an average distance of 149.6 million kilometers (93 million miles) and speeds around 30 km/s (66,620 mph), it travels approximately 2.6 million kilometers in a single day.

This distance translates to a need for about 361° of rotation to align with the Sun's position, explaining why our 24-hour days are roughly 4 minutes longer than Earth's axial rotation period.

Next, we consider the number of days in a calendar year. The tropical year—how we measure seasons—does not align perfectly with Earth's 360° orbit around the Sun. We must take into account the subtle shifts in Earth’s axial tilt, which dictate the seasons.

To maintain seasonal alignment with our calendar, we calculate the length of a tropical year to be approximately 365.242189 days. Thus, a calendar system must accommodate this fractional day to prevent seasonal drift.

Historically, the Julian calendar was introduced over 2000 years ago. Initially based on lunar months, it was later adjusted to a solar calendar by Julius Caesar, fixing the year at 365 days with an extra day every four years to account for leap years.

The Julian calendar was effective until the discrepancies in solstice and equinox dates prompted reforms. By the late 16th century, the Gregorian calendar emerged, refining the leap year system. Now, a year is a leap year if divisible by four, with the exception of years ending in "00," which must also be divisible by 400.

This adjustment brought the calendar into better alignment with Earth's seasonal changes. The Gregorian calendar averages 365.2425 days per year, significantly closer to the actual tropical year than the Julian system.

The nuances of the Gregorian calendar mean:

• Each standard year has 365 days, resulting in equinoxes and solstices occurring later by about 0.242189 days annually.
• Leap years, with 366 days, cause these events to occur 18 hours, 11 minutes, and 14.87 seconds earlier than the previous year.

Thus, the summer solstice in 2024 is predicted to be roughly 45 minutes earlier than in 2020.

Although many countries adopted the Gregorian calendar in 1582, others transitioned much later, leading to discrepancies in date recordings across nations. Consequently, the timing of equinoxes and solstices gradually shifts over the years.

This means that the earliest solstices and equinoxes will occur four years before the next non-leap century, while the latest will happen three years after the last non-leap century. Those who lived in the 20th century may recall the summer solstice typically occurring on June 21, with some rare instances on June 22. As we progress through the 21st century, these events will continue to shift earlier.

Enjoy the summer solstice and its accompanying events, knowing they are the earliest Earth has witnessed since the late 18th century. By the 2060s, these occurrences on leap years will surpass anything seen in the 1700s, continuing through the 21st century. Only those reaching the 2100s will witness the end of this ongoing trend.

Starts With A Bang is written by Ethan Siegel, Ph.D., author of Beyond The Galaxy, Treknology, and The Littlest Girl Goes Inside An Atom. New titles, including the Encyclopaedia Cosmologica, are forthcoming!