2024’s summer solstice is Earth’s earliest since 1796

On average, the summer solstice occurs on June 21 of most years, as the Earth’s north pole is tilted maximally toward the Sun at a particular moment on that day. As the Earth revolves around the Sun over the course of a year, its axis remains pointed in the same direction, so that the orientation of Earth in space goes through a cycle:

• where the Earth’s north pole is maximally pointed toward the Sun (summer solstice),
• where the Earth’s north pole is aligned with the motion of Earth around the Sun (autumnal equinox),
• where the Earth’s north pole is maximally pointed away from the Sun (winter solstice),
• and where the Earth’s north pole is anti-aligned with the motion of Earth around the Sun (spring equinox).

The equinoxes and solstices given here apply to the northern hemisphere; in the southern hemisphere, seasonal equinoxes and solstices are opposed, as seasons are determined by the direction that Earth’s south pole points, rather than the north pole.

But this year, in 2024, the summer solstice will be a little earlier than usual: on June 20 for most of the world. In fact, the time at which the solstice occurs, 20:51 UTC (or 4:51 PM EDT/1:51 PM PDT), marks the earliest summer solstice for planet Earth since the year 1796: when George Washington, the very first President of the United States of America, was still president. There’s a fascinating scientific reason for this, with enormous implications for the rest of the century.

The Earth, moving in its orbit around the Sun and spinning on its axis, always defines “noon” and “midnight” in the same fashion: where the Sun’s height above or below the horizon is maximized. This moment in time doesn’t correspond to when Earth has rotated 360 degrees from the prior day, but rather closer to 360.9856 degrees, owing to the added effects of Earth’s motion around the Sun.

As the Earth orbits the Sun, it does several things at once.

• It revolves around the Sun in the shape of an ellipse, where a full orbital revolution takes 365 days, 6 hours, 9 minutes and 10 seconds.
• It rotates about its axis, which it does each and every day, where a complete rotation takes 23 hours, 56 minutes and 4.09 seconds.
• And the direction in which Earth’s axis points precesses, where it takes more than 20,000 years for Earth’s axis to make one full rotation with respect to the Sun.

There are many different ways to mark the passage of time, but for our purposes as denizens of Earth, we use precisely none of these as our measure.

Instead, we use the common idea of a “day” and a “year” to mark our calendars, where every single day has exactly 24 hours to it, and where a year is defined to have either 365 or 366 days, dependent on whether we assign that particular year to be a leap year (like this one) or not. The difference between what the Earth and Sun are doing in space and between how humans mark time on Earth is of tremendous importance in figuring out precisely when the solstice occurs.

From one day to the next doesn’t correspond to just a 360 degree rotation of the Earth, but enough extra rotation to correspond to the Sun returning to the same position in Earth’s sky. Similarly, the duration of a lunar month isn’t simply the time it takes the Moon to revolve 360 degrees around Earth, but to return to the same position relative to the Sun as seen from Earth.

The reason a day (24 hours) doesn’t correspond to the time it takes Earth to complete one rotation about its axis (23 hours, 56 minutes and 4.09 seconds) is illustrated above. Sure, if Earth made a full 360° rotation and remained stationary with respect to the Sun, we would have marked time differently; that rotation would correspond to the length of a day, as the Sun would return to the same apparent point in Earth’s sky, day after day, every 23 hours, 56 minutes, and 4.09 seconds.

But the Earth is not stationary. In fact, it orbits the Sun at a mean distance of 149.6 million kilometers (93 million miles) with an orbital speed of around 30 km/s (66,620 mph), meaning that over the course of what we conventionally call a “day,” it migrates a whopping 2.6 million kilometers through space.

That extra distance corresponds to almost 1° in the orbit of Earth around the Sun, and so it isn’t a full 360° rotation that’s needed to go from day-to-day, but rather more like 361° of rotation. That explains why a day, as we mark it, is approximately 4 minutes longer than the time it takes Earth to rotate once about its axis.

This unique fisheye timelapse image of the Sun shows its path through the sky on the Summer Solstice (top), Winter Solstice (bottom), and equinox (middle) days. Although the path of the equinox or solstice Sun doesn’t vary from year to year, the date/time on which that path occurs will vary significantly over time.

Then we have to ask the next question: based on how we keep track of each day, how many days are there in each calendar (also known as tropical) year? This, once again, is not the same as the amount of time it takes for Earth to return to the same exact position in space after completing a 360° orbit around the Sun. We have to account for the subtle, small changes in Earth’s axial tilt, as it’s the orientation of Earth’s axis returning to the position it was in a year prior that determines the onset and changing of the seasons, and hence, the duration of a year. Whether we go from:

• summer solstice to summer solstice,
• autumnal equinox to autumnal equinox,
• winter solstice to winter solstice,
• or spring equinox to spring equinox,

is not the important part of the equation. What is important is that we design our calendar in such a way that the seasons don’t drift with respect to the dates of our calendar, and in order to do that, it’s important to calculate how many days — that is, 24 hour periods, or days as we commonly use them — there are in a tropical (calendar) year. In an ideal world, that number would be nice and even, as that way every year would have the same number of days in it. But according to the best way of calculating this that we know of, there are actually 365.242189 days in a year. And so, we needed to develop a calendar system that could accommodate this offset between “days” and how we experience a year on Earth.

Just 800 years ago, perihelion and the winter solstice aligned. Due to the precession of Earth’s orbit, they are slowly drifting apart, completing a full cycle every 21,000 years. Over time, the Earth drifts slightly farther from the Sun, the precession period increases, and the eccentricity varies as well. The most accurate measure of the “year,” as experienced by Earthlings, is to go from either equinox-to-equinox or solstice-to-solstice, but this does not correspond to a single 360 degree revolution of Earth around the Sun.

Our first attempt now dates back more than 2000 years: what we know as the Julian calendar. (Although recent evidence shows that it owes its origin to the Egyptians, nearly 1000 years before the time of Caesar.) Prior to Julius Caesar, the Roman Republic kept time based on a lunar calendar, where there were typically 12 lunar months assigned to every year. The time from full moon to full moon (or new moon to new moon), known as a lunar month, is only 29.53 days, however. Most years, then, would only have about 354 days to them. However, in order to keep the seasons in line with the year, around every three years they would insert an additional 13th month to the year: an intercalary month.

As consuls (an annual political position) in the Roman Republic sought to retain power for longer, they would “change the laws” to add an intercalary month to their consul years: an oft-abused system. To eliminate this abuse, Julius Caesar enacted the reform of letting the calendar instead be governed by the Sun, to free it from the concerns of human tampering. The result was the Julian calendar. Now, every year would have 365 days to it, and every fourth year — that is, every year whose number was exactly divisible by 4 — would get one extra day: 366 days. This would average out to 365.25 days per year: the best estimate for keeping the solar calendar aligned with the seasons at the time.

Today, in the year 2024, Polaris makes an excellent “North star,” as it’s located within about 1 degree of the celestial north pole. As Earth’s axis precesses, the presence and properties of a pole star change; 5000 years ago, the star Thuban in the constellation of Draco was an excellent pole star for humanity. This precession plays an important role in determining the true number of days in an actual calendar year, but plays practically no role in the total amount of heat that Earth receives from the Sun.

For many centuries, that system was “good enough” for most. But a few hundred years ago, the fact that the solstices and equinoxes had migrated by several days could not be ignored. That difference between the “true value” of 365.242189 days in a year and the “assigned” value of 365.25 days in a year results in a mismatch of just under one day per century. By the middle of the second millennium, solstices and equinoxes were occurring in the first half of March, June, September, and December, rather than on their original dates. Additional calendrical reforms were required, and finally arrived in the late 16th century with a new system: the Gregorian calendar.

Now, instead of every fourth year being a leap year, the math was a little bit different. The subtle change was that a year would be a leap year if it was divisible by four and — as an extra stipulation — that if the year ended in “00” (i.e., was a turn-of-the-century year), it must also be divisible by 400 to remain a leap year. The year 2000, which is both divisible by 4 and 400, was a leap year, but the years 1900, 1800, and 1700 were not. There may be no one alive who remembers the year 1900, but some of us living today will live to see the next turn-of-the-century, and experience the year 2100: which will not be a leap year.

The presence or absence of a February 29 on the calendar determines with great significance whether the equinox (or solstice) shifts forward or backward in time from the prior year’s equinox (or solstice). 2020 marked the first year since 1896 where the entire United States experienced a March 19 equinox, and 2024 will mark another year with a leap day and the earliest summer solstice since 1796. Leap days occurred every 4 years under the Julian calendar, but the Gregorian calendar revised that to remove some of the leap days (on years ending with “00” but not divisible by 400) to better keep track of time.

This reform brought the calendar that we use much closer into alignment with the actual way that planet Earth’s seasons change over time. With the advent of the Gregorian calendar, we were now accounting for 365.2425 days per year, on average, as opposed to the true value of 365.242189, which was a much better approximation than the Julian calendar’s 365.25 days. Whereas the Julian calendar caused us to be offset by a calendar year to the tune of about 1 day per century, the Gregorian calendar will only be off by about 1 day for every 32,000 years that go by.

But the intricacies of how the Gregorian calendar works means the following.

• Every normal (non-leap) year only experiences 365 days, meaning the dates of the equinoxes and solstices fall 0.242189 days (5 hours, 48 minutes, and 45.13 seconds) later in the year than the prior year.
• Every leap year experiences 366 days, with the extra day coming in on February 29, meaning the dates of the equinoxes and solstices fall 18 hours, 11 minutes, and 14.87 seconds earlier in the year than the year prior.
• And that every four years in non-century years, cumulatively, we count 365.25 days per year for four years rather than 365.242189 days per year, meaning that we keep time “too fast” by about a total of 45 minutes.

That last aspect means that the summer solstice in 2024 is about 45 minutes earlier than the summer solstice from four years prior: in 2020.

Although a great many countries first adopted the Gregorian calendar in the year 1582, it wasn’t until the 18th century that it was adopted in England, with many countries making the transition even later. As a result, the same date, as recorded in different countries, often corresponds to a different point in time. Whereas the first countries to adopt it in 1582 only needed to remove 10 days from their calendar, Russia, whose calendar was revised in 1918, needed to remove 13 such days.

This means that very gradually, the solstices and equinoxes migrate to slightly earlier times than they were four years prior. However, every time we experience a non-leap century (e.g., 1700, 1800, 1900, 2100, etc.), the fact that we only have 365 days in those years means that the equinoxes and solstices now fall almost a full 24 hours later in the year than they did compared with four years ago. In other words, the solstices and equinoxes obey:

• a four-year cycle,
• with roughly six hour (later) differences between solstices and equinoxes in successive non-leap years,
• where leap years bump solstices and equinoxes to roughly 18 hours earlier than the prior year,
• where most four year periods regress by about 45 minutes (earlier) than the year prior,
• and where non-leap centuries let the calendar “catch up” by having the solstices and equinoxes occur 23 hours and 15 minutes later (by the absence of a day) during four year periods that include those centuries.

In other words, we can fully expect that even though 2024 gives us the earliest solstices and equinoxes since 1796, that they’ll be even earlier in coming leap years: 2028, 2032, 2036, and so on.

The moment of summer solstice from Vancouver, BC, Canada, from the 1890s through the early 2100s. The summer solstice of 2020 was the earliest one since 1896, but four years later, in 2024, is earlier than any summer solstice from the 1800s; one must go all the way back to 1796 to find an earlier summer solstice.

This will continue throughout the remainder of the 21st century, until its final leap year: 2096. In 2096, Earth will experience the earliest summer solstice since the adoption of the Gregorian calendar: on June 20th at 06:32 UTC. It will be the only summer solstice that people in the Pacific time zone of the Americas will experience on June 19 for several hundreds of years!

Then, however, because 2100 is not a leap year, the years 2097 through 2103 will all be non-leap years. Over that time interval, by measuring time in UTC, the summer solstice will migrate from early on June 20th (in 2096) to late on June 21st (in 2103), and then the first leap day of the 22nd century will bump the solstice to around 18 hours earlier for 2104.

There’s a lovely pattern that emerges if you plot the time of the same “solstice” or “equinox” year-over-year, where you can see the four year pattern very clearly, but on longer timescales, you can notice how the solstices and equinoxes drift to earlier times, only to be bumped back to later times in the year when the non-leap centuries occur.

The Gregorian Calendar, adopted in 1752 by the United Kingdom and its territories (including what would become the USA), has a specific prescription for how to keep time. Here, the occasion of the summer solstice is marked, in terms of fractions-of-a-day in June, on the y-axis, while the x-axis shows years from 1750-2250: a 500 year span.

Furthermore, it explains why the earliest solstices and equinoxes always occur four years before the first “non-leap century” after a leap century (2096, 2496, 2896, etc.), whereas the latest equinoxes and solstices occur just three years after the third and final non-leap century passes in a cycle (1903, 2303, 2703, etc.). For those of us who grew up in the 20th century, we might remember that the summer solstice usually occurred on June 21, and some of us who are older and/or who live in the far west may even remember experiencing a few summer solstices on June 22. But as we progress throughout the 21st century, equinoxes and solstices will migrate later and later, destined to reach their latest times in 2096.

Enjoy the summer solstice and all the solstices and equinoxes of this year knowing they’ll be the earliest ones that planet Earth has experienced since the end of the 18th century. By the time the 2060s arrive, our solstices and equinoxes on leap years will be earlier than even anything the 1700s had to offer, and that will continue through the last leap year of the 21st century. With every four years that goes by, the solstices and equinoxes will continue to migrate earlier and earlier. Only those of us who make it into the 2100s will live long enough to see this trend come to its inevitable end!