On June 20, 2024, the summer solstice will occur at its earliest point in 228 years – but this is just the start. Over the next 72 years, the annual event will get progressively earlier every four years. So what’s going on?

The summer solstice tends to occur on June 21 every year. This is the point when the Earth’s north pole has its maximum tilt towards the Sun, which also leads to that day having the most hours of sunlight and the shortest night. This event is caused by the Earth’s axis, which is tilted to 23.5 degrees with respect to its orbit around the Sun. 

In contrast, as the Northern Hemisphere experiences its longest day, the Southern Hemisphere has its shortest day – its winter solstice. 

This year, however, something interesting is happening. The summer solstice will occur earlier than usual, on the 20 June at 8:50 pm UTC (4:50 pm EDT/1:50 pm PDT). This will be the earliest summer solstice since 1796. Or, to put it another way, the last time the solstice occurred this early was at a time when George Washington was president, and the French Revolution was still in full force. 

Essentially, the variation happens because of the Gregorian calendar and how we mark the transition of time. To be clear, the Gregorian calendar is pretty good, especially when compared to its predecessor, the Julian calendar. 

The Earth’s journey around the Sun each year is not exactly 365 days. Instead, it is more like 365.242189 days. To account for this, the Gregorian calendar spaces leap years to make the average year 365.2425 days long. 

In every normal year – non-leap year – we experience 365 days, which means that the solstices (and equinoxes) fall a little later each year than they did on the previous ones. Then, every four years, we have a leap year which is 366 days long – the extra day appearing on February 29. This means the dates of the solstices and the equinox are 18 hours, 11 minutes, and 14.87 seconds earlier in the year than the year before. 

So far so good, but then there is another complication when it comes to how the Gregorian calendar accounts for leap years. To make it work, the system was set up so that every leap year is one that is divisible by four. But if the year ends in “00”, as in the turn-of-the-century year (1800, 1900, 2000), then it can only be a leap year if it is divisible by 400. 

The year 2000 was one such year. It was divisible by both 4 and 400, but 1900, 1800 and 1700 were not. By not including leap years for these centuries – which amounts to dropping three leap years every 400 years – we are able to correct for the actual speed of the planet hurtling through space. 

What this means is that every four years in a century that does not start with a leap year, we cumulatively count 365.25 days per year for four years and not 365.242189 days per year. This results in us keeping time “too fast” by around 45 minutes. 

As such, the summer solstice this year is about 45 minutes earlier than the one in 2020. It also means that this trend will continue through the century until 2096 when the solstice will occur at 06:32 am UTC on June 20. After this, the solstice will flip back to being later in the year as the cycle resets.

[H/T: Big Think]