It is 2024, which means we have the Summer Olympics, a US presidential election, and a leap day.
However, unlike the other two, one doesn't technically happen every four years - leap year.
This year, leap day occurs on Thursday, followed by the start of autumn for us here in Australia on Friday.
We also have an early Easter at the end of March - and all of these things are part of the history of determining leap day.
For millennia, cultures around the world based their calendar on the moon's movement around the Earth, and the visible monthly cycles its appearance goes through.
The lunar months are based on the phases - the time between each new or full moon. It takes the moon about 29.53 days to go through the phases - two days longer than how long it takes the moon to go around Earth.
Twelve of these cycles or months, gives us 354.56 days. Lots of cultures had measured that the solar year, Earth's orbit around the sun, was closer to 365.25 days, and created systems to account for this.
The Roman calendar systems were a mess. They wanted 12 months like the lunar calendar, but one based on the sun, that would average to 365.25 days. These previous Roman systems devised that the calendar year would change between 355 days, 377 days, 355 days, 378 days, and then repeat.
While this makes the average year 366.25 days, they had a solution. Over a period of eight years, they would shorten one of the eighth years, by seven days, to be closer to the 365.25 average.
This is the reason leap day is in February - as it was previously the last month added into the old calendar systems and was the shortest month when they needed to have less days in this cycle.
How do you determine when this eighth year occurs? The politicians got to choose, which quickly became a political tool and predictably, didn't work.
Julius Caesar introduced the concept of leap year - and that every year which is divisible by four is a leap year - to average to 365.25 days.
This would be OK, except the Earth's path around the sun is closer to 365.2422 days, and so over a few hundred years, we'd end up with extra days.
The implementation was also not smooth.
Over time, the Julian Calendar ended up with too many days, and the calendar ended up being off from seasonal equinoxes and solstices.
This caused Easter, which is determined by first full moon after the northern hemisphere spring equinox, to occur later and later.
In 1582, Pope Gregory, created a new system to fix this - the modern-day leap year system.
This system adds an extra day in February every four years.
Unlike the Julian calendar, leap year is skipped if the year is divisible by 100, and this rule is skipped in years divisible by 400, which makes the average year over a long period, closer to the real 365.2422 days.
For instance, the year 2000, also a summer Olympic year and US presidential election, was a leap year as it was divisible by 400.
For those who will be around in 2100, it won't be.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.
