Just like Neil deGrasse Tyson will be forever known as Pluto’s discombobulator – for everyone who still thinks Pluto is a planet, even though it was the International Astronomical Union who was responsible for demoting Pluto – via VOTING, Sosigenes of Alexandria might be accountable for leaplings despite what we think of Julius Caesar.
Leaplings, according to the dictionary, are people who have their birthday once every four years on the 29th of February. And all of that because of one man.
So what did Sosigenes do?
According to Pliny (Roman author, naturalist, natural philosopher, naval and army commander and now a beer),
… There were three main schools, the Chaldaean, the Egyptian, and the Greek; and to these a fourth was added in our country by Caesar during his dictatorship, who with the assistance of the learned astronomer Sosigenes brought the separate years back into conformity with the course of the sun.
Pliny, Natural History 18.211
A mathematician of the famous Alexandrian School, Sosigenes, knew his maths. He figured out Mercury’s orbit, in a long line of scientists who studied it, finishing with Albert Einstein. Sosigenes confirmed Babylonian astronomer Kidinnu’s observations: Mercury is never further than 22° from the Sun. How much work he did for Caesar, however, it is unclear. That’s because Caesar really liked astronomy (Dialetis, D. (2007). Sosigenes of Alexandria. In: Hockey, T., et al. The Biographical Encyclopedia of Astronomers. Springer, New York, NY.)! He liked astronomy so much that even wrote a treatise. “De Astris” (About Stars), was like a really good farmer’s almanac, a hit with the astronomy fans of the season.
The Genesis of Leap Years
The story of leap years begins in 45 BCE. The with the Roman calendar was in disarray, having drifted significantly from the seasonal cycle it was meant to track. It wasn’t tracking, actually. The amount of time it takes the Earth to go around the Sun is 365.2422 days. The amount of time it takes to have a lunar year is 12 lunar months, about 354 days. How did the Romans cope?
Before Julius Caesar’s reform, the Roman calendar had months that varied in length. The year was typically 355 days. February was chopped and changed a couple times. Here and there they would sneak in an intercalary month, called Mercedonius or Intercalaris, to align with the solar year.
Solar vs Lunar years
Many cultures had to deal with this. In the 3rd century BCE by Ptolemy III Euergetes suggested a similar arrangement as Sosigenes and Caesar later implemented. His subjects had refused to follow it. Some cultures who would have had enough of the Moon nonsense switched to a solar year. They were in dire need to measure the middle of winter accurately so they could plan their food supplies and not starve. The winter solstice is a popular date among those who experience a snowy season.
Out of sync
The amount of time takes Earth to go around the Sun once does not match 365 full days. Nor 366. It’s like the Earth needs 365 days and a morning. Or a good afternoon. Preciselly 365.2422 – that is 365 days, 5 hours, 48 minutes, and 46 seconds. This is the length of a tropical year (or a solar year).
90 days were added to the calendar to bring the year back in sync.
So the legend goes that Sosigenes proposed a simple yet revolutionary solution. Add a day to the calendar every four years. It accounts for the extra approximately six hours it takes the Earth to orbit the Sun beyond the 365-day year. This leap day, added to February, brought the calendar back in alignment with the Earth’s annual journey around the Sun.
Leaplings: Born into Rarity
Leaplings, those born on the elusive the 29th of February, owe their unique birthday to this ancient adjustment. With the day occurring only once every four years, leaplings find themselves in a rarefied group. The odds of being born on the 29th of February are about 1 in 1,461. Leaplings are a distinct minority in the global population.
Famous leaplings:
Jack Lousma – NASA astronaut (https://en.wikipedia.org/wiki/Jack_R._Lousma), Tony Robbins (motivational speaker) and Gioachino Antonio Rossini (composer) and the most famous of all, Superman – all are celebrated on the 29th of February.
Jack Lousma was the Pilot on Skylab 3, the second crew aboard America’s first space station – July 28 to September 25, 1973. This was NASA’s longest manned mission at the time.
Tony Robbins is famous for being a motivational speaker, Gioachino Rossini composed the William Tell overture,
and Superman…is well, Superman (known best by people born before 1978 when the movie was produced).
Cultural and Legal Quirks
The leap year phenomenon has spawned a wealth of cultural traditions and legal oddities around the world. In some places, Leap Day has been seen as a time when the normal rules of courtship were upended. Women could propose marriage to men. Meanwhile, legal systems have had to adapt to the peculiarities posed by leaplings. They had their own approaches to determining their legal birthdays in common years.
Why did the Julian Calendar need to be replaced?
The Julian year had a flaw too. Sosigenes overestimated the length of the year by about 11 minutes. This might seem minor, but over centuries, it added up. By the time of Pope Gregory XIII in the 16th century, the Julian calendar had drifted by approximately 10 days.
This was bad for calculating Easter – important religious event based on the vernal equinox. By the 1580’s, the equinox, marking the start of spring, had shifted from March 21 (as observed by the Council of Nicaea in AD 325) to a date 10 days earlier. This was disrupting the Christian liturgical calendar.
10 days were skipped
To correct this drift and realign the calendar with the seasons, Pope Gregory XIII introduced the Gregorian calendar in 1582. With the assistance of a commission of astronomers and mathematicians, the calendar got two adjustments. They skipped ten days to bring the calendar back in alignment with the astronomical seasons. This is known as the Correction of the Drift. October 4, 1582, was followed directly by October 15, 1582, in countries that adopted the Gregorian calendar immediately.
A new Leap Year Rule was added
To prevent future drifts, the Gregorian calendar refined the leap year rule. A year would still be a leap year if it is divisible by 4. However, years divisible by 100 would not be leap years, unless they are also divisible by 400. This adjustment reduced the average year length to approximately 365.2425 days, closely matching the solar year.
Happy Birthday to our Leaplings
Leaplings, as they are affectionately known, navigate a world where their official birthdays come only once every four years. They have a unique perspective on the passage of time and the celebration of personal milestones. The mere existence of the 29th of February is a subject that is at once a scientific endeavor, a cultural curiosity, and a reflection of the human condition. It is a topic that invites us to look skyward and inward, pondering our place in the cosmos and the ways we choose to mark the moments of our existence on this spinning blue orb we call home.
What’s next?
There is still a minor discrepancy of about 26 seconds per year, or roughly 1 day every 3,236 years in the Gregorian calendar. That’s because the average year length in the Gregorian calendar is 365.2425 days, closely matching the solar year’s length of approximately 365.2422 days. So it would be around 10,000 years before the Gregorian calendar drifts by approximately three days relative to the current alignment with the solar year.
However, this estimate depends on the assumption that the current length of the solar year remains constant, which it does not. Due to gravitational interactions among the Earth, Moon, and other bodies in the solar system, the Earth’s rotation is gradually slowing, and the length of the solar day is increasing over long time scales. Additionally, the Earth’s orbital period itself can change due to various gravitational influences and other factors affecting the dynamics of the solar system.
In practical terms, the Gregorian calendar’s alignment with the seasons is stable enough for most foreseeable human timekeeping needs. Any need for adjustment is likely to be so far in the future that current calendar systems may have evolved or been replaced by then.
Glossary
- Tropical Year: The tropical year, also known as the solar year in the context of Earth’s seasons, is the time it takes for the Sun to return to the same position in the cycle of seasons, such as from vernal equinox to vernal equinox. It’s the basis for most calendar systems, including the Gregorian calendar, because it corresponds to the cycle of seasons. The length of the tropical year is approximately 365.2422 days. This measurement is crucial for agricultural planning and understanding climate patterns.
- Solar Year: The term “solar year” can be used more broadly to refer to the time it takes for the Earth to complete one orbit around the Sun relative to any fixed point. However, when discussing the basis of calendar systems and the cycle of seasons, it’s synonymous with the tropical year. In other contexts, “solar year” might be specified as:
- Sidereal Year: The time it takes for the Earth to complete one orbit around the Sun relative to the fixed stars, which is about 365.25636 days. This is slightly longer than the tropical year because of the precession of the equinoxes, a slow, top-like wobble in Earth’s rotational axis.
- Anomalistic Year: The time between two consecutive passages of the Earth through perihelion, the point in its orbit closest to the Sun. The anomalistic year is about 365.2596 days, slightly longer than the sidereal year due to the gradual movement of the perihelion caused by gravitational interactions, primarily with Jupiter.
The distinction between these types of years is important in astronomy for understanding the Earth’s orbit and for precise timekeeping. For most practical purposes, including calendar design and the marking of seasons, the tropical year is the relevant measure.
