History remembers Julius Caesar as a conqueror. It rarely remembers the man who actually fixed the clock. That man was Sosigeness of Alexandria.
He was a Greek astronomer and mathematician. Probably born in Alexandria. He worked in the shadow of one of the most powerful figures in Rome. His job? Sort out the mess that was the Roman calendar.
Caesar was not just any general. He was in the midst of a brutal civil war (49-45 BC). The dates were all over the place. Different regions used different systems. It was chaos. To rule an empire, you need a shared time. Sosigeness proposed a solution.
Egyptian solution with a twist
Sosigeness looked north, towards Egypt. They had a solar calendar. It lasted 365 days. Simple. Accurate.
Caesar adopted it. But he added a rule. Add one day every four years. Leap year.
It wasn’t entirely new. Ptolemy III Euergetes tried a similar trick in the 3rd century BC. At the time, the Egyptians refused to participate. So they stuck with the old lunar-lunisolar mess. Caesar had the power to enforce the change. Sosigeness provided the math.
To make it work immediately, they had to reset the clock. They added 90 days to the year 46 BCE. This moved the vernal equinox to the end of March. It adjusted the calendar according to the seasons. The earlier Roman calendars had drifted considerably. The equinox was around March 25. Now it was back where he belonged.
Leap Year Glitch
This is where things get interesting. The implementation was not perfect.
There was a misunderstanding. Big. Sosigeness said “every four years”. Roman scribes read “every three years.”
Why? Inclusive counting. The Romans counted the starting year as year one. So 1, 2, 3… that’s three years. They skipped a beat. For a while, leap days were added too often. The calendar ran fast.
It took decades to fix the problem. Augustus eventually corrected the error. He canceled several leap years to reset the drift. Sosigeness’ theory was correct. The execution was Roman. Sloppy.
Sosigeness may also have written the astronomical almanac that accompanied the new calendar. It helped people understand the change. The Julian calendar is basically the same as what we use today as the Gregorian calendar with minor tweaks. You are reading this on a timeline that Sosigeness helped create.
Lost Writings and Mercury’s Orbit
We don’t have much of Sosigeness’ own works. He is said to have written three treatises on calendrical science. They are lost. Gone.
What we know comes from other sources. the Roman scholar Pliny the Elder mentioned Sosigenes. In particular, he noted that Sosigeness agreed with the Babylonian astronomer Kidinnu.
Kidinnu had noticed something about Mercury. The planet never strays from the Sun by more than 22 degrees. It’s always close.
Some historians have jumped to conclusions about this. They suspect that Sosigeness taught that Mercury orbits the Sun. That’s probably wrong. The evidence is weak.
