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Light Leap Years
When Pope Gregory XIII briefly shortened the light-year in 1582, it led to navigational chaos and the loss of several Papal starships.
Title text: When Pope Gregory XIII briefly shortened the light-year in 1582, it led to navigational chaos and the loss of several Papal starships.


Ambox notice.png This explanation may be incomplete or incorrect: Created by A FLEET OF PAPAL STARSHIPS FROM ANNO DOMINI MDLXXXII - Please change this comment when editing this page. Do NOT delete this tag too soon.

The comic portrays Cueball and Ponytail updating astronomical distances in some sort of database, noting how long and unpleasant the process is; the caption reveals that the reason is that leap years "make light-years 0.27% longer" (366/365 = 1.0027397...). This makes the distance to Alpha Centauri "0.27% shorter". 2024 is a leap year in the Gregorian calendar, and leap day (February 29) was just over one week away when this comic was released.

The joke of this strip is based on the fact that "one year" isn't a precise unit of measurement: there have been different definitions, evolving over time, of what constitutes a year. The Gregorian calendar (the one most commonly used in modern times) includes a system of leap years in which an additional day is added every fourth year (with some exceptions) to make up for incompatibilities between day and year cycles. This temporarily changes the length of a year from 365 to 366 days.

A light year is a unit of distance, commonly used in astronomy, equal to the distance light travels in a vacuum in one year; the year used is the Julian year, or 365.25 days. This results in a light year which is standardized at 9,460,730,472,580.8 km, no matter how long the calendar year may be. However, in this comic, a light year has been defined based on the length of the current year, and consequently becomes longer during leap years, meaning databases with astronomical distances have to be adjusted. Thankfully, most systems of measurement do not change continually, and even those that do (eg. DST) usually are setup to automatically update when necessary.

The title text jokes that Pope Gregory XIII, the originator of the Gregorian calendar, "briefly shortened the light-year in 1582." What really occurred in 1582 was that the Pope decided to advance the previously Julian calendar by 10 days to make up for an accumulated excess of past leap days and bring the subsequent Gregorian one more into line with astronomical measurements. Not all places went with the change, at that time. Some of the later adopters had to skip yet other days once they did, while others continue to use a calendar with an offset factor. In the world of the comic, this change led to "navigational chaos and the loss of several Papal starships". This is of course ludicrous since there have not (yet) been any (known earthly) starships, nor any church-funded space programs that might create a "Papal starship",[citation needed] still less in the 16th century. (There have been vehicles named 'Starship', but these do not meet the common definition of large craft that can travel between star systems.) Furthermore, the light-year wasn't developed as a unit of measurement until 1838. Indeed, it wasn't known that the speed of light is finite until Rømer's determination of the speed of light in 1676. Navigational chaos has been a cause of shipwrecks, notably the Scilly naval disaster of 1707 in which 4 ships were lost and over 1,400 sailors died.

The joke is that the evolving and somewhat loose and changing definitions of early calendars had significant impacts on the units of measurement we still use today. Such changes did serve to catalyze political and religious conflicts in some instances, and raised temporary issues around matters such as taxes, rents, etc., but as technology has advanced and become increasingly reliant on precise and consistent measurements, they could be significantly more disastrous.

The values given for Proxima Centauri's distance from the Sun, 4.2377 light-leap-years and 4.2493 light-nonleap-years, are consistent with a distance of 4.2464 actual light-years as described by the International Astronomical Union, which is only minutely different from 4.2465 light-years, the value given by Gaia Data Release 3 in 2020. Though tiny on an interstellar scale, the difference between 4.2377 and 4.2493 light-years, 0.0116 light years, equals 109.7 billion km (68.2 billion miles), about 730 times the average distance between the Earth and the sun (150 million km or 93 million miles).

Ironically, this kind of change would not actually bother astronomers in the slightest. Astronomical distances on scales larger than the solar system are universally (or rather, globally: we do not know how things are done in other parts of the universe) measured with the parsec (one of these being approximately 3.24 light years; founded upon anthropogenic ideas of scale and angle, but lacking reliance to our time conventions) or multiples thereof (kpc, Mpc, or Gpc). While light years are common in science popularizations, they are essentially not used at all in astronomy and astrophysics research.


[Cueball is sitting at a desk with a laptop on it and leaning to the back of his office chair, while having his other hand on the laptop. He is looking at Ponytail standing behind him. The text from the laptop screen is shown above it, indicated with a zigzag line.]
Cueball: It took until February, but I finally got all the distances updated!
Ponytail: I really wish we didn't have to do this.
[Laptop screen:]
Proxima Centauri
Distance: 4.2493 ly 4.2377 ly
[Caption below the panel:]
Astronomers hate leap years because they make light-years 0.27% longer.

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