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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 PHOTON MOVING AT EXACTLY 9,478,164,812,659,200 M/LEAP YEAR - Please change this comment when editing this page. Do NOT delete this tag too soon.

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. 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, which could be taken to change the length of a light year. In this comic, Randall assumes that a light year is based on the length of the current year, which means that during leap years, it's based on 366 days, and during non-leap years, 365 days. That means that at the start and end of leap years, databases with astronomical distances have to be adjusted.

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 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".

Standardized systems of measurement naturally don't change continually. As the comic points out, the difficulty in having to regularly update every reference to these units would be enormous and pointless. In real life, a light year is defined by the Julian year, defined as 365.25 days, with each day being 86,400 SI seconds in length. 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.

The title text jokes that Pope Gregory XIII, the originator of the Gregorian calendar, "briefly shortened the light-year in 1582." In reality he shortened the year, as he decided to advance the Julian calendar by 10 days to make up for excess past leap days. This change led to "navigational chaos and the loss of several Papal starships". This is of course ludicrous since there were no starships in the 16th century, there's never been a "Papal starship",[citation needed] and 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. 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 were of only minor significance at the time, but as technology has advanced and become increasingly reliant on precise and consistent measurements, such changes could be disastrous.

The values given for Proxima Centauri's distance from the Sun, 4.2377 light-years during leap years and 4.2493 light-years otherwise, are consistent with a distance of 4.2464 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. This error happens to be consistent with Randall potentially wrongly assuming that the light-year is defined using the Gregorian year (365.2425 days) rather than the Julian year (365.25 days).


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
[Cueball is sitting at his laptop 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: [in red, crossed out] 4.2493 ly
[in green] 4.2377 ly
[Caption below the panel:]
Astronomers hate leap years because they make light-years 0.27% longer.

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