Editing 2897: Light Leap Years

{{comic
| number    = 2897
| date      = February 21, 2024
| title     = Light Leap Years
| image     = light_leap_years_2x.png
| imagesize = 288x389px
| noexpand  = true
| titletext = When Pope Gregory XIII briefly shortened the light-year in 1582, it led to navigational chaos and the loss of several Papal starships.
}}

==Explanation==

The comic features [[Cueball]] and [[Ponytail]] updating astronomical distances in a database. The caption imagines a world in which {{w|leap year}}s, which add an extra day to the year, making it 366 days long instead of 365, purportedly extend light-years by 0.27% due to the additional day (366/365 = 1.0027397...). This adjustment ostensibly reduces the number of light years to celestial bodies like Alpha Centauri by a corresponding percentage — a relatively small amount, but one that corresponds to approximately 730 times the average Earth-sun distance. The comic was released about a week before the leap day of 2024, a leap year.

The joke hinges on the fact that in most common usages years have a variable length, with the {{w|Gregorian calendar}}'s leap year system — adding a day every four years to align the calendar year with the astronomical year — being the current civil standard in most of the world. A {{w|light year}}, defined in astronomy as the distance light travels in a vacuum over a Julian year (365.25 days), remains constant at 9,460,730,472,580.8 km, unaffected by the Gregorian calendar's leap years. However, the comic amusingly suggests that leap years lengthen light years, necessitating database updates for astronomical distances.

The title text imaginatively claims {{w|Pope Gregory XIII}}, who introduced the Gregorian calendar in 1582, inadvertently affected the length of the light-year. This is not due to 1582 being a leap-year (it was [https://www.customcalendarmaker.com/leap-years ''not'' a leap-year]), but because of the calendar days that had to be skipped to remove the timing error built up when using the prior (and less correct) Julian method of leap-years. Those adopting the system in 1582 had to shorten this year by ten days. {{w|List of adoption dates of the Gregorian calendar by country|Later adopters}} may have had to shorten the year that they ''did'' change by up to 13 days, i.e. up to three extra days for every four whole centuries spent on the 'wrong' calendar.

A year of effectively 355 days, rather than 365, would therefore lead (by this comic's premise) to potential misunderstandings/misapplications of distance approaching 3%, leading to "navigational chaos" and the loss of "Papal starships." This satirizes the significant historical impact of calendar reforms on navigation and measurement, despite the anachronism, as the light-year wasn't defined until 1838 and the concept of a finite speed of light only emerged in 1676 with {{w|Rømer's determination of the speed of light}}, whilst practical starships (papal or otherwise) that would usefully rely upon light-year measurements, have yet to be developed (on Earth, at least, the only place where such light-year measurements might originate). However, navigational chaos ''has'' been a cause of maritime shipwrecks, such as the notable {{w|Scilly naval disaster of 1707}} in which four ships were lost and over 1,400 sailors died due to navigational errors.

Alternatively, the title text could be interpreted as a joke about how the light-year in astronomy is based on the Julian year (365.25 days) rather than the mean Gregorian year (365.2425 days). The pope may have briefly changed that definition, leading to "navigational chaos". Although the difference between a Julian light-year and a Gregorian light-year is only about 20 parts per million, it still amounts to about 194 million km (121 million mi) per light year.

The comic implies the distance to Proxima Centauri varies slightly between leap years and non-leap years, according to the leap year calculation. In reality, astronomers wouldn’t be bothered by this change: they use the {{w|parsec}} for interstellar distances, a unit based on angular measurements unrelated to Earth's calendar.

This is the second comic in the last 10 about how differing interpretations of standard units could have absurd real-world implications, the other being [[2888]].

===Discussion of the use of light year values in the comic===
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 {{w|International Astronomical Union}}, which is only minutely different from 4.2465 light-years, the value given by {{w|Gaia catalogues|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 {{w|parsec}} ("''pc''", or useful multiples such as ''kpc'', ''Mpc'', or ''Gpc''). One of those is approximately 3.24 light years, so has a [[2205: Types of Approximation|similar astronomical magnitude]], but is founded upon common interpretations of distance and angle instead of time. (Both partly rely upon baseline measures that are complementary aspects of Earth's orbit, i.e. its periodicity and radius, which theoretically make for a globally agreeable system; but highly unlikely to match whatever equivalent any non-terran scientists would independently develop.)  While light-years, and {{w|Light-year#Related units|related units}}, are common in publications intended for non-astrophysicists and for the benefit of laypersons, they are generally considered as secondary usefulness to parsecs within the actual fields of astronomy and astrophysics research. As such, it is highly likely that the clearly exacting database that Cueball and Ponytail are in the process of modifying is not even keyed to any light-units, making leap-/non-leap-light-years already an automatic conversion that the system may pander for without such a direct interaction.

==Transcript==
:[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, who is 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:]
:<u>Proxima Centauri</u>
:Distance: [in red, crossed out] <span style="color:red"><s>4.2493 ly</s></span>
:[in green] <span style="color:green">4.2377 ly</span>

:[Caption below the panel:]
:Astronomers hate leap years because they make light-years 0.27% longer.

{{comic discussion}}

[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Comics with color]]
[[Category:Astronomy]]
[[Category:Time]]
[[Category:Calendar]]
[[Category:Comics featuring real people]]