Editing 1930: Calendar Facts

{{comic
| number    = 1930
| date      = December 18, 2017
| title     = Calendar Facts
| image     = calendar_facts.png
| titletext = While it may seem like trivia, it (causes huge headaches for software developers / is taken advantage of by high-speed traders / triggered the 2003 Northeast Blackout / has to be corrected for by GPS satellites / is now recognized as a major cause of World War I).
}}

==Explanation==
This is the second comic using [[:Category:Facts|Facts]] in the title.

[[Randall]] presents what appears to be a generator of 156,000 facts [20 x 13 x (8 + 6 x 7) x 12] (780,000 if including the title text) about calendars, most of which are false or have little meaning{{Citation needed}}. The facts are seeded by a mishmash of common tidbits about the time of year.

The formula for each generated fact goes as follows: "Did you know that '''[a recurring event]''' '''[occurs in an unusual manner]''' because of '''[phenomena or political decisions]'''? Apparently '''[wild card statement]'''." The title text adds on as follows: "While it may seem like trivia, it '''[real-life consequence]'''."

This is [[:Category:Supermoon|the fifth time]] that Randall has referred to the phenomenon of a {{w|supermoon}}, which he typically makes fun of, most prominently in [[1394: Superm*n]].

The title text continues the chart with supposed real-life consequences of the trivia in the comic.

There are multiple online generators of Calendar 'facts' using this formula [https://perchance.org/xkcd-1930 here] and [http://yahel.com/calendarfacts/ here].

All 156 000 possible combinations can be found [https://www.dropbox.com/s/866fwtpwvd0z9hq/combinations%20xkcd%201930.txt?dl=0 here], lovingly assembled by hand (or rather, by a Python script) for your entertainment. A random fact generator (including title text), written in Python, can be found [https://gist.github.com/petersohn/6c8f9d124bd961e909d2dc9a967ade2e here].

==Elements==
{| class="wikitable"
! Entry
! What it is
! Relation to other entries
|-
! colspan="3" | Recurring Events
|-
| The [Fall/Spring] {{w|Equinox}}
| The time of year at which the apparent position of the overhead sun passes the equator. During the equinox, the time that the Sun is above the horizon is 12 hours across the globe.
| Before the adoption of the {{w|Gregorian calendar}} in 1582, the equinoxes fell on earlier and earlier dates as the centuries went by, due to the {{w|Julian calendar}} year being 365.25 days on average compared to the tropical Earth year of 365.2422 days. {{w|Pope Gregory}}'s decision to remove the leap days on years that were multiples of 100 but not 400 corrected the average length of the calendar year to 365.2425 days.
|-
| The [Winter/Summer] {{w|Solstice}}
| The time of year when the apparent position of the overhead sun reaches its most extreme latitude. During the Winter and Summer solstices the days are the shortest and longest respectively.
| Similar to the equinoxes, the solstices were also falling on earlier dates every year before the Gregorian calendar.
|-
| The [Winter/Summer] {{w|Olympics}}
| The Olympic Games occur during the summer and the winter, alternating between the two seasons every two years.
| The Olympic Games do not have any set dates, and seem to only be included humorously as something else that alternates between occurring during the summer and winter.
|-
| The [latest/earliest] [sunset/sunrise]
| The extremes of times that the sun crosses a horizon according to a clock that keeps a fixed 24 hours as opposed to varying with the sun like a sundial.
| The latest sunset and earliest sunrise occur around the summer solstice; the latest sunrise and earliest sunset occur around the winter solstice. They do not occur exactly on these dates due to the {{w|equation of time}} causing drift in the times that sunsets and sunrises occur.
|-
| Daylight [saving/savings] time
| {{w|Daylight saving time}}, commonly referred to as daylight savings time, is the practice of setting clocks ahead, typically by one hour, during the summer months of the year. 
| Daylight saving time will push the time of certain events such as sunrise and sunset past their "natural" times. For example, solar noon will occur around 1:00 PM instead of 12:00 noon when daylight saving time is active, making it the "wrong" time.
|-
| Leap [day/year]
| Because the durations of celestial events are not generally nice multiples of each other, they will tend to fall out of sync with each other. Leap days are days inserted into specific years to bring the calendar back into sync, and the years on which these {{w|leap day}}s occur are called {{w|leap year}}s.
| 
|-
| {{w|Easter}}
| Easter is a holiday celebrating the death and resurrection of {{w|Jesus}}. Traditionally it was defined as the Sunday after {{w|Passover}}; today, it is the first Sunday after the first full moon after the spring equinox. This complicated formula has a long tradition behind it, known as {{w|Computus}}.
| When Pope Gregory decided to change the calendar in 1582, it was because the spring equinox was putting Easter on unexpectedly early dates.
|-
| The [harvest/super/blood] moon
|
* The {{w|harvest moon}} is the full moon that appears closest to the autumnal equinox.
* The {{w|supermoon}} is a phenomenon in which the moon is full at its closest approach to the Earth.
* The {{w|blood moon}} is a moon that appears tinted red during a total lunar eclipse because of light refracted from the Earth's atmosphere. It can also refer to the {{w|hunter's moon}}, the full moon directly after the harvest moon.
| Each of these lunar events happens approximately once a year.
* The harvest moon appears exactly once because it has a particular definition based on the time of year.
* The cycle of the distance of the full moon lasts about 13.5 months (14 full moons). However, because a supermoon is defined as any full moon that is within 10 percent of the closest relative distance possible (with 0 being perigee and 1 being apogee), it happens multiple times a cycle, for a total of usually 3 to 4 times per year.
* The blood moon during a lunar eclipse appears between zero to two times a year. The hunter's moon appears exactly once like the harvest moon.
|-
| Toyota Truck Month
| Toyota offers a discount for {{w|Toyota Tacoma|Tacoma}} trucks one month a year. Mainly notable because radio and television ads hype this discount up as "Toyota Truck Month".
| 
|-
| {{w|Shark Week}}
| Every year, the {{w|Discovery Channel}} dedicates a week during the summer to programming featuring or about sharks.
| 
|-
! colspan="3" | Unusual manners in which the events occur
|-
| happens [earlier/later/at the wrong time] every year
| colspan="2" | The solstices and equinoxes happened earlier every year ''before'' the decree by Pope Gregory in 1582. The earliest sunrise happens one hour later than it "should" happen due to daylight saving time having turned the clocks forward one hour.
|-
| drifts out of sync with the [sun/moon]
| colspan="2" | The Sun and Moon are generally what calendars are based on. If something were to drift out of sync, some corrective mechanism would have to be put in to put it back. This is the motivation behind leap years, leap months (in countries with lunisolar calendars) and leap seconds.  
|-
| drifts out of sync with the zodiac
| colspan="2" | The dates on which the Sun crosses the constellations in the traditional zodiac has shifted in the past centuries due to the precession of the Earth's axis. In the period of time traditionally known as {{w|Aries}} (March 21–April 20), for example, the Sun actually points to {{w|Pisces}} instead.
|-
| drifts out of sync with the [Gregorian/Mayan/lunar/iPhone] calendar
| colspan="2" | 
*The {{w|Gregorian calendar}} is a solar calendar with a mean calendar year length of 365.2425 days. 
*The {{w|Mayan calendar}} is based on two cycles or counts, with a 260-day count combined with a 365-day "vague" solar year.
*A {{w|lunar calendar}} is based on Moon's phases, with each {{w|lunation}} being approximately 29.5 days, and a lunar year lasting roughly 354 days. An example of a lunar calendar is the {{w|Hebrew calendar}}.
*The {{w|iPhone calendar}} is listed humorously due to its data synchronization issues.
|-
| drifts out of sync with the atomic clock in {{w|Colorado}}
| colspan="2" | 
{{w|NIST-F1}} is an {{w|Atomic clock}} used as a reference for official time in the USA.
|-
| might [not happen/happen twice] this year
| colspan="2" | Some events may have a period of slightly more or slightly less than one year. If an event has a period of slightly less than one year (e.g. the Islamic calendar), it can occur twice in the same solar year (e.g. the year 2000 had two {{w|Eid al-Fitr}}s—one on January 8, and one on December 28). If an event has a period of slightly more than one year, there can be a year in which it does not occur at all, instead occurring near the end of the previous year and the beginning of the next.
|-
! colspan="3" | Cause (phenomena or political decisions)
|-
| time zone legislation in [Indiana/Arizona/Russia]
| Some states or provinces have time zone legislation that sets the standard time to something other than what the natural longitude of that location would suggest.
|
* The state of {{w|Arizona}} generally does not observe daylight saving time, keeping their clocks on {{w|UTC-7:00}} Mountain Standard Time year round. However, the {{w|Navajo nation}} reservation inside Arizona does observe it, causing the two regions to have different times in the summer and the same time in the winter.
* Time zones in Russia are all one hour ahead of what their longitude would suggest, which puts them in a "permanent" state of daylight saving time. (For example, {{w|St. Petersburg}} is 30°E, which means that its natural time zone is {{w|UTC+2:00}}, but its time zone is actually {{w|UTC+3:00}}.) From 1981 until 2011 Russia used to have the daylight saving time on top of it as well. The other changes include the abolition of the one-hour shift in 1991 and a return it back in 1992, and an increase to two hours in 2011 and a restoration back to one hour in 2014.
* {{w|Indiana}} has {{w|Time in Indiana|a complicated history}} with daylight saving time, likely related to the state being split between two time zones.
|-
| a decree by the Pope in the 1500s
| In 1582, Pope Gregory introduced the Gregorian Calendar, the calendar we use today, to replace the Julian Calendar. The calendar applied retroactively to the birth of Jesus Christ, which means that they had to skip 10 days, going straight from October 4 to October 15, 1582, during the switchover.
| The introduction of the Gregorian calendar brought Easter and the dates that months started back in sync with what they were in the 3rd century AD.
|-
| the precession of
| The Earth's axis is slowly changing position, in a phenomenon called the {{w|Axial precession|precession of the equinoxes}}. 
| The precession of the equinoxes causes the seasons to occur about 20 minutes earlier than would be expected with the Earth's position relative to the stars, which could be construed as the equinox happening "later every year" if you use the stars as your frame of reference.
|-
| the libration of
| The Moon is {{w|tidal locking|tidally locked}} to its orbit around the Earth, which means that the same side of it tends to face the Earth at any given point in time. However, there are slight variations in the angle over the course of a month, which are known as {{w|libration}}.
| The libration of the Moon does not affect anything else in the chart, and seems only be included humorously as another example of a celestial phenomenon.
|-
| the nutation of
| Besides precession, there is also a smaller wobbling effect called {{w|Astronomical nutation|nutation}}.
| 
|-
| the libation of
| A {{w|libation}} is a drink, often used in the context of a ritual offering of liquid to a deity by pouring it onto the ground or into something that collects it.
| This entry seems to have been included simply as a humorous misspelling of the word "libration". Certainly libation of any of the entities listed would be inadvisable.{{Citation needed}}
|-
| the eccentricity of
| Orbital eccentricity is the deviation of a body's orbit from a perfect circle. Orbital travel is faster when it's closer to the body being orbited and slower when farther away.
| The Earth's eccentric orbit causes the equinoxes and solstices to occur at irregular intervals. For example, summer in the northern hemisphere lasted 93 days in 2017, while fall only lasted 90 days.
|-
| the obliquity of
| The tilt of the Earth's axis relative to the ecliptic is also known as its obliquity.
| 
|-
| the Moon
| The Moon is the primary satellite of the Earth.
| 
|-
| the Sun
| The Sun is the star that the Earth orbits around.
| The Sun is the basis for many timekeeping events, such as the day and year.{{Citation needed}}
|-
| the Earth's axis
| The Earth's axis of rotation defines the Geographic North and South Pole, as well as the lines of latitude.
| 
|-
| the Equator
| The Equator is the line on the Earth's surface which is equidistant from both poles of the Earth's axis.
| 
|-
| the Prime Meridian
| The Prime Meridian is the line that starts at the geographic North Pole, runs through the {{w|Royal Observatory, Greenwich|Greenwich Royal Observatory}} in London, and ends at the South Pole. It is the basis for longitude when calculating coordinates for positions on the surface of the Earth.
| The Prime Meridian (and in particular the Greenwich Observatory) gives us Greenwich Mean Time (GMT), which is the basis for UTC and the time zone system.
|-
| the International Date Line
| The International Date Line is a line on the opposite side of the Earth as the Prime Meridian that separates regions that use time set behind UTC versus regions that are set ahead of UTC. It has many irregularities due to political changes that put certain countries or islands on either side of the divide contrary to their natural longitude.
| The irregular shape of the International Date Line means that certain regions of the Pacific Ocean (such as Kiribati) are more than 24 hours ahead of some other regions (such as Baker Island and American Samoa), which may cause problems with timekeeping.
|-
| the Mason-Dixon Line
| The Mason-Dixon line is a line delineating a portion of the border between Pennsylvania, Maryland, and Delaware.
| The Mason-Dixon line is included as a humorous example as another imaginary geographic line.
|-
| magnetic field reversal
| The Earth's magnetic field has been reversed several times in its geologic history, so that what we would currently call the "magnetic North Pole" was near the geographic South Pole about 780,000 years ago, before the most recent reversal.
| 
|-
| an arbitrary decision by Benjamin Franklin
| Benjamin Franklin wrote [http://www.webexhibits.org/daylightsaving/franklin3.html a letter to the Journal of Paris in 1784] in which he advised them to rise with the sun in order to save candlelight, after he observed that the Parisians were getting up at the same time by the clock and burning a lot of candles in the winter as a result.
An "arbitrary decision by Benjamin Franklin" also likely refers humorously to Franklin having defined positive charge to be that which is left on a glass rod by rubbing it with silk. As described in [[567: Urgent Mission]], this had the unfortunate consequence of assigning a negative value to the charge of the electron, which was later identified as the fundamental carrier of electric charge.
| Benjamin Franklin is often touted as "the father of daylight saving time", despite him never actually proposing to alter the clocks.
|-
| an arbitrary decision by Isaac Newton
| Possibly a reference to how Newton divided the color spectrum into the now-familiar seven colors of the rainbow, on a somewhat arbitrary basis. Newton did spend time working on the problem of calendar reform, but it's unlikely that any decisions he made as a result would affect anything, since he never published his work, and by the time it gained attention the Gregorian Calendar had been widely adopted.
| The spectrum fact is one of those standard bits of trivia of the kind the chart alludes to. Although it has nothing to do with time-keeping, Newton is the sort of person who seems like he should have made decisions like this. 
|-
| an arbitrary decision by FDR
| Franklin Delano Roosevelt set all time zones one hour ahead year-round during World War II. The law was repealed after the war ended.
Additionally, he changed the date of Thanksgiving from the last Thursday in November to the third Thursday in November as a way to increase the length of the Christmas shopping season. It was later changed to the fourth Thursday after his death.
| Setting the time permanently one hour ahead would make everything happen at the "wrong" time celestially.
|-
! colspan="3" | Related 'fact'
|-
| It causes a predictable increase in car accidents.
| colspan="2" | The week following daylight saving time, car accidents increase by about 5-7%.<ref>http://www.cbc.ca/news/canada/end-of-daylight-saving-time-2015-6-eye-opening-facts-1.3296353</ref>
|-
| That's why we have leap seconds.
| colspan="2" | Leap seconds occur because the time required for one rotation of the Earth is actually slightly longer than the 86,400 seconds in a standard UTC day. The Earth's rotation is slowing down by about 2 × 10<sup>-5</sup> seconds every year due to tidal friction caused by the Moon's gravity; however, this is not one of the possible entries in the list of phenomena.
|-
| Scientists are really worried.
| colspan="2" | 
|-
| it was even more extreme during the [Bronze Age/Ice Age/Cretaceous/1990s].
| This may be reference to debates over climate change, where global temperature changes during these periods are frequently cited as supposedly proving / disproving human-related change.
| Solar events, such as sunspot activity, are often invoked as explaining temperature change in these debates. However, while there are a number of potential sun-related 'facts' that could be generated, none touch on sunspots.
|-
| There's a proposal to fix it, but it [will never happen/actually makes things worse/is stalled in Congress/might be unconstitutional].
| colspan="2" | Time zone reform is surprisingly a very controversial and politicized issue, with special interests on either side looking to modify it to fit their needs. Examples of proposals to modify the scheme include:

* Extend the duration of daylight saving time by one month, which was done in 2007 in many states as part of an energy-saving proposal by George W. Bush.
* Reduce the duration of daylight saving time back to its original span, or further.
* Eliminate daylight saving time altogether, going back to using standard time.
* Abolish daylight saving time but advance the time zone by one hour, effectively instating daylight saving time year round. This was done during World War II, and is also done in certain areas of Canada like Saskatchewan (which from 1966 onwards has observed Central Standard Time despite the entire province being squarely in the Mountain longitudes).
* Abolish daylight saving time and advance the time zone by 30 minutes, splitting the difference between the current standard time and daylight saving time.
* Abolish daylight saving time, but make government offices open one hour earlier in the summer, encouraging private businesses to do the same. This was done by Warren G. Harding in 1922 because he felt that changing the clocks was a "deception", but was rolled back the next year as it caused mass chaos in terms of what businesses decided to do to adapt to the change in business hours.
* Reduce the number of time zones in the United States to two, consolidating Pacific time into Mountain time (UTC-7:00), and Eastern time into Central time (UTC-6:00). This was proposed in a [https://qz.com/142199/the-us-needs-to-retire-daylight-savings-and-just-have-two-time-zones-one-hour-apart/ 2013 article in Quartz] by Allison Schrager.

At best, these time zone proposals will be fraught with controversy, with both sides arguing for the benefits of their time system. Some proposals, such as the 30-minute and 20-minute suggestions, would put the minute hands of the entire United States out of sync with the rest of the world, defeating the purpose of time zones with hourly UTC offsets in the first place, which could be construed as "making things worse".
|-
| It's getting worse and no one knows why.
| colspan="2" | 
|-
! colspan="3" | Title Text: Consequences
|-
| causes huge headaches for software developers
| colspan="2" | Trying to support time zones correctly for all dates present and historic is a mishmash of different regional laws, time zones, and DST changes. The headache is best exemplified in [https://www.youtube.com/watch?v=-5wpm-gesOY this video] by Tom Scott.
|-
| is taken advantage of by high-speed traders
| colspan="2" | A leap second must be taken into account by trading software, and may cause bugs if not accounted properly. Because leap seconds happen at midnight UTC, it might happen in regular trading hours for somebody living in Seattle, where the time zone is UTC-08:00. Somehow, a high-frequency trader may try to take advantage of any bugs in the software if they are not built to handle this particular case. This scenario is relatively unlikely because the market software can keep its own "market-official time" and synchronize with the correct time while the market is closed.
|-
| triggered the 2003 Northeast Blackout
| colspan="2" | The {{w|Northeast blackout of 2003}} was caused by a race condition in the energy management software at a power plant in Ohio. In a race condition the result of a computation is different depending on the order of completion of the operations, even though the result is supposed to be independent of that order.  Race conditions can theoretically be caused by mismatched timestamps.
|-
| has to be corrected for by GPS satellites
| colspan="2" | Because {{w|Global Positioning System}} (GPS) satellites are further from the earth than surface receivers, their clocks run faster than clocks on the surface due to general relativity. But they are also slower because they are moving faster than surface receivers, as explained by special relativity.  Also, their clocks are not updated for leap seconds. All these factors mean that GPS satellites have a different timekeeping standard than clocks on the ground which are generally synchronized to Greenwich solar time.
|-
| is now recognized as a major cause of World War I.
| colspan="2" | Daylight saving time was first implemented in World War I as a fuel-saving measure. Randall seems to be humorously implying that World War I was started in order to implement these fuel-saving measures during peacetime as well.
|-
|}

==Examples of true complete statements==

# Did you know that '''the spring equinox''' '''drifts out of sync with the zodiac''' because of '''the precession of the Earth's axis'''? Apparently '''it was even more extreme during the Ice Age'''.
# Did you know that '''daylight saving time''' '''might happen twice this year''' because of '''time zone regulation in Russia'''? Apparently '''there's a proposal to fix it, but it actually makes things worse'''. (True in Russia in 1981)
# Did you know that "leap year" "might not happen this year" because of "a decree by the pope in the 1500s"? Apparently "there's a proposal to fix it, but" "it will never happen". While it may seem like trivia, it "causes huge headaches for software developers". (The Pax calendar proposes that 2018 be a leap year. If anyone finds a calendar in which 2017 is a leap year, I'd love to see it!)

==Transcript==

:<big>-Calendar Facts-</big>

:[Shown below is a branching flow chart of sorts that begins at the phrase "Did you know that", then flows through various paths to build up a sentence. (Note that the "→" arrow symbol is used below to indicate a new branch with no intermediate text from a previous branch.)]

:Did you know that:
::the ( Fall | Spring ) Equinox
::the ( Winter | Summer ) ( Solstice | Olympics )
::the ( Earliest | Latest ) ( Sunrise | Sunset )
::Daylight ( Saving | Savings ) Time
::Leap ( Day | Year )
::Easter
::the ( Harvest | Super | Blood ) Moon
::Toyota Truck Month
::Shark Week
:→
::happens ( earlier | later | at the wrong time ) every year
::drifts out of sync with the
:::Sun
:::Moon
:::Zodiac
:::( Gregorian | Mayan | Lunar | iPhone ) Calendar
:::atomic clock in Colorado
::might ( not happen | happen twice ) this year
:because of
::time zone legislation in ( Indiana | Arizona | Russia )
::a decree by the pope in the 1500s
::( precession | libration | nutation | libation | eccentricity | obliquity ) of the 
:::Moon 
:::Sun 
:::Earth's axis 
:::equator 
:::prime meridian 
:::( International Date | Mason-Dixon ) Line
::magnetic field reversal
::an arbitrary decision by ( Benjamin Franklin | Isaac Newton | FDR )
:?
:Apparently
::it causes a predictable increase in car accidents.
::that's why we have leap seconds.
::scientists are really worried.
::it was even more extreme during the
:::Bronze Age.
:::Ice Age.
:::Cretaceous.
:::1990s.
::there's a proposal to fix it, but it
:::will never happen.
:::actually makes things worse.
:::is stalled in congress.
:::might be unconstitutional.
::it's getting worse and no one knows why.

== References ==
<references/>

{{comic discussion}}

[[Category:Calendar]]
[[Category:Charts]]
[[Category:Facts]]
[[Category:Comics featuring real people]]
[[Category:Daylight saving time]]
[[Category:Time]]
[[Category:Science]]
[[Category:Astronomy]]
[[Category:Supermoon]]
[[Category:Sharks]]