https://www.explainxkcd.com/wiki/api.php?action=feedcontributions&feedformat=atom&user=MathEconLawComp 2026-04-19T23:00:16Z User contributions MediaWiki 1.30.0 https://www.explainxkcd.com/wiki/index.php?title=2172:_Lunar_Cycles&diff=176214 2019-07-05T18:47:04Z

<p>MathEconLawComp: /* Explanation */</p> <hr /> <div>{{comic<br /> | number = 2172<br /> | date = July 5, 2019<br /> | title = Lunar Cycles<br /> | image = lunar_cycles.png<br /> | titletext = The Antikythera mechanism had a whole set of gears specifically to track the cyclic popularity of skinny jeans and low-rise waists.<br /> }}<br /> <br /> ==Explanation==<br /> {{incomplete|Created by a BOT. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}<br /> <br /> This comic shows a mixture of real, scientific lunar cycles and cycles that are comedic or fictional in nature.<br /> <br /> The comic shows that the moon's apsidal precession follows an 8.9 year cycle. This is true. What this means is that the major axis of the moon's orbit - the line between the periapsis (nearest point) and apoapsis (furthest point from Earth) of its orbit - [https://en.wikipedia.org/wiki/Lunar_precession precesses] eastward, taking about 8.9 years to complete a 360-degree cycle.<br /> <br /> The comic shows that distance has a 27.5 day cycle. This is true. A sidereal month, which is essentially the time it takes the moon to go around the earth, is approximately this length (although the mean time for a sidereal month may be closer to 27.3 days). Because the moon's orbit around the earth is elliptical and not perfectly circular, with a periapsis of about 362600 km earth-moon distance and an apoapsis of about 405400 km earth-moon distance, the distance to the moon essentially follows this cycle with times of peak distance about 27 and a half days apart.<br /> <br /> The comic also shows that phase has a 29.5 day cycle. This is true. This is the synodic month. Because the lunar cycle is based on how the sun's light shines on the moon as seen from earth, it is determined by the relative positions of the earth, moon, and sun. Because the earth has moved, very roughly, 1/12 of the way around the sun during a lunar month, it takes extra time for the moon to reach the point where it is is fully lit again, so this is longer than a sidereal month. A synodic month, or the time from one full moon to another, is about 29.5 days as stated.<br /> <br /> The idea that there is a cycle in which the moon becomes larger than the earth, or a cycle in which the moon becomes a square and then a circle again, is a joke&lt;sup&gt;[[http://lmgtfy.com/?q=does+the+moon+get+bigger+than+the+earth&amp;s=d ''citation needed'']]&lt;/sup&gt;.<br /> <br /> The final image shows many different cycles superimposed on each other. While the popularity of skinny jeans does change over time, the idea that this is connected to a lunar cycle is also a joke.<br /> <br /> ==Transcript==<br /> {{incomplete transcript|Do NOT delete this tag too soon.}}<br /> <br /> {{comic discussion}}</div>

MathEconLawComp https://www.explainxkcd.com/wiki/index.php?title=2172:_Lunar_Cycles&diff=176212 2019-07-05T18:29:40Z

<p>MathEconLawComp: /* Explanation */</p> <hr /> <div>{{comic<br /> | number = 2172<br /> | date = July 5, 2019<br /> | title = Lunar Cycles<br /> | image = lunar_cycles.png<br /> | titletext = The Antikythera mechanism had a whole set of gears specifically to track the cyclic popularity of skinny jeans and low-rise waists.<br /> }}<br /> <br /> ==Explanation==<br /> {{incomplete|Created by a BOT. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}}<br /> <br /> This comic shows a mixture of real, scientific lunar cycles and cycles that are comedic or fictional in nature.<br /> <br /> The comic shows that distance has a 27.5 day cycle. A sidereal month, which is essentially the time it takes the moon to go around the earth, is approximately this length (although the mean time for a sidereal month may be closer to 27.3 days). Because the moon's orbit around the earth is elliptical and not perfectly circular, with a periapsis of about 362600 km earth-moon distance and an apoapsis of about 405400 km earth-moon distance, the distance to the moon essentially follows this cycle with times of peak distance about 27 and a half days apart.<br /> <br /> The comic also shows that phase has a 29.5 day cycle. This is the synodic month. Because the lunar cycle is based on how the sun's light shines on the moon as seen from earth, it is determined by the relative positions of the earth, moon, and sun. Because the earth has moved, very roughly, 1/12 of the way around the sun during a lunar month, it takes extra time for the moon to reach the point where it is is fully lit again, so this is longer than a sidereal month. A synodic month, or the time from one full moon to another, is about 29.5 days as stated.<br /> <br /> The idea that there is a cycle in which the moon becomes larger than the earth, or a cycle in which the moon becomes a square and then a circle again, is a joke.<br /> <br /> The final image shows many different cycles superimposed on each other. While the popularity of skinny jeans does change over time, the idea that this is connected to a lunar cycle is also a joke.<br /> <br /> ==Transcript==<br /> {{incomplete transcript|Do NOT delete this tag too soon.}}<br /> <br /> {{comic discussion}}</div>

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