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Revision as of 02:01, 20 June 2013

Welcome to the explain xkcd wiki!

We have an explanation for all 5 xkcd comics, and only 0 (0%) are incomplete. Help us finish them!

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Earth Orbital Diagram
You shouldn't look directly at a partial eclipse because of the damage that can be caused by improperly aligning the solar-lunar orbital plane with the orbital bones around your eye.
Title text: You shouldn't look directly at a partial eclipse because of the damage that can be caused by improperly aligning the solar-lunar orbital plane with the orbital bones around your eye.


Ambox notice.png This explanation may be incomplete or incorrect: Created by a BOT - Please change this comment when editing this page. Do NOT delete this tag too soon.

This comic is the third consecutive comic published in the week before the solar eclipse occurring on Monday, August 21, 2017 which is a total solar eclipse and visible in totality within a band across the contiguous United States from west to east. The other comics are 1876: Eclipse Searches and 1877: Eclipse Science.

The comic claims that the reason that eclipses don't happen every month is simple to understand by looking at an orbital diagram. Ironically, the cartoon has so many parts and labels that it is far more difficult to understand than is implied. While the graph itself is based on astronomical definitions, all the labels are nonsense in this context. In effect, the comic is a parody of a common joke in which a person asks a scientist a question, the scientist begins by saying "It's really quite simple", then proceeds to give a very lengthy and highly technical explanation that non-scientists would not be expected to understand.

All of the labels in the diagram are complicated words or phrases. Some are related to orbital mechanics (e.g. "equinox" and "perihelion"), while others are wholly unrelated or even made up. Each label is nonsensical in its place in the diagram. Compare/contrast with the standard Kepler Orbit diagram.

The title text references warnings to not look directly into the sun, but parodies those warnings by referring to 'orbit', the anatomical term for the eye socket.

Labels and Their Astronomical Meanings

  • Arctangent is the inverse function of the tangent function of trigonometry. You can determine a non-right angle of a right triangle by taking the arctangent of the length of the opposite side divided by the length of the adjacent side.
  • The angle shown in the comic has no astronomical meaning.
Astral plane
  • The Astral plane is a plane of existence in various esoteric theories. It features prominently in Dungeons and Dragons cosmology, connecting the various other planes of existence.
  • The picture shows the lunar orbital plane, the plane in which the Moon orbits the Earth, tilted about 5.1 degrees from the ecliptic.
  • Declension is the inflection of nouns in a language.
  • In astronomy, the declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system. It is measured north or south of the celestial equator, like the geographical latitude on Earth. But in the picture the label is at the angle for the axial tilt of the Earth.
Determinant of the date of Easter
  • In Western Christianity Easter always falls on the first Sunday after the first ecclesiastical full moon after the beginning of spring (equinox). The ecclesiastical full moon is determined by a calendar that approximates the actual time of the full moon, Thus the date of easter is defined by a combination of a solar and a moon calendar. The position of that angle isn't that bad but it should be not more than 30 degrees (slightly more than one month.)
  • In mathematics, the determinant is a function of numerical matrices. In this context, however, it apparently refers to something that directly determines the date of Easter.
Dimples of Venus
  • The Dimples of Venus are indentations sometimes visible on the human lower back.
  • In astronomy the Belt of Venus is a shadow cast by the Earth visible in its atmosphere.
  • Enceliopsis are small genus of flowering plants in the daisy family, appropriately known as "sunrays".
  • In astronomy this point has also no specific meaning. But Enceladus is a moon around Saturn.
Equinox / Solstice

Equinox and Solstice have very different meanings:

  • An Equinox is one of two instants in the year when the sun is exactly over the equator; the length of day and night are very nearly equal that day at all locations on the planet, and (in the United States) it is the first day of Spring or Autumn, depending on the time of year.
  • A Solstice is one of two instants in the year when the sun's angle is maximally far from Earth's equator; when one occurs, the length of the day or night is shortest or longest (depending on whether one is in the northern or southern hemisphere), and (in the United States) it marks the first day of summer or winter.

Both types occur because the Earth's rotation axis is tilted (at 23.4 degrees) from its orbital plane (ecliptic) about the Sun.

Jokingly insisting that two different terms are American/British variants of the same word has been the topic of 1677: Contrails.

  • Hypothecate is a legal verb that means something similar to "make a mortgage".
  • The hypotenuse is the longest side of a right-angled triangle. Here it is a length with no astronomical term. In terms of this comic, it's approximately the diameter of the earth's orbit times the errata.
  • Obsequity means the state of being obsequious (showing an indecorous willingness to obey or serve, or "sucking up").
  • In astronomy the correct word is Obliquity, meaning an axial tilt.
  • This is a portmanteau of helix and perihelion.
  • The perihelion is the point in a elliptical solar orbit that is closest to the Sun.
  • A Prolapse is a medical condition in which an internal organ is slipped forward or down.
  • Retrograde and prograde motion are terms used to describe the apparent motion of celestial objects through the sky.
Sagittal plane
  • The Sagittal plane is an anatomical plane, dividing the body in left and right.
  • The correct label in the picture would be the Ecliptic plane. The plane the Earth orbits the Sun.
  • Sagittarius is one of the stellar constellations of the Zodiac. The center of the Milky Way lies in this constellation.
Solar plexus
  • The Solar plexus is a network of nerves located in the abdomen. It was the name of 64: Solar Plexus.
  • Solar is an adjective referring to the Sun, the star in our solar system.
  • The Tropopause is the boundary in our atmosphere between the troposphere and stratosphere, defined as the boundary where air ceases to cool with increasing elevation. It is 9-17 km above sea level, not the thousands of kilometers as depicted here.
Angle between the Astral and the Sagittal Planes
  • The angle depicted is the inclination of the moon orbit. The planes are marked with greek letters, the angle is marked with the greek letter phi (ϕ).
  • Errata are corrections in a published text (e.g. a newspaper article) issued after the publication.
  • The angle depicted as errata is half the angular size of the sun, which has to match the lunar angular size to cause a solar total eclipse.

Explanation for "Why isn't there a (solar) eclipse every month?"

If the plane of where the Earth orbits the Sun and where the Moon orbits the Earth were completely aligned, then there would be a solar eclipse at every new moon (once every 29.5 days) and a lunar eclipse at every full moon (half a lunar period about 14.7 days after a New Moon). However, the plane in which the Moon orbits the Earth is tilted with an inclination of 5 degrees relative to that of the ecliptic plane (the plane defined by the Earth's orbit around the Sun). Eclipses are only possible during two eclipse seasons each year (half a year apart) where for a period of 31 to 37 days the Sun is nearly aligned with the two points in the tilted Earth-Moon plane where the Moon crosses the ecliptic plane. During an eclipse season at the time of a new moon there will be solar eclipses visible from certain locations and during full moons there will be lunar eclipses.

Eclipse Diagram.jpg

The real explanation of eclipses is evident from this xkcd comic, but is labeled with a fictional character similar to a Greek phi but with two vertical lines; the remaining labels also do not contribute to this explanation and exist only to distract or misinform the reader.


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
[An orbital map of the Earth is shown. The Sun is in the center, the Earth is at the right bottom, and the Moon is left below the Earth.]
Why isn't there an eclipse every month?
This is a common question! The answer is made clear by a quick look at the Earth's orbital diagram:
[Label Sun:]
Solar plexus
[Label on the Earth's plane:]
Sagittal plane
[Labels on Earth's orbit (beginning at the Earth counterclockwise):]
Perihelix, Declension, Obsequity, Hypothecate, Enceliopsis, Equinox (Solstice in British English)
[Two angles in the plane are labeled as:]
Determinant of the date of Easter, Arctangent
[The plane of the Moon is pictured in a small angle to the Earth's plane and named Astral Plane. The angle is presented between two lines (Greek Nu or Gamma and a double Greek Chi) and identified by a "Game Of Thrones" 'O' (a character that looks similar to a Greek Phi but with two vertical lines).]
[The labels at the Moon's path are:]
Tropopause, Prolapse, Errata.
[An arrow points to the Earth at the zero meridian on the equator. The label reads:]
Dimples of Venus

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