1878: Earth Orbital Diagram

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Earth Orbital Diagram

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.

Explanation

This comic is the third of five consecutive comics published in the week before and during the solar eclipse occurring on Monday, August 21, 2017 which was visible as a total solar eclipse within a band across the contiguous United States from west to east and visible as a partial eclipse across the entire contiguous United States and beyond. The other comics are 1876: Eclipse Searches, 1877: Eclipse Science, 1879: Eclipse Birds, and 1880: Eclipse Review.

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 new take on 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. Diagrams for eclipses commonly include things that laypeople may not find relevant, without explanation, such as the umbra and penumbra.

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

All items are not drawn to scale. Neither the sizes of the celestial objects are that similar as shown nor the orbits are. The real scales are shown in this table:

	real (in km)	to scale
Sun (radius)	695,700	basketball
distance Earth-Sun	149,600,000	length of a large truck (26 meters)
Earth (radius)	6,371	pinhead (1 mm)
Moon (radius)	1,737	pin (0.3 mm)
distance Earth-Moon	384,399	small necklace (6.6 cm)

When the distance Sun-Earth is scaled to one meter or below neither Moon nor Earth can be seen by the human eye.

Arctangent

• 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 & 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

• Declension is the inflection of nouns in a language. In Latin declension and declination are both called Declinatio. In this comic, however, it might be a portmanteau of declination and (right) ascension.
• 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.
• And the right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question.

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 lunar calendar. The position of that angle isn't that bad but it should be not more than 30 degrees (slightly more than one lunar 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

• Enceliopsis are small genus of flowering plants in the daisy family, appropriately known as "sunrays".
• The element "encel-" might also be a reference to Enceladus, a moon around Saturn.
• The elements "-elio-" and "-psis" are also found in many technical orbital terms such as aphelion, perihelion, apsis, apoapsis and periapsis. See apsis.
• The point depicted on the diagram has no specific meaning.

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 it is potentially the first day of Spring or Autumn, depending on the time of year, in which hemisphere (Northern vs Southern) the observer is located, and which definition of seasons one uses.
• 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

• 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 an unrelated length, approximately equal to the diameter of the sun (half the angular size of the sun times twice the distance to it).

Obsequity

• 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.

Perihelix

• This is a portmanteau of helix and perihelion.
• The perihelion is the point in a elliptical solar orbit that is closest to the Sun.

Prolapse

• A Prolapse is a medical condition in which an internal organ is slipped forward or down.
• The word might be a reference to the apoapsis, which is the point of a body's elliptical orbit about the system's centre of mass where the distance between the body and the centre of mass is at its maximum. The periapsis is the point where the distance between the body and the centre of mass is at its minimum. In the specific case of the Moon’s orbit, these points are called apogee and perigee. On the diagram, the prolapse is not shown as a point, but as an angle of the Moon’s orbit.
• 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.

Tropopause

• 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.
• The label appears to point at the orbit of the moon.

Angle between the Astral and the Sagittal Planes

• The angle depicted is the inclination of the moon orbit. The planes are marked with nonexistent symbols, derived from Greek letters. The lunar orbit plane is labeled by a mixture of a nu (ν) and a gamma (γ), the ecliptic is labeled with a double chi (χ), and the angle between is marked with a phi (ϕ) but having two vertical lines.

Errata

• Errata are corrections in a published text (e.g. a newspaper article) issued after the publication.
• The word might be a reference the words aberration, eccentricity or anomaly, which all have both a technical astronomical definition and a common definition meaning "something wrong or strange". Of the three, the term "aberration" is the closest looking to "errata", but, unlike eccentricity and anomaly, it is not the name of an orbital parameter.
• The angle depicted lies between the direction from Earth to the Sun in the ecliptic and the line where the lunar orbit plane crosses the ecliptic. When this angle would be zero AND the Moon is between the Sun and Earth a total eclipse would occur. This is they only part of the diagram fulfilling slightly Randall's promise on top of the picture.

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.

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 or entertain the reader. Thus, there is some truth behind the statement, "The answer is made clear by a quick look," assuming a quick look means only a glance at the diagram/drawing without taking the time to read the labels.

Transcript

[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 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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Discussion

I guess first off, we should note the "solstice" is *not* the Bristish equivalent of "equinox" -- they are actually opposites. The equinoxes occur in April and September, when the day & night are equal length, and the solstices occur in June and December, when the length of daylight and nightime, respectively, are at their longest. JamesCurran (talk) 15:30, 18 August 2017 (UTC)

Technically, the opposite of solstice is the other solstice. Solstice and equinoxes are orthogonal. -- Hkmaly (talk) 00:01, 20 August 2017 (UTC)

Being picky but the Equinox/Solstice section refers to equinoxes marking the start of either spring or autumn, but actually both equinoxes mark the beginning of both spring and autumn in opposite hemispheres. ExternalMonolog (talk) 22:08, 22 August 2017 (UTC)

"Determinant of the date of Easter" refers to the fact that in the Catholic Church (and possibly other Christian denomiations) the date of Easter is the first Sunday after the first full moon of Spring, which means it is an astronomical calculation, but completely unrelated to the indicated angle. JamesCurran (talk) 16:28, 18 August 2017 (UTC)

Well, everyone celebrates Easter on the same day, right? So it's the first Sunday after the first full moon for everybody. Berets (talk) 23:20, 18 August 2017 (UTC)

No, not all denominations agree on the date of Easter; a particular example being the Orthodox church, which usually has Easter a week after the Catholic church, but sometimes as much as five weeks later. The difference is caused by the two denominations using different idealized calendars, both lunar and solar, as well as a slight difference in the definition. 162.158.69.57 18:41, 19 August 2017 (UTC)

"Astral planes" might as well be a Unicode reference, taking into account Randall's occasional mention of emoji, since emoji reside on one of the astral planes: https://en.wikipedia.org/wiki/Plane_%28Unicode%29 141.101.105.168 (talk) (please sign your comments with ~~~~)

That supplementary planes humorously refer also to Astral planes as mentioned in this explanation.--Dgbrt (talk) 19:54, 18 August 2017 (UTC)

Solstice comes from the Latin ... Sol = Sun .. Sistere = Stand still. It literally means the day the sun stands still and refers to the longest day of the year (summer) and the shortest day of the year (winter). So how does the Sun "stand still". On those days the Earth reaches either end of orbital ellipse and returns around the other side. If you stick a pole in the ground and observe its shadow every day at Noon you will see the shadow grow longer every day from winter to summer and grow shorter every day from summer to winter. The shadow is shortest when the Sun is highest in the sky at mid-summer and the shadow is longest when the sun is lowest in the sky at mid-winter. The sun is either getting higher in the sky or lower in the sky every day. When the Earth is at the end of the ellipse and the transition takes place the shadow will not make any noticeable change from one day to the next and one could say that the "Sun has stood still". Rtanenbaum (talk) 19:36, 18 August 2017 (UTC)

I think that Solstice/Equinox thing is a reference to Randall having a hobby of spreading linguistic misinformation, as seen in 1677:Contrails. 172.68.26.251 00:32, 19 August 2017 (UTC)

I'm not sure what the labels of the planes are, but they certainly aren't Greek letters. They look like alchemical symbols to me. 162.158.74.243 00:56, 20 August 2017 (UTC)

Part of the humor of Declension is that it's a portmanteau of right ascension and declination. Right now only declination is mentioned. 162.158.75.40 02:43, 20 August 2017 (UTC)

Arctangent is also a music festival in the UK, happening when this comic came out. --162.158.234.46 22:30, 21 August 2017 (UTC)