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Thread: Callisto cartography and so forth

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    Post Callisto cartography and so forth

    After reading so many papers on Callisto, I became interested in the cartographic elements that were mentioned. Astronomers are interested in different hemispheres (faces, sides) of the large moons in the outer Solar System, because the leading hemisphere is not like the trailing one, etc. I'm into Callisto at present, so I made a list of the terms most often used and tried to figure out the terms for other map points.

    Attached are five drawing I labeled, using a small public domain illo from Wikipedia.

    Coordinate maps 1 & 2 show the basic cartographic reference points for Callisto, but these are generally common for all large moons with captured rotation.

    The H1, H2, and H3 made show the names for the six major hemispheres with the most commonly used or most accepted (by IAU) names.

    I have a lot of notes but will save them for later. Left out for now are the sunward/sunlit and [unnamed dark hemispheres, anti-solar I guess] hemispheres, which are not in line with the other cartographic hemispheres.

    Comments and corrections welcome. I saved the sources used for these, if anyone wants to know where these came from. As it turns out, many Solar System planets and the Moon have independent cartographic systems that do not overlap, as they were created for different projects then grandfathered into the IAU system. Mercury and the Moon, for instance, operate off completely different mapping systems.

    The system shown here is not really my suggestion for "official" cartographic points. It's more a compilation of what terms astronomers use most often.
    Attached Images Attached Images
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    It was interesting that there are three types of paired opposing fixed points on giant satellites (and, really smaller ones too).

    1. Poles: North and South.
    2. Points: Toward the major planet and away from it (taken from "subsolar points", I guess). Here, the sub-Jovian (or sub-Jupiter) point, and the anti-Jovian/anti-Jupiter point.
    3. Apices: The leading apex in the direction of the orbital motion, taken from the solar apex in Hercules I guess, and the trailing antapex, the "most trailing" point that looks exactly backward along the orbit.

    Nice how that worked out, I thought.

    Confusingly, longitude is counted WESTWARD from the Prime Meridian, which I did not expect. Will post the source in a moment.
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    Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015
    https://astropedia.astrogeology.usgs...015reprint.pdf
    Received: 3 October 2017 / Accepted: 27 October 2017
    Springer Science+Business Media B.V., part of Springer Nature (outside the USA) 2018

    QUOTES: The 326◦ meridian of Callisto is defined by the crater Saga.

    In astronomical tradition, planetographic coordinates (commonly used on maps) may not be identical with traditional spherical coordinates. Planetographic coordinates are defined by guiding principles contained in a resolution passed at the 14th General Assembly of the IAU in 1970. These guiding principles state:
    1. The rotational pole of a planet or satellite which lies on the north side of the invariable plane will be called north, and northern latitudes will be designated as positive.
    2. The planetographic longitude of the central meridian, as observed from a direction fixed with respect to an inertial system, will increase with time. The range of longitudes shall extend from 0◦ to 360◦. Thus, west longitudes are used when the rotation is direct, and east longitudes are used when the rotation is retrograde. The origin is the center of mass. The Earth, Sun, and Moon do not traditionally conform to this definition. Their rotations are direct and longitudes run both east and west 180◦, or positive to the east 360◦. NASA missions have adopted the use of 0◦–360◦ east longitude for the Moon (LRO 2008). For planets and satellites, latitude is measured north and south of the equator, with north latitudes designated as positive. The planetographic latitude of a point on the reference surface is the angle between the equatorial plane and the normal to the reference surface at the point. In the planetographic system, the position of a point (P) not on the reference surface is specified by the planetographic latitude of the point (P) on the reference surface at which the normal passes through P and by the height (h) of P above P.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Here's a point I would need clarified, about East and West on Callisto.

    From Jupiter's perspective, looking at Callisto, it seems that West lies in the direction of Callisto's orbital motion, so the Leading Apex is at 90◦ [West] longitude, and the Trailing Antapex is at 270◦ [West] Longitude.

    This sound right to any of you, given the IAU Working Group definition above?
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Found a sentence in another paper that confirms the Leading Apex at 90 W.

    http://cdsads.u-strasbg.fr/abs/2017Icar..286...56N
    The near-surface electron radiation environment of Saturn's moon Mimas
    Nordheim, T. A., et al. (4/2017)
    QUOTE: We predict a lens-shaped electron energy deposition pattern that extends down to ∼cm depths at low latitudes centered around the apex of the leading hemisphere (90 W).


    I also kind of figure the term "west" will go out of use for the nonrotating moons.
    Last edited by Roger E. Moore; 2019-May-08 at 11:55 PM.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Also found an interesting cartographic term: the octant.

    https://en.wikipedia.org/wiki/Octant_(solid_geometry)
    http://mathworld.wolfram.com/Octant.html

    (Wikipedia) The octant in solid geometry is one of the eight divisions of a Euclidean three-dimensional coordinate system defined by the signs of the coordinates. It is similar to the two-dimensional quadrant and the one-dimensional ray. The generalization of an octant is called orthant.

    By my figuring, you identify an octant for a large moon in this order: North/South, Jovian/anti-Jovian, Leading/Trailing.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by Roger E. Moore View Post
    I also kind of figure the term "west" will go out of use for the nonrotating moons.
    They are all rotating, just not relative to the planet.

    But why, out of use?

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    Quote Originally Posted by grapes View Post
    They are all rotating, just not relative to the planet.

    But why, out of use?
    It occurred to me that if the term "west" was relative to the direction of orbital motion, would it apply to the anti-Jovian hemisphere as well? In other words, east and west would flip over once you get to the anti-Jovian side.

    WAIT: maybe I have that wrong, thought I read somewhere... guess not. Never mind.
    Last edited by Roger E. Moore; 2019-May-09 at 12:59 PM.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by Roger E. Moore View Post
    It occurred to me that if the term "west" was relative to the direction of orbital motion, would it apply to the anti-Jovian hemisphere as well? In other words, east and west would flip over once you get to the anti-Jovian side.

    WAIT: maybe I have that wrong, thought I read somewhere... guess not. Never mind.
    No, it seems east is defined so that the sun rises in the east on that moon. More or less.

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    Cute. Found out that Ganymede looks a tiny bit like a football itself. Wonder if Callisto and the other large Jovian moons are similar distorted.

    https://www.sciencedirect.com/scienc...007?via%3Dihub
    Planetary and Space Science, Volume 117, November 2015, Pages 246-249

    New Ganymede control point network and global shape model
    A. Zubarev, I. Nadezhdina, J. Oberst, H. Hussmann, A. Stark

    We confirm that Ganymede has a pronounced ellipsoidal shape, approximately aligned with the Jupiter-direction, in agreement with Ganymede being in tidal equilibrium. The point heights, suffering from large individual errors, do not reveal any large-scale topography below our typical error levels (97% <5 km). By analysis of data residuals we search for, but cannot detect Ganymede longitudinal forced librations. We conclude that libration amplitudes cannot be larger than 0.1 (corresponding to a lateral displacement of 4.6 km at the equator).

    LATE ADD: This would add another cartographic feature, not sure what to call it, the "football tip" aimed at Jupiter, with the counter-tip on the other side of the world.
    Last edited by Roger E. Moore; 2019-May-09 at 01:15 PM.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Also found that Titan has winter and summer hemispheres, just like Earth. Also cute.


    By way of comparison, here is the cartographic system for the Moon. Quotes are from the Wikipedia page.

    Selenographic coordinates
    https://en.wikipedia.org/wiki/Selenographic_coordinates

    The selenographic colongitude is the longitude of the morning terminator on the Moon, as measured in degrees westward from the prime meridian. The morning terminator forms a half-circle across the Moon where the Sun is just starting to rise. As the Moon continues in its orbit, this line advances in longitude. The value of the selenographic colongitude increases from 0 to 359 in the direction of the advancing terminator.

    Longitude on the Moon is measured both east and west from its prime meridian. When no direction is specified, east is positive and west is negative.

    Moon's prime meridian, which is the line passing from the lunar north pole through the point on the lunar surface directly facing Earth to the lunar south pole.

    Anything past 90E or 90W would not be seen from Earth, except for libration, which makes 59% of the Moon visible.

    Sunrise occurs at the prime meridian when the Lunar phase reaches First Quarter, after one fourth of a lunar day. At this location the selenographic colongitude at sunrise is defined as 0. Thus, by the time of the Full Moon the colongitude increases to 90; at Last Quarter it is 180, and at the New Moon the colongitude reaches 270. Note that the Moon is nearly invisible from the Earth at New Moon phase except during a solar eclipse.

    The selenographic longitude of the evening terminator is equal to the colongitude plus 180.

    Roughly speaking, the Moon's prime meridian lies near the center of the Moon's disc as seen from Earth. For precise applications, many coordinate systems have been defined for the Moon, each with a slightly different prime meridian. The IAU recommends the "mean Earth/polar axis" system, in which the prime meridian is the average direction (from the Moon's center) of the Earth's center

    ===

    NASA, ESA, and other spaceflight missions have adopted the use of 0◦ to 360◦ east longitude for the Moon (see below, LRO 2008). This system, developed by the IAU, is specific only to spacecraft missions to the Moon, and not necessarily for purposes of other astronomers. This is quite a different take from the above, so you might want to read the document linked below.

    https://lunar.gsfc.nasa.gov/library/451-SCI-000958.pdf
    A Standardized Lunar Coordinate System for the Lunar Reconnaissance Orbiter
    LRO Project White Paper Version 4 2008 May 14
    Goddard Space Flight Center, Greenbelt, Maryland
    Last edited by Roger E. Moore; 2019-May-09 at 01:12 PM.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
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    Quote Originally Posted by grapes View Post
    No, it seems east is defined so that the sun rises in the east on that moon. More or less.
    Okay, I can visualize that now. Thank you!
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    UPDATE: Per my reading, Callisto is not known to be distorted gravitationally. It is assumed at present to be spherical. (see below link)

    Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015
    https://astropedia.astrogeology.usgs...015reprint.pdf
    Received: 3 October 2017 / Accepted: 27 October 2017

    diameter of Callisto is 2410.3 1.5 km radius all over
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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    The Sunward Hemisphere of Callisto is not equivalent to any of the other hemispheres named above, obviously because the world's orbit is tilted to Jupiter's orbit by a small amount (can't find the reference now, it wasn't to the ecliptic...).

    The point on Callisto at which the Sun is at zenith, directly overhead at 90, is the Sub-Solar Point. The spot on the surface directly opposite the Sun is the Anti-Solar Point. You can't use the word "apex" because the Solar Apex and Solar Antapex are different things entirely.

    Generally speaking, papers name the side of Callisto (or any large moon) facing the Sun as the Sunward or Sunlit Hemisphere. I don't usually see a name for the night side, and "Anti-Sunward" sounds clunky. Dark Hemisphere? Ugh. Night Side? Better. Whatever.

    The Sunward Hemisphere would not necessarily reach a full 180 hemisphere, because the horizon at the 90E and W mark would block it.

    Maybe just skip "hemisphere" and use Sunward Side and Night Side.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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