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    Eclipses
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    Apprentice Greytalker

    Joined: Nov 11, 2003
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    From: The Nexus

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    Fri Jun 26, 2020 8:04 am  
    Eclipses

    I'm trying to figure out how lunar eclipses would work in Oerth. In spite of the geocentric nature of Oerth's solar system, Celene and Luna should at intervals block the sun, causing an eclipse.

    This should happen regularly but I'm not big on astronomy and need help figuring out what that might look like.

    Can anyone shed some light on this?
    GreySage

    Joined: Jul 26, 2010
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    From: LG Dyvers

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    Fri Jun 26, 2020 10:27 am  
    Re: Eclipses

    Theala_Sildorian wrote:
    Can anyone shed some light on this?


    Sorry, Theala, but I'm in the dark on this one. Wink

    Icarus and others have spoken at length about this topic in other threads, though.

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    GreySage

    Joined: Aug 03, 2001
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    Fri Jun 26, 2020 11:13 am  

    In the real world, you get an eclipse season twice a year, and in that time you get a solar eclipse during the new moon and a lunar eclipse during the full moon. Each eclipse season on our world is separated by five and a half months, so they happen on different months each year. If you want eclipse seasons to sync more tightly to Oerth's years, you can just pick the months when Luna's eclipse seasons always happen, but there's nothing in canon that suggests Luna's eclipses are that regular, and they kind of become less special and auspicious if they happen every single year on the same date like clockwork.

    Celene would have its own synodic months, logically less frequent than Luna's. Perhaps a Celenian eclipse season is separated by five and a half seasons (Celenian cycles), rather than five and a half months. So there'd be a season (say, spring of 576) when Celene had both a solar and Celenian eclipse, then about a year and a half would pass before another Celenian eclipse season (so in this example, maybe fall of 577).
    Journeyman Greytalker

    Joined: Feb 01, 2005
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    From: Columbus, Ohio

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    Fri Jun 26, 2020 6:07 pm  

    Real-world eclipses are elusive to predict without proper mathematic calculations. They do not appear regularly over specific locals. For example, Philadelphia last experienced a full solar eclipse in the mid-1400s and will not do so again for 200 years. Other places have one every 99 years. The current proximity of the moon, the season, the cloud cover, and the location all play into an eclipse occurring.

    The only canon note on eclipses appears in TAB under the description of the festivals. One moon eclipses the other on the 4th of Growest over Greyhawk each year. It is unlikely this happens in Swartzenbruin to the same extent on the same day.

    The best choice: pick a date that suits your game's needs. Of course, you do have two moons to consider.

    Earth's axis is tilted 23.4%.
    Oerth's axis is tilted 30% (Or the plane of the solar system is to Oerth).

    This has other effects on Oerth, like hotter days in summer and colder days in winter than what is listed. Of course, Oerth is 100 million miles for Liga, while Earth is 93 million from Sol so maybe it's a wash.

    I just looked into this last week, so I can list websites that will help you better understand all this if you like. The take away for me was to use whimsy since there is no base positioning from which to start for either moon.
    Apprentice Greytalker

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    Sat Jun 27, 2020 4:03 pm  

    Skech wrote:

    The only canon note on eclipses appears in TAB under the description of the festivals.


    Do we regard Spelljammer as canon? If so then in Greyspace it states that Luna & Celene are both in the same orbital plane as Liga, making eclipses relatively common.

    Liga, at 100 million miles distant, orbits Oerth once every 365 days.
    Celene, at 10 million miles, orbits Oerth every 91 days.
    Luna, at 30 million miles, orbits Oerth every 28 days.

    I'm no expert on celestial mechanics but I think we just need to divide 365 by the orbital period of each body to find out how many times they eclipse Liga in the year. So 13 times a year for Celene & 4 times a year for Luna.
    GreySage

    Joined: Aug 03, 2001
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    Sat Jun 27, 2020 5:57 pm  

    [quote="DriveByPoster"]
    Skech wrote:

    I'm no expert on celestial mechanics but I think we just need to divide 365 by the orbital period of each body to find out how many times they eclipse Liga in the year. So 13 times a year for Celene & 4 times a year for Luna.


    Yeah, if the orbital planes are always lined up, you'd get a solar eclipse every time there was a new moon, and a lunar eclipse every time there was a full moon. Though they wouldn't necessarily be visible everywhere on Oerth.

    I think making them rarer is more interesting.
    Apprentice Greytalker

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    Sat Jun 27, 2020 10:53 pm  

    rasgon wrote:
    I think making them rarer is more interesting.


    Well, that's a request that's easy enough to oblige. Going back to Oerth Journal #3, Roger Moore states the circumference of Oerth as roughly 25,200 miles, so divide that number by the number of eclipses gives us 1,938 miles. Which is the distance between locations where the eclipse can be seen in full.

    So. let's say, for the sake of argument, on Day 0 everything is all lined up in eclipse mode & that the eclipse can be seen in full in the longitude of the Free City. Then we set our mental Orrery in motion & the effect of the next eclipse will be seen some 1,938 miles to the west. Counting hexes directly west from the City of Greyhawk on the Darlene map, this puts it somewhere near the Ullsprue Mountains which (rather excitingly) is also the longitude of Torvag Baragu.

    After that there wouldn't be another full eclipse in the Flanaess until the final one of the year, which occurs in the longitude of Rel Astra.

    Of course this is very simplified & doesn't take into account Oerth's spin or inclination to the plane of the ecliptic but at this point basic maths fails me & my brain turns to strawberry jam. If someone wants to create a simulated Orrery that would be great!
    Apprentice Greytalker

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    Sat Jun 27, 2020 11:02 pm  

    The above all relates to Luna, I should have mentioned. For Celene they would be very much rarer. And to get both moons eclipsing at the same time. Well...
    GreySage

    Joined: Aug 03, 2001
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    Sun Jun 28, 2020 6:45 am  

    A double solar eclipse would only happen on Goodmonth 11.
    Novice

    Joined: Nov 04, 2004
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    From: Vancouver, Canada

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    Sun Jun 28, 2020 9:41 am  

    Interesting side note: if Greyspace is considered canon, then there's another variable.
    In spelljammer, Liga is a hundred million miles from Earth, similar to the 93 million miles that the sun is from Earth. However Liga is only 100,000 miles across, compared to 800,000 miles diameter for Sol. Apparently Oerth has a very tiny sun! More susceptible to eclipses?
    Journeyman Greytalker

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    Tue Jun 30, 2020 9:35 pm  

    I must have overlooked Greyspace noting Liga's size as 1/512th the size of Sol. 1/8 in diameter - 1/512 volume.

    Sol is 93 Million miles from Earth
    Liga is 100Million miles from Oerth

    That places Oerth with a comparative distance to Liga (based on it's smaller size) as if it were 860 million miles away. That would place our Earth in the equivalent orbit of Saturn. That would mean a daytime average temp of -300F.

    I think Gary Gygax thought "Liga" would be comparable to Sol with the same light and heat benefits.

    Perhaps it's best to think of Liga's noted size as a misprint, and 100,000 miles should be read as 800,000 miles.

    https://www.enchantedlearning.com/subjects/astronomy/planets/
    GreySage

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    Wed Jul 01, 2020 9:52 am  

    Yeah, I think we're meant to assume that Liga appears the same size in Oerth's sky as Earth's sun appears from Earth, and somebody just didn't do the math.
    Apprentice Greytalker

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    Wed Jul 01, 2020 4:30 pm  

    rasgon wrote:
    Yeah, I think we're meant to assume that Liga appears the same size in Oerth's sky as Earth's sun appears from Earth, and somebody just didn't do the math.


    I have to agree. I've been plugging some of the Greyspace figures into a triangle calculator in order to determine the apparent size in the sky of Oerth of each body (calculated in degrees of angle). Taking the Greyspace figures gives the following:-

    Liga = 0.04297
    Celene = 0.00057
    Luna = 0.00764

    Compare with the Sun which = 0.5329

    Meaning that Liga would look like a large, bright star in the sky & the two moons would be tiny dots passing in front of it.

    Liga would need to be a factor of 10 bigger, as rasgon notes, in order to occupy a similar amount of the sky as the sun does on earth.

    Celene would need to be a factor of 10 bigger & a factor of 100 closer (i.e. 1,000 mile diameter & 100,000 miles distant) in order to occult Liga.

    Luna, at 4,000 miles diameter, would need to be roughly 500,000 miles distant (rather than 30 million) in order to do the same.

    Playing D&D takes you to some strange places on occasion!
    Apprentice Greytalker

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    From: The Nexus

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    Thu Jul 02, 2020 3:49 pm  

    rasgon wrote:
    A double solar eclipse would only happen on Goodmonth 11.


    Why that day?
    Apprentice Greytalker

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    From: The Nexus

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    Thu Jul 02, 2020 3:51 pm  

    DriveByPoster wrote:
    rasgon wrote:
    Yeah, I think we're meant to assume that Liga appears the same size in Oerth's sky as Earth's sun appears from Earth, and somebody just didn't do the math.


    I have to agree. I've been plugging some of the Greyspace figures into a triangle calculator in order to determine the apparent size in the sky of Oerth of each body (calculated in degrees of angle). Taking the Greyspace figures gives the following:-

    Liga = 0.04297
    Celene = 0.00057
    Luna = 0.00764

    Compare with the Sun which = 0.5329

    Meaning that Liga would look like a large, bright star in the sky & the two moons would be tiny dots passing in front of it.

    Liga would need to be a factor of 10 bigger, as rasgon notes, in order to occupy a similar amount of the sky as the sun does on earth.

    Celene would need to be a factor of 10 bigger & a factor of 100 closer (i.e. 1,000 mile diameter & 100,000 miles distant) in order to occult Liga.

    Luna, at 4,000 miles diameter, would need to be roughly 500,000 miles distant (rather than 30 million) in order to do the same.

    Playing D&D takes you to some strange places on occasion!


    And this is why I asked the question ... this kind of math makes my head explode! Shocked

    The real answer is I'm going to have to come up with my own system! Whimsy as someone said.

    Thanks, guys, for your help :)


    Last edited by Theala_Sildorian on Thu Jul 02, 2020 3:52 pm; edited 1 time in total
    GreySage

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    Thu Jul 02, 2020 3:51 pm  

    Theala_Sildorian wrote:
    rasgon wrote:
    A double solar eclipse would only happen on Goodmonth 11.


    Why that day?


    Because it's Dark Night, the only night of the year that has a double new moon. For a moon to eclipse the sun, it has to be new (otherwise the sun isn't behind it).


    Last edited by rasgon on Thu Jul 02, 2020 3:52 pm; edited 1 time in total
    Apprentice Greytalker

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    From: The Nexus

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    Thu Jul 02, 2020 3:52 pm  

    OH, that makes so much more sense! Duh!
    GreySage

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    Thu Jul 02, 2020 4:36 pm  

    There's a Greyhawk tides model that's pretty interesting, I thought. Contra Greyspace, it assumes that Celene is both larger and further away than Luna.
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