I was lucky enough to witness the total solar eclipse crossing the US this past August, but while it was impressive, it was also fairly short - lasting just under two minutes at my location. I'd like to increase the intensity of the experience for a less technologically advanced civilization by drawing out the duration of the eclipse as much as possible.

I see that total eclipses on Earth can last up to approximately seven and a half minutes when the Earth is near aphelion, the moon is near perigee, and the eclipse happens near the Equator.

But can we draw this out even longer?

Given a planet and moon roughly like Earth and our Moon, are there modifications we can make to the orbit, size, or rotational speed of either body that could extend this out, while still maintaining a stable system?

My target is 15 minutes for a single location on the planet to witness an eclipse, ideally while able to see the sun's corona. Preference is for the eclipse to be caused by a moon rather than a large planet.

Bonus points if this long total eclipse can occur on a relatively frequent basis for a given area, where "frequent" is defined as anywhere between a month and a decade (so that a generation of people would experience multiple such events during their lifetime).

  • $\begingroup$ If it doesn't have to be the earth-moon system, just consider a really distant, massive moon, right? $\endgroup$ – Wen Sep 9 '17 at 4:25
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    $\begingroup$ I'd prefer to keep the "earth" planet's radius within 75%-125% of earth's, if possible. How massive of a moon could you have before it's no longer considered "just a moon"? $\endgroup$ – Troyen Sep 9 '17 at 4:35
  • $\begingroup$ Yes that's if you're on a JET plane flying at anywhere between 1500mph to 2400mph depending on your location, it may travel exactly the diameter of Earth if ur luck is peaked. BTW what's it gotta do with worldbuilding? $\endgroup$ – user6760 Sep 9 '17 at 5:27
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    $\begingroup$ @L.Dutch It's off-topic on astronomy because it's a hypothetical question. They send those here. $\endgroup$ – Troyen Sep 9 '17 at 8:17
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    $\begingroup$ This question is being discussed on meta. $\endgroup$ – Monica Cellio Sep 10 '17 at 4:25

The easiest and more effective is to pull your moon nearer to planet so it will have a larger apparent size and you'll get both more frequent and longer lasting eclipses.

You will be limited by Roche limit, but there's ample margin as this, for real Earth/Moon system is a scant 34,638km radius while real moon orbit is about 384,399km away, which is 21 times the Roche limit.

Note that, as you pull Moon in you'll get three effects together:

  • it will occupy a larger portion of the sky (eclipses will last longer and will happen more often)
  • it will appear to move slower (if it is still outside GEO, otherwise its apparent speed will rise again, in the other direction) (eclipses will last longer)
  • tidal waves will be much higher (situation may become unstable if Moon is too much inside GEO due to energy dissipation from tides pulling it toward Earth).

I think your best bet would be to have a moon near to GEO that will have rather high tidal waves moving very slowly.

About frequency of eclipse: this is strictly dependent on angle between orbital planes for the Sun-Earth and Earth-Moon systems; if this angle is less than apparent size of the moon then you'll have an eclipse every "New Moon" phase.

If you tell us your target we can try to compute if this is compatible with orbital physics.


I'll try to compute the exact parameters (I'm not familiar with orbital math, so it might take a bit); in qualitative terms what I think we can do is:

  • start from "real" Earth-Moon-Sun system.
  • pull moon nearer to planet (so it's apparent movement will be slower; need to about half the apparent speed; note: "lunar months" will be accordingly longer).
  • reduce moon diameter so that its apparent size will remain the same (~= .5°; needed to see Sun corona).
  • reduce angle between planet orbital plane and moon orbital plane so that you will have an eclipse somewhere at each "new moon" (this angle should be less than angular dimension of moon ~= .5°).
  • I would avoid to modify too much rotational speed of planet (it would change day length).
  • I have no idea (suggestions welcome) about how to compute effects of changes on tidal waves (pulling nearer would increase effect, but shrinking satellite would decrease it; square/cube might apply since gravity decreases with inverse square while mass is cube of dimensions and we can assume $sin \alpha \simeq \alpha$)
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  • $\begingroup$ If you get the moon closer, it will also move faster in the sky. $\endgroup$ – L.Dutch - Reinstate Monica Sep 9 '17 at 10:43
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    $\begingroup$ @L.Dutch: no, the moon is currently "falling back" as rotational Earth movement is "faster" than orbital speed; as you move it closer it will appear to slow down in the sky, in spite of it actually moving "faster" against fixed star reference system. When at GEO will appear fixed and then will start moving again in the other direction (west to east). $\endgroup$ – ZioByte Sep 9 '17 at 11:03
  • $\begingroup$ @ZioByte I've updated with some time targets. Goal is to drag out the eclipse for a single location. Aiming for an eclipse that lasts for 15 minutes, ideally while still showing the sun's corona. $\endgroup$ – Troyen Sep 9 '17 at 18:17
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    $\begingroup$ If you increase the apparent size of the moon, you won't get the same sort of total eclipse - that is, with the moon just masking the sun so you see the corona &c. You'll just have a period of darkness. $\endgroup$ – jamesqf Sep 10 '17 at 4:34

There are are a number of ways in which this can be attained:

  • First, you can make the rotation speed of the planet very slow and the revolution speed of the moon slow to get this effect. But I hardly think that if you do that you can get a solar eclipse for a very long time. Also if you do this it will mean that the hours in a day for that planet will increase severely. And also getting this effect again and again so that a person can experience it in an interval of month is very unlikely. A decade is more of a reasonable time gap.

  • Secondly you can get this effect by making planets come in front of the planet. Note- the planet coming in front has to large enough to cause a solar eclipse cause planets have large distance between them. So a planet equal to the size of The Planet (where eclipse is happening) coming in front of the planet will not work. That is why Mercury doesn't cause a solar eclipse when it comes in between the sun and earth, but if saturn or jupiter comes between sun and earth it could cause a solar eclipse. Also by using this process you can get the solar eclipse many times. Just make different planet come in front of it again and again.

Hope it helps.

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