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How often does the moon orbit intersect with sunset?

  1. Apr 23, 2017 #1
    Bear with me, this is a very specific question for a scifi book I'm writing.

    If we had a ring in place of the moon, and you lived somewhere in the United States, how often would the ring intersect with the sun when it's setting (or rising, if it makes it easier).

    I know that it can take up to 40 years for a total eclipse of the sun in the US. But if we're talking about the whole moon orbit, it's more frequent than that. But adding the restriction of the 30 minutes when it's setting may make it harder.
    Last edited: Apr 23, 2017
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  3. Apr 23, 2017 #2

    jim mcnamara

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    I'm confused. A lunar eclipse happens when the moon is in the shadow of the Earth. Right? So why would the sun be in the same place as the moon for a lunar eclipse?

    So, you are asking how often (without the moon being there for some reason) is there an eclipse of the sun at sunset in the US - any where in there I guess. Do I get what you want?
  4. Apr 23, 2017 #3
    Sorry if I wasn't clear.

    Imagine that instead of the moon we had in its place a ring of debris that somehow stays in place (there are other problems such as rocks closer to Earth would fall down, etc., but let's ignore it for simplicity.) But it follows the orbit of the Moon.

    Now, imagine you're somewhere in the contiguous United States.

    How often would this ring intersect with the sunset (or sun rising) from the point of view of someone on Earth, living in the USA?

    So this is not an eclipse, no. And by the way, I meant an eclipse of the sun, not of the moon.
  5. Apr 24, 2017 #4
    The answer, I think, would be...


    Earth's moon does not orbit over Earth's equator, as most other moons and Saturn's rings do. Instead it orbits in the plane of the planets. So, if the moon retains this orbit, but is crushed into a ring, then you'd have a ring that would always be edge-facing the sun. Rings are very thin, so this ring would cast no visible shadow on the Earth. The ring would only be visible from times and places where it could not intersect the sun - The poles for example. It would not be visible whenever it did intersect the sun, except perhaps as a very thin line.

    Note: I could be wrong about the moon's orbit, and that would through the rest of this off.
  6. Apr 24, 2017 #5

    So, as you said, since it's in the same plane as the planets, that should be mathematically impossible, unless you're on the equator, as you said. The Moon does intersect the sun (and we have eclipses), but we'd expect the ring to be very thin.

    Here's an article with art showing what Earth would like with Saturn-like rings.
  7. Apr 24, 2017 #6


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    The Moon orbits at ~5 degrees to the ecliptic.(otherwise we would get a solar eclipse during every new moon)
  8. Apr 24, 2017 #7
    Dammit Moon, straighten up!
  9. Apr 24, 2017 #8

    jim mcnamara

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    Still confused. What do saturn-like rings in Earth's sky have to do with what was originally asked? By ring did you mean a ring that was made of debris orbiting Earth? @Janus answer (thank you) implies that it is possible to have periodic eclipses. We knew that. But another poster may not have, I guess. And yes, eclipses can be at different times in North America. There used to be books or tables called ephemerides that let you look this stuff up. All I got from Google that looked interesting was some JPL software. Which I knoiw from experience is "pretty interesting" to use, as in many hours.

    So. This is as close as I could get. https://en.wikipedia.org/wiki/List_of_solar_eclipses_visible_from_the_United_States
    There is no obvious periodicity for sunrise eclipses in the US that I can see from this. You can try.

    So where are we? If you are a programmer, try one of Jean Meeus books, and see if you can calculate when the next eclipse of the flavor you want will occur.

    Astronomical Algorithms will be in a college library or you can find one on Amazon.
    Last edited by a moderator: May 8, 2017
  10. Apr 24, 2017 #9


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    As far as the question as to how often the Moon's orbital path intersects with the Sun:
    This happens when ever the Sun is in line with either the ascending or descending node of the Moon's orbit (where the moon's orbit and the ecliptic cross)
    If the orientation of the Moon's orbit never changed this would occur 1/2 a year apart. However, this is not the case, as due to the gravitational influence of the Sun the orbit undergoes a nodal precession with a period of ~18.6 years. This increases the time between alignments to ~ 0.528 years, or about 10 days longer.
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