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Two Keplerian closed orbits always intersect:

Kepler orbits
Source: Astronomy SE answer by Flater

As seen from Earth, Venus appears larger than Mercury; this makes sense because Venus is both larger and closer than Mercury (as measured by minimal distance; by average distance all planets are equally distant), but can also be readily observed by comparing solar transits for either planet (or wait for 26 July 69,163, when both transit simultaneously).

Unless prevented by orbital resonance, that means that sometimes, Mercury should be eclipsed by Venus (occultation) when, seen from the Earth, Mercury is exactly behind Venus; and Mars should pass in front of the outer planets occasionally, but I'm not sure which body appears larger from Earth (Mars or Jupiter).

How often do occultations between planets occur?

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  • $\begingroup$ In either case, we'd call it a transit, not an eclipse. My guess is that the odds of all 3 bodies being in sufficient alignment for a perfect transit is pretty small, unless you get lucky with the orbital periods. I touched on that topic in my answer about Venus solar transits: astronomy.stackexchange.com/a/36489/16685 Obviously, solar transits are a lot more likely, since you only need 2 orbiting bodies to come into alignment, and the Sun's a rather large target. ;) $\endgroup$
    – PM 2Ring
    Nov 11, 2020 at 2:23
  • $\begingroup$ @PM2Ring I thought it'd be called a transit if the nearer body appear smaller (such that part of the further body remains visible) and an eclipse if the nearer body appears larger (such that the further body is hidden / eclipsed). $\endgroup$
    – gerrit
    Nov 11, 2020 at 8:50
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    $\begingroup$ Occultation is the term when the nearer body appears larger than the distant body. $\endgroup$
    – JohnHoltz
    Nov 11, 2020 at 17:45
  • $\begingroup$ @JohnHoltz Thanks, I've edited the question. $\endgroup$
    – gerrit
    Nov 11, 2020 at 22:47

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What you describe is a mutual planetary occultation (or transit), and yes, it has happened—in 1737—and will happen again—in 2133 (both dates are for Venus/Mercury events; there are other possible instances of planets passing in front of each other; for example, in 2065, Venus will pass in front of Jupiter). I would suggest giving a look at https://en.wikipedia.org/wiki/Planetary_transits_and_occultations

What is interesting with Mercury and Venus is that, considering the respective positions of these planets as well as the Earth, it is technically possible for Mercury to pass in front of Venus! The last time it happened was in 796, and it won’t happen again at least until the year 2500. See More Mathematical Astronomy Morsels by Jean Meeus or the link above for more information as well as for the frequency at which mutual planetary transits occur.

A “near-miss“ will happen on 27 April 2022 at 19:11 UT, when Neptune will be very close to Venus—about 25″ depending on where you will be on the Earth at that moment…

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    $\begingroup$ They're a lot more common that I had expected! $\endgroup$
    – gerrit
    Nov 21, 2020 at 9:11

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