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Is there a single term that completes this sentence: "at its _____, Pluto is about 27AU from Earth." peri- something...?

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Usually the term 'closest approach' is used. It applies to any two bodies in motion. Perigee is a term occasionally used, but the dictionaries say it only applies to things orbiting Earth. Use your discretion.

In the case of the planets that orbit farther from the sun than us their closest approaches occurs when they are on the opposite side of Earth that the sun is, which is known as an opposition. For Venus and Mercury, it happens when they align with the sun from our viewpoint, and that is known as an inferior conjunction. From Wikipedia - Opposition (planets):

diagram of opposition, conjunction, elongation, and quadrature

Pluto has an inclination of 17o to the ecliptic, and an orbital eccentricity of 0.25. Either one of those things is enough to mean that when Pluto is closest to the sun, that brings it much closer to us, and the change in distance between when we are in front of or behind the sun in relation to it each year is a much smaller factor by comparison. For instance, right now it is about 33 AU from the sun, while on average it is 39 AU. When it is in opposition, that puts us about 32 AU away, and 34 AU when it is in conjunction. However, the orbit of Pluto is 248 years, so for all practical purposes the point each year when it is in opposition is what matters, unless you are willing to wait for as much as that time for it to get closer. The same holds true for all the possible dwarf planets in the Kuiper Belt.

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    $\begingroup$ The last paragraph sounds wrong. An Kuiper-belt object, say, with an perihelion farther out than Uranus, if it moves slowly enough to be in a bound orbit, will be within a distance of (perihelion + 1AU) from the sun for at least a handful of years at a time -- so the closest approach to Earth within that period would surely happen at one of the times when Earth passes between it and the sun, that is, at opposition. $\endgroup$ – Henning Makholm Jul 17 '15 at 15:56
  • $\begingroup$ @HenningMakholm It depends on the eccentricity of the orbit, and the inclination of the orbit to the plane of the ecliptic. But i have been thinking about that and wondering about doing some calculations. $\endgroup$ – kim holder Jul 17 '15 at 15:59
  • $\begingroup$ x @briligg: Assuming that our hypothetical object orbits (very) roughly in the ecliptic plane, I don't think eccentricity matters. Even if the exact perihelion happens at conjunction, if the object moved fast enough to get away from Earth faster than Earth goes around half its orbit, it would have more than solar escape velocity! $\endgroup$ – Henning Makholm Jul 17 '15 at 16:06
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    $\begingroup$ I've done some calculations. The most sensitive configuration seems to be if the dwarf planet's orbit is at right angles to the ecliptic and it reaches perihelion right when the Earth is farthest from the dwarf's orbital plane. Even then, the annual motion of the Earth will dominate the Earth-dwarf distance in the short term and make the closest approach depend on when the Earth is at the closest location in its orbit rather than when the dwarf is at perihelion -- as long as the dwarf's perihelion distance is at least 11 AU or thereabouts, no matter what the eccentricity. $\endgroup$ – Henning Makholm Jul 17 '15 at 16:45
  • $\begingroup$ @HenningMakholm that doesn't sound right to me, but i am not very familiar with the math. I am going to ask a new question on the topic. But please check if the edit i made on the answer satisfies your concern. I put it in very general terms to cover all cases. The eccentricity and inclination of Uranus and Neptune are small enough to not be more important than opposition. $\endgroup$ – kim holder Jul 17 '15 at 16:56
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Not quite what you want, but very closely related to what you want are the terms "opposition" (for the outer planets) and "inferior conjunction" (for Venus and Mercury).

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Perigee would be the closest, but that may be abusing the term. Perigee means "closest to Earth", but generally it's used for objects that are in Earth's orbit.

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  • $\begingroup$ Yeah, it is only for things orbiting around Earth - merriam-webster.com/dictionary/perigee $\endgroup$ – kim holder Jul 17 '15 at 14:50
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    $\begingroup$ @brillig - Not necessarily. in-the-sky.org/news.php?id=20140414_13_100 $\endgroup$ – David Hammen Jul 17 '15 at 14:52
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    $\begingroup$ @DavidHammen i checked a few other dictionaries and they agree on this. If it is used differently occasionally than it is defined in the dictionary, then the world is descending into chaos! $\endgroup$ – kim holder Jul 17 '15 at 15:07
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    $\begingroup$ @kimholder The term dates from the pre-Copernican age of astronomy in which all objects orbited the Earth. It is therefore a historical hold-over to use it in reference to astronomical objects which do not orbit the Earth. $\endgroup$ – called2voyage Oct 17 '16 at 16:16
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MOID. Minimum Orbit Intersection Distance.

Usually used for Near Earth Objects when assessing risk of impact.

But Wikipedia writes "It is defined as the distance between the closest points of the osculating orbits of two bodies. " So perhaps MOID could have a more general use than just the minimum distance between an NEA and earth.

In general the minimum distance to an outer body would occur at opposition and when the outer body is at perihelion. Minimum distance to an inner body would occur at inferior conjunction when inner body is at aphelion. But even these general rules could have exceptions if the orbits are inclined to one another and/or resonant.

For example a Hilda asteroid might have a 5.19 A.U. aphelion and Jupiter's orbit is 5.2 A.U.. Does this mean that a Hilda could get within .01 of Jupiter? No. Because of a Hilda's resonance with jupiter, aphelions will only occur near Jupiter's L4, L5 and L3 regions. Finding a Hilda Jupiter MOID is an interesting geometry problem. I suspect it doesn't occur at opposition or inferior conjunction. If I have time and energy to play with this puzzle, I'll may add to my answer

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