Editing 3753 Cruithne (section)

== Dimensions and orbit ==
[[File:Animation of 3753 Cruithne orbit.gif|thumb|left|Animation of 3753 Cruithne orbit from 1600 to 2500<br />{{legend2| Yellow| Sun}}{{·}}{{legend2| RoyalBlue| Earth}}{{·}}{{legend2|Magenta| 3753 Cruithne}}]]
[[Image:Orbits of Cruithne and Earth.gif|frame|left|Cruithne and Earth seem to follow each other because of a 1:1 [[orbital resonance]].]]
[[Image:Horseshoe orbit of Cruithne from the perspective of Earth.gif|frame|left|Cruithne appears to make a bean-shaped orbit from the perspective of Earth.]]
Cruithne is approximately {{convert|5|km|0}} in [[diameter]], and its closest approach to Earth is {{convert|12|e6km|AU mi}}, approximately thirty times the separation between Earth and the [[Moon]]. From 1994 through 2015, Cruithne made its annual closest approach to Earth every November.<ref name=jpl-close>{{cite web |type=2008-10-25 last obs |title=JPL Close-Approach Data: 3753 Cruithne (1986 TO) |url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=3753;cad=1#cad |access-date=2009-06-28}}</ref>

Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has probably been synchronized with Earth's orbit for a long time. There is no danger of a collision with Earth for millions of years, if ever. Its orbital path and Earth's do not cross, and its orbital plane is currently tilted to that of Earth by 19.8°. Cruithne, having a maximum near-Earth [[apparent magnitude|magnitude]] of +15.8, is fainter than [[Pluto]] and would require at least a {{convert|12.5|in|mm|adj=on|order=flip}} [[reflecting telescope]] to be seen.<ref>{{cite web |url=http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66 |title=This month Pluto's apparent magnitude is m=14.1. Could we see it with an 11" reflector? |publisher=Singapore Science Centre |access-date=2007-03-25 |archive-url=https://web.archive.org/web/20070930160349/http://www.science.edu.sg/ssc/detailed.jsp?artid=1950&type=6&root=6&parent=6&cat=66 |archive-date=2007-09-30 |url-status=dead }}</ref><ref>{{cite web |url = http://www.icq.eps.harvard.edu/MagScale.html |title = The astronomical magnitude scale. |publisher = The ICQ Comet Information Website |access-date = 2007-09-26}}</ref>

Cruithne is in a normal [[elliptic orbit]] around the Sun. Its [[orbital period|period of revolution]] around the Sun, approximately 364 days in the early 21st century, is almost equal to that of Earth. Because of this, Cruithne and Earth appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".<ref name="space.com">{{Cite web |last=Lloyd |first=Robin |publisher=[[Space.com]] |url=http://utstaging.space.com/scienceastronomy/solarsystem/second_moon_991029.html |archive-url=https://archive.today/20121208143923/http://utstaging.space.com/scienceastronomy/solarsystem/second_moon_991029.html |url-status=dead |archive-date=2012-12-08 |title=More Moons Around Earth? }}</ref> However, it does not orbit Earth and is not a moon.<ref>Meeus, reference above, writes "we may not deduce that Cruithne is a "companion" of the Earth, as some authors wrote, and certainly it is not a satellite! The object simply ''cannot'' be a satellite of the Earth, as it moves from nearly the orbit of Mercury to outside that of Mars, and because sometimes it is in ''superior'' conjunction, at the far side of the Sun as seen from the Earth".</ref> In 2058, Cruithne will come within 0.09&nbsp;[[Astronomical unit|AU]] ({{convert|13.6|e6km|e6mi|disp=or|abbr=off}}) of Mars.<ref name=jpl-close/>

Due to a high [[orbital eccentricity]], Cruithne's distance from the Sun and orbital speed vary a lot more than Earth's, so from Earth's point of view Cruithne actually follows a [[kidney bean|kidney-bean]]-shaped [[horseshoe orbit]] ahead of Earth, taking slightly less than one year to complete a circuit of the "bean". Because it takes slightly less than a year, Earth "falls behind" the bean a little more each year, and so, from the point of view of an observer on Earth, the circuit is not quite closed, but rather like a spiral loop that moves slowly away from Earth.{{citation needed|date=April 2021}}

After many years, Earth will have fallen so far behind that Cruithne will then actually be "catching up" on Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to Earth and [[gravity assist|gravitationally exchange orbital energy]] with Earth; this will alter Cruithne's orbit by a little over half a million kilometres—while Earth's orbit is altered by about {{convert|1.3|cm}}—so that its period of revolution around the Sun will then become slightly more than a year. The kidney bean will then start to migrate away from Earth again in the opposite direction—instead of Earth "falling behind" the bean, it is "pulling away from" the bean. The next such series of close approaches will be centred on the year 2292—in July of that year, Cruithne will approach Earth to about {{convert|12.5|e6km|AU mi}}.{{citation needed|date=April 2021}}

After 380 to 390 years or so, the kidney-bean-shaped orbit approaches Earth again from the other side, and Earth, once more, alters the orbit of Cruithne so that its period of revolution around the Sun is again slightly less than a year (this last happened with a series of close approaches centered on 1902, and will next happen with a series centered on 2676). The pattern then repeats itself.<ref>{{Cite web |date=2022-12-28 |title=3753 Cruithne, la «deuxième Lune» dont vous ignoriez l'existence |url=https://korii.slate.fr/et-caetera/astronomie-3753-cruithne-deuxieme-lune-espace-terre-satellite-orbite-ellipse-soleil |access-date=2024-10-17 |website=korii. |language=fr-FR}}</ref>