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Cruithne also known as Asteroid number 3753 is an asteroid in orbit around the Sun. Due to its unusual orbit relative to that of the Earth, it is a periodic inclusion planetoid. It is sometimes called "Earth's second moon", although it is not a satellite of the Earth.


Cruithne was discovered on October 10, 1986, by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. The 1983 apparition (1983 UH) is credited to Giovanni de Sanctis and Richard M. West of the European Southern Observatory in Chile. It was not until 1997 that its unusual orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Toronto, and Seppo Mikkola, working at the University of Turku in Finland.


The asteroid is named after the Cruithne people (also known as the Priteni or the Picts) who inhabited Scotland and parts of Ireland between 800 BCE and 1000 CE; the name may specifically refer to their legendary first leader, also called Cruithne.


Cruithne is approximately 5 km in diameter, and its closest approach to Earth is approximately 30 times the separation between Earth and the Moon (12 Gm or million kilometres). Although Cruithne's orbit is not thought to be stable over the long term, calculations by Wiegert and Innanen showed that it has likely been in sync with Earth's orbit for a long time. There is no danger of a collision with Earth for millions of years, if ever. Cruithne, having a maximum opposition magnitude of +15.8, is fainter than Pluto and would require at least a 12.5 inch reflecting telescope to attempt to be seen.


Cruithne is in a normal elliptic orbit around the Sun. However, because its period of revolution around the Sun is almost exactly equal to that of the Earth, they appear to "follow" each other in their paths around the Sun. This is why Cruithne is sometimes called "Earth's second moon".Cruithne's distance from the Sun and orbital speed vary a lot more than the Earth's, so from our point of view Cruithne actually follows a kidney bean-shaped horseshoe orbit ahead of the Earth, taking slightly less than one year to complete a circuit of the "bean". Because it takes slightly less than a year, the Earth "falls behind" the bean a little more each year, and so from our point of view, the circuit is not quite closed, but rather like a spiral loop that moves slowly away from the Earth.


After many years, the Earth has fallen behind far enough that Cruithne is now actually "catching up" on the Earth from "behind". When it eventually does catch up, Cruithne will make a series of annual close approaches to the Earth, and gravitationally exchange orbital energy with Earth; this will alter Cruithne's orbit by a little over half a million kilometres (whilst Earth's orbit is altered by about 1.3 centimetres) so that its period of revolution around the Sun is now slightly more than a year. The kidney bean now starts to migrate away from the Earth again in the opposite direction — instead of the Earth "falling behind" the bean, the Earth is now "pulling away from" the bean. The next such series of close approaches will be centred around the year 2292 — in July of that year, Cruithne will approach Earth to about 12.5 million km.


After 380 to 390 years or so, the kidney-bean-shaped orbit approaches Earth again from the other side, and the 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 centred on 1902, and will next happen with a series centered on 2676). The pattern then repeats itself.


A graphic of Cruithne’s orbit



Cruithne has a very interesting orbit which is heavily influenced by Earth.


The simulation shows Cruithne in a rotating frame. Watch as it orbits the Sun in a kidney bean shape orbit. The Earth has a noticable effect on Cruithne's orbit, sometimes repelling it as it gets too close. Cruithne's kidney bean-shaped "orbit" then spends centuries travelling all the way around the Sun, just to meet the Earth from the other side. Earth once again repels the kidney bean-shaped orbit. Cruithne's kidney-bean orbit repeats this process over and over again.



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