• Cruithne, but it's actually an asteroid.
  • We only have one Moon. But we do have lots of little artificial satellites buzzing around, I don't really know the names of them though.
  • Moon Unit Zappa. She sang a song about valley girls and appeared in one of the Vacation films.
  • 3753 Cruithne (pronounced /ˈkrɪnjÉ™/, from Old Irish IPA: [ˈkrɪθnÉ›]; Modern Irish IPA: [ˈkrɪhnʲə] or [ˈkrɪnʲə]) is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. 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 orbits the Sun, not 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 and the Isle of Man between 800 BCE and 1000 CE[citation needed]; 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 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 the Earth by 19.8°. 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 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".However, it does not orbit the Earth and is not a moon. Cruithne's distance from the Sun and orbital speed vary a lot more than the Earth's, so from the Earth's 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 slightly more than a year. The kidney bean then starts to migrate away from the Earth again in the opposite direction — instead of the Earth "falling behind" the bean, the Earth 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 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. Here is an image,
  • The Earth only has one moon...THE Moon. Cruithne is sometimes called Earth's second moon, but as Bad Astronomy's Phil Plait said in an article for Discover Magazine, " actually orbits the Sun, so it’s not a moon of ours. Same goes for the other three objects discovered, too." Yup, there are apparently 4 large objects besides the Moon that stick pretty close to Earth, Cruithne being the largest, but none of them are moons.

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