The Kordylewski clouds, sometimes called the lunar libration clouds, are sparse clouds of cosmic dust that trail ahead and behind the Moon. They sit at the L4 and L5 Lagrange points, two theoretically stable regions of space in the Earth–Moon system. Their relative stability inspired Polish astronomer Kazimierz Kordylewski to search their vicinity for co-orbital objects of the Moon. He first spotted the clouds in 1961 from the Tatra Mountains in former Czechoslovakia. Following Kordylewski's discovery, inconsistent observations by other astronomers led to the clouds' existence becoming controversial. Observation attempts were complicated by the clouds' exceedingly dim nature, making them difficult to discriminate against gegenschein and atmospheric airglow even in very dark skies. Observations from the ground, air, and space reported both positive and negative detections, and a 1991–92 encounter from Japan's Hiten spacecraft failed to find the clouds. In 2018, they were tentatively confirmed to exist via polarimetry, by a team of Hungarian astronomers of Eötvös Loránd University. Although the lunar L4 and L5 points are relatively stable, perturbations from the Sun are able to destabilize dust particles in the clouds. They are likely replenished by material from the interplanetary dust cloud as they are temporarily trapped in the lunar Lagrange points. Dust particles remain there for decades, forming large, rapidly-evolving bands within the clouds. Eventually, perturbations from the Sun lead to their escape back into interplanetary space. Due to their elusiveness they are sometimes nicknamed ghost moons.
History
Discovery Following French astronomer Frédéric Petit's spurious report of a second moon of Earth in 1846, other astronomers began searching for undiscovered moons. Between 1953 and 1956, a team headed by Clyde Tombaugh planned to search for small natural satellites near the Moon's Lagrange points—dynamically stable regions of space—but were prevented by poor weather. In 1951, Polish astronomer Kazimierz Kordylewski began his own search for trojan satellites at the lunar L4 and L5 Lagrange points. He was also unsuccessful, but in 1956 Józef Witkowski suggested to Kordylewski to instead search for faint, diffuse dust clouds. The clouds were first observed with the naked eye by Kordylewski in October 1956, at the Skalnaté pleso Observatory in the Tatra Mountains of former Czechoslovakia. Even with very dark skies, the clouds were difficult to observe. They appeared as slight brightenings near the lunar L4 and L5 points at least 2° in diameter and one to two magnitudes fainter than the brightest gegenschein. On 6 March and 6 April 1961, Kordylewski successfully photographed two distinct clouds at the lunar L5 point from Kasprowy Wierch's summit observatory. The photographs were taken using a Jupiter 3 Leica camera, with an exposure times of 11–12 minutes. Kordylewski photometrically analyzed the photographs and published his results in the journal Acta Astronomica in 1961, and an International Astronomical Union circular announced the clouds' discovery on 23 May of that year.
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