Occultation

A phenomenon in which the moon comes in front of a star or planet, hiding it. It is also called a stellar eclipse, as opposed to a solar eclipse or lunar eclipse. Occultations of bright stars rarely occur, but first-magnitude stars such as Regulus, Spica, Antares, and Aldebaran, which are close to the ecliptic path of the moon, are occulted relatively often. Occultations of stars up to about magnitude 6 can be seen dozens of times each year. When the moon is bright, occultations of faint stars are difficult to see, and they are easy to see before the full moon because they are in the dark edge of the moon's waning, but after the full moon, they are difficult to see because they are in the bright edge. Stars appear almost like point sources of light, and they disappear and reappear momentarily, so measuring the time when they do so has been important in the study of the position and movement of the moon. However, the shape of the moon is not a perfect circle, and there are unevenness with mountains and valleys, so the problem is complicated. In recent years, the unevenness of the moon's rim has been studied in great detail, and observations of so-called contiguity, in which a star passes by the northern or southern edge of the moon, have been given importance.

Sometimes a planet is hidden by the moon, and sometimes a planet hides a star. These are also types of occultations. When a planet with a thick atmosphere, such as Jupiter or Saturn, occults a star, the star gradually becomes darker, which has been useful in studying the properties of the atmosphere, and in 1977, when Uranus hid a star, thin rings were discovered around Uranus. Furthermore, occultations of stars by asteroids have been actively observed in recent years, and some asteroids with moon-like structures have been discovered.

[Sadao Murayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

月が恒星や惑星などの前にきて、それらの天体を隠す現象。日食、月食に対して星食ともよぶ。明るい恒星の掩蔽はめったにおこらないが、月の通り道である白道に近いレグルス、スピカ、アンタレス、アルデバランなどの1等星は比較的多く掩蔽される。6等星ぐらいまでの恒星の掩蔽は毎年数十回見ることができる。月が明るいと暗い星の掩蔽は見にくく、満月前は月の欠けた暗い縁に入るので見やすいが、満月後は明るい縁に入るので見にくい。恒星はほとんど点光源のように見え、瞬間的に隠れ、また現れるので、その時刻の測定は、月の位置や運動の研究上重要視されてきた。しかし月の形は真円ではなく、山や谷の凹凸があるので、問題は複雑である。近年は月の周縁の凹凸がかなり詳しく調べられており、月の北や南の端を恒星がかすって通るいわゆる接食の観測が重要視されている。

　惑星が月に隠されることもあり、また惑星が恒星を隠すこともある。これらも掩蔽の一つである。木星や土星のように濃い大気をもつ惑星が恒星を掩蔽するときは、恒星がしだいに暗くなるようすから大気の性質の研究に役だったこともあるし、1977年には天王星が恒星を隠したときの観測から、天王星に細い環が発見された。さらに小惑星による恒星の掩蔽も近年盛んに観測され、そのなかから衛星のようなものがある小惑星も発見されている。

［村山定男］

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