Maximum angle of separation

The distance that the inner planets Mercury and Venus are furthest away from the Sun to the east or west. When they are furthest to the east of the Sun, this is called greatest eastern elongation, and is most often seen in the western sky after sunset around this time. Conversely, when they are furthest to the west, this is called greatest western elongation, and they are seen highest in the eastern sky at dawn around this time. Mercury's maximum distance from the Sun is about 28 degrees, and Venus' is about 47 degrees, but due to their eccentricity and orbital inclination, this can vary greatly from time to time, especially for Mercury. When viewed through a telescope around the time of greatest elongation, Mercury and Venus appear to be crescent-shaped.

[Sadao Murayama]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

内惑星である水星や金星が太陽の西や東にもっとも離れること。太陽の東にもっとも離れるのを東方最大離角といい、そのころ日没後の西天にもっともよく見られる。反対に西にもっとも離れるのが西方最大離角で、そのころ明け方の東天にもっとも高く見える。水星では太陽から最大28度、金星では47度くらいになるが、離心率や軌道傾斜のため、とくに水星はときによって大きく変化する。なお最大離角のころ、水星や金星を望遠鏡で見れば、半月状に見える。

［村山定男］

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