In November 1677, Halley observed a transit of the innermost planet, Mercury, from the desolate island of St Helena in the South Pacific.

1677年11月，哈雷在南太平洋荒凉的圣赫勒拿岛上观测到了水星（最靠近太阳的行星）的凌日现象。

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June 2004 saw the first passage, known as a "transit", of the planet Venus across the face of the Sun in 122 years.

2004年6月，金星的投影穿越太阳表面，这是122年一遇的天象，被称为“凌日”。

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When you see a planet by a transit, the orbit has to be in your line of sight.

当你通过经纬仪看到行星，其轨道应该在你的视线中。

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This was because they saw the black-drop effect in a transit of Mercury, a planet that has no atmosphere.

ECONOMIST: Transits of Venus

During such a transit, the observed brightness of the star drops slightly because the planet blocks a part of the starlight.

BBC: New robotic telescope in Chile set for planet hunt

The transit method for finding planets searches for the small drop in brightness of a star that occurs when a planet passes in front of it and blocks some of its light.

ENGADGET: 9-gigapixel image of the Milky Way reminds us just how small we truly are