Halley realised that Mercury was so far away that its parallaxangle would be very difficult to determine.
哈雷意识到水星离地球如此之远,以至于很难确定它的视差角。
2
As Venus was closer to the Earth, its parallaxangle would be larger, and Halley worked out that by using Venus it would be possible to measure the Suns distance to 1 part in 500.