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THE APPEARANCE OF MERCURY
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revolution; that is to say, the arc MN is a little less than one third of the circle.

Now suppose that when the earth is at E, Mercury, instead of being at M is near the highest point A of the orbit as represented in the figure. It will then be at its greatest apparent distance from the sun as we see it from the earth; or, in technical language, at its greatest east elongation. Being east of the sun it will

Fig. 29.—Elongations of Mercury.

then set after the sun, by a time generally between an hour and a quarter and an hour and a half. This is the most convenient time for seeing it. If the sky is clear, it will readily be seen in the twilight from half an hour to an hour after sunset. At the opposite elongation, near C, it is west of the sun; then it rises before the sun and may be seen in the morning twilight.

The Surface and Rotation of Mercury

The best time to make a telescopic study of Mercury is late in the afternoon, when it is near east elongation.

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