National Astronomical Observatory of Japan



Occultation of Mercury on July 25 (July, 2017)

The occultation of Mercury at major locations in Japan
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Let's play hide-and-seek with Mercury!

On July 25, an “occultation of Mercury” occurs when Mercury gets concealed by the thin 2-day old Moon. During sunset, this phenomenon can be seen in the lower western sky from all over Japan, except for part of the Nansei Islands. Conditions are great because 5 days after that Mercury will reach its greatest eastern elongation, when Mercury seems to be the farthest away from the Sun.

Mercury disappears behind the dark part of the Moon and reappears from the thin bright edge of the Moon. (It is called “disappearance” when an object is concealed behind the Moon, and “reappearance” when it emerged from behind the Moon.) It is difficult to observe the time when Mercury disappears behind the Moon because the sky is still bright. On the other hand, when Mercury reappears the sky will have become dark and you can observe Mercury. However, the altitudes of the Moon and Mercury become very low all over Japan during the reappearance.

Let's observe the reappearance of Mercury with binoculars or a telescope in a place where the lower western sky is unobstructed. It is good to start observing a little earlier than the actual reappearance time, so that you don’t miss the moment of reappearance.

The predicted times for the occultation of Mercury at major locations in Japan are as follows.

The Occultation of Mercury at Major Locations in Japan
LocationDisappearance StartsDisappearance CompleteReappearance StartsReappearance Complete