Mercury, the innermost planet, is now heading towards greatest eastern elongation (GEE), which it reaches on April 1, 2017. From mid-March, observers at northern-based locations should be able to spot the elusive planet, low down above the western horizon just after sunset. Each subsequent evening it improves in visibility until GEE is reached. After that, Mercury sinks gradually back towards the horizon until about 10 days later when it becomes lost to the bright twilight sky. This also happens to be the best evening apparition of the year from northern locations.

From southern latitudes, Mercury is unsuitably placed for observation this time.

Mercury as seen by the MESSENGER space probe (credit:- freestarcharts)

To spot Mercury, a clear unobstructed view of the western horizon is essential. At the time of GEE the planet will shine at mag. -0.2 and be positioned 19 degrees from the Sun. From mid-latitude northern locations, it appears 10 degrees above the western horizon, 45 minutes after sunset. Observers should note that the planet's brightness decreases during the visibility period. For example on March 17th, it will shine at mag. -1.4, but be down to mag. +0.8 by April 6th.

The diagram below shows the altitude of Mercury 45 minutes after sunset as seen from latitude of 51.5N (e.g. London, England). A similar view exists at other northern temperate latitudes.

Mercury 45 minutes after sunset from mid-latitude northern locations (credit:- freestarcharts)


Sky Highlights - April 2017

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