See Mercury at its best in the evening sky

By Ade Ashford

Mercury attains a greatest easterly elongation of 19 degrees from the Sun on 1 April. This solar separation combined with a favourable inclination of the ecliptic to the western horizon an hour after sunset means that the next ten days or so offer the best evening showing of the innermost planet for Northern Hemisphere observers for the entire year. In the looping animation above, which simulates a naked-eye view of the UK western horizon an hour after sunset from 25 March—1 April, note that planet Mars (magnitude +1.5) lies about three-quarters of a span of an outstretched hand at arm’s length to the upper left of Mercury. AN animation by Ade Ashford.
Mercury passed through superior conjunction on 7 March making it an evening object, but UK observers had to wait until around 18 March before it moved far enough from the Sun to be glimpsed with difficulty low in the west soon after sunset. Now that we are in the last week of March, the innermost planet finally ascends high enough into the twilight sky of the British Isles to be seen with the unaided eye against the constellation backdrop of Pisces, moving into Aries at the end of the month.

For a convenient celestial guide to the innermost planet, on Wednesday 29 March look very low due west an hour after sunset (8:40pm BST for the heart of the UK, click here for other locations) to see magnitude -0.4 Mercury 8 degrees to the right of a 43-hour-old slim crescent Moon.

Mercury begins April as a magnitude -0.1 object in the constellation Aries at its greatest easterly elongation of 19 degrees from the Sun, in what is the best evening apparition of the year for Northern Hemisphere observers owing to the favourable inclination of the ecliptic to the western horizon.

As seen from the heart of the British Isles at 9pm BST, the innermost planet can be found almost 6 degrees above the west-northwest horizon from 1-8 April. Thereafter, Mercury’s brightness drops rapidly as its orbit brings it between us and the Sun. The innermost planet is at inferior conjunction on the morning of 20 April and crosses the border into Pisces the following day.

Inside the magazine

For a comprehensive guide to observing all that is happening in the coming month’s sky, tailored to Western Europe and North America, obtain a copy of the April 2017 edition of Astronomy Now.

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