With the demise of NASA’s highly successful MESSENGER spacecraft in a spectacular April 30th crash on the surface of Mercury still firmly in our minds, observers in the British Isles wishing to see the innermost planet for themselves need only wait until early evening twilight since the little world is best placed for Northern Hemisphere observers in the first ten days of May.
Mercury isn’t a difficult naked-eye object if (1) the sky is transparent enough, (2) you select an observing location that gives you an unobstructed view of the horizon from west through northwest, and (3) you time your observation just right — the secret lies in leaving it just late enough for dusk twilight to fade to a level that the planet is visible, but not so late that it’s too close to the horizon and lost in the haze.
The optimum time to view Mercury with the naked eye is when the Sun is about 10° below the horizon, which is currently ~75—80 minutes after local sunset in the UK. Our online Almanac will provide you with this information for your nearest large city which is accurate enough for our purposes. For observers in the centre of the British Isles, the best time would be 9:55 pm on 1st May and 10:15 pm by 10th May, the Sun reaching an altitude of -10° about 2 minutes later with each passing day (all times BST).
Fortunately, Mercury has a very convenient beacon to aid its location over the next week or so in the form of dazzling Venus. Both Venus and Mercury maintain an almost constant angular separation of around 22° as they move against the background stars from 1st through 10th May. Since 22° is about the span of an outstretched hand at arm’s length, one only needs to offset this amount to the lower right of Venus to find Mercury at the stated times. Caution: if you choose to sweep for Mercury with binoculars, please only do so after the Sun has set. Clear skies and happy hunting!
Inside the magazine
You can find out more about Mercury in the May edition of Astronomy Now in addition to a full observing guide to the night sky.
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