10 December 2024

Mercury’s best early evening twilight show for 2015

By Ade Ashford

1 May 2015 Ade Ashford

The best evening apparition of innermost planet Mercury for Northern Hemisphere observers during 2015 occurs in the first ten days of May, the planet reaching greatest easterly elongation from the Sun on the 7th. This looping animation shows a 45-degree wide view from west through northwest when the Sun is 10° below the horizon, 2nd-10th May. This equates to 75—80 minutes after sunset in the British Isles. Let Venus be your guide to Mercury as the pair remain approximately 22° apart — the span of an outstretched hand at arm’s length — during this period. AN graphics by Ade Ashford.

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!