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Astronomy Now See innermost planet Mercury’s favourable dawn show

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Ade Ashford


Observers in the British Isles should find an observing location offering an unobscured east-northeast horizon around the beginning of civil twilight (40 minutes before sunrise in the heart of the UK) between now and 9 September to catch a glimpse of Mercury. The innermost planet attains a westerly elongation of 18°from the Sun on 26 August. This looping animation covering 18 mornings is about 60 degrees wide, or three times the span of an outstretched hand held at arm’s length. Note that the Moon’s apparent size is enlarged for clarity. Caution: only look for Mercury with binoculars and telescopes before sunrise. AN graphic by Ade Ashford.
Have you ever seen all five of the bright naked-eye planets from the British Isles? If not, then the last week of August presents a good opportunity to do so. Four of the planets currently reside in the evening sky. Venus, magnitude -4.4, is very low in the west-southwest by civil dusk (currently about 40 minutes after sunset for the UK) when you’ll also find magnitude -2.0 Jupiter low in the southwest. By 10pm BST, ringed planet Saturn (magnitude +0.3) is low in the south, while Mars, still big and bright at magnitude -2.3, may be found low in the south-southeast.

If you wish to add Mercury to your tally then you’ll need to be something of an early riser over the next 2½ weeks or so. You need to find a viewing location that offers an unobstructed view from northeast through east around 40 minutes before sunrise (hint: our interactive online Almanac provides this information) when you stand your best chance of catching a glimpse of the innermost planet with the unaided eye or binoculars some 8 degrees (or slightly less than the span of your fist at arm’s length) above a level horizon in the growing twilight. One word of caution: never use binoculars or telescopes to search near the horizon if sunrise is imminent — it’s not worth risking your eyesight.

Mercury attains a westerly elongation of 18 degrees from the Sun on 26 August. The planet is magnitude zero on this date and grows steadily brighter over subsequent mornings, moving from the constellation of Cancer into Leo on 29 August. Mercury is at its highest in the east-northeast at the beginning of civil twilight from 27-31 August. By 3 September, Mercury is shining at magnitude -1.

At civil dawn in the UK (about 5:50am BST) on Thursday, 6 September, Mercury passes just 1 degree – or twice the apparent width of the full Moon – north of Regulus, the brightest star in the constellation of Leo. The slim waning crescent of a 27-day-old Moon lies to the upper right of Regulus and Mercury on the UK morning of Saturday, 8 September.

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