Mercury’s transit of the Sun on 11 November is still fresh in the memory, but it doesn’t take long for the innermost planet’s orbital motion to carry it far enough west of the Sun to be visible low above the southeastern horizon in dawn twilight. Mercury attains its greatest westerly elongation of 20 degrees on the UK morning of 28 November. In fact, for Northern Hemisphere observers, the remainder of the month into early December offers Mercury’s best morning viewing prospects for the entire year.
Any opportunity to get a glimpse of this elusive and fast-moving planet is well worth getting up a little earlier for, particularly when – as now – you get a chance to see Mars nearby at the same time. As with any observation made in the eastern sky during dawn twilight, timing is everything: you need to view late enough that Mercury gets a chance to rise high enough above the horizon murk, but not so late that impending sunrise makes the sky too bright to see it. (Never look anywhere near the Sun with an unfiltered optical instrument after it has risen.)
Observers in the British Isles need to find a location that offers an unobscured view of the southeast horizon about three-quarters of an hour before sunrise between now and the first week of December. Our interactive online Almanac gives you the time of sunrise for your nearest town or city, so just subtract 45 minutes from that.Mercury is located in the constellation of Libra for the period illustrated in the animation at the top of the page. The planet lies about 9 degrees (almost the span of a fist held at arm’s length) above the southeast horizon at the optimal viewing time between 23 November and the beginning of December. The Red Planet sits midway between Mercury and the first-magnitude star Spica, the brightest in the constellation of Virgo, at UK dawn on 24 November.
Magnitude +1.7 Mars remains in Virgo until the morning of 1 December when it crosses the constellation border to join Mercury in Libra. Mercury brightens more than fourfold from magnitude +1 to -0.6 during the 18 November to 3 December observing window. If clear, don’t miss the binocular highlights of 24 and 25 November at dawn when the old waning crescent Moon lies 4° above Mars and 3° to the lower left of Mercury, respectively.