Observe Mercury as its morning best – Astronomy Now

Mercury is now right at the start of its most favourable morning apparition of the year for observers at mid-northern latitudes. After inferior conjunction on the nearside of the Sun on 19 August, Mercury has sped west of the Sun at across a flat eastern horizon it’s possible to see it until around 18 September (from the south of England). 

Mercury is often troublesome to locate and difficult to observe, as, outside of observing it in broad daylight, it’s visible only close to dawn or dusk. As the closest planet to the Sun it never strays too far from our star’s glare.

On 31 August, Mercury lies 16.5° west of the Sun and lies around 6° to 7° up at the onset on civil twilight, some 35 minutes before sunrise. Shining at magnitude +0.8 it should be bright enough to spot, provided the eastern horizon is relatively free rom haze and significant light pollution, though atmospheric extinction will knock a bit off Mercury’s lustre. A pair of binoculars might come in handy otherwise.

At the beginning of September, Mercury shines at magnitude +0.56 and from the south of England lies 8° high at 5.40am BST, some 35 minutes before sunrise (Mercury lies progressively lower the farther north you go; in Edinburgh just over 4° altitude at about the same time). It has the company of the very slender crescent of an old moon lying above. 

On 5 September Mercury reaches greatest western elongation (18°) from the Sun. The planet now shines at magnitude –0.23 and in London peaks 10° high at about 5.45am. A small telescope shows Mercury’s fast-changing phase, changing from a 29.2 per cent illuminated crescent on 1 September to a gibbous 67 per cent phase by 10 September. 

Try adding colour filters to improve contrast of dusky marking seen on Mercury. An orange or light red filter (Wratten 21 or 23A) is best for small-aperture telescopes, while red or deep-red filters (Wratten 25 or 29) are more useful for a larger telescopes. Observers have reported that violet, green and light-blue filters (Wratten 47,68 and 80A) are useful also.

At mid-September, Mercury shines at mag. –1.13, but has descended to just 6° altitude in London, at about 6am, just about bringing this apparition to an end. Happily, this is not the last chance to catch Mercury this year. Although there are no further favourable evening outings, the planet offers a reasonable pre-dawn showing for the second half of December.