Good Morning Mercury

The Night Sky

The crescent Moon will be a good signpost for Mercury on 22 October. AN graphic by Greg Smye-Rumsby.
The crescent Moon will be a good signpost for Mercury on 22 October. AN graphic by Greg Smye-Rumsby.

Forget about Mercury’s evening appearance in the first half of this month – it is its morning apparition towards the end of October and into November that is the cause of celebration.

Although Mercury was at greatest elongation on 21 September (23 degrees east of the Sun) and does not reach inferior conjunction until 16 October, this particular evening apparition is very unfavourable for observers based in the UK because of its decreasing brightness and very low altitude in the twilight skies. However, throughout the second half of October, Mercury heads ever west of the Sun as it shoots into the morning skies to make a very favourable showing. By 22 October Mercury is already eleven degrees from the Sun; rising before 7am it is a good eight degrees high at sunrise an hour later. Although it is only magnitude +2 on this date, the presence of an old and narrow waning crescent Moon (28 days old) less than seven degrees to the west of Mercury allows the user of 7 × 50 binoculars to spot both objects in the same field-of-view, given a reasonably unobstructed eastern horizon and tolerably transparent skies. Importantly, never sweep for any object so close to the Sun if the Sun is directly visible above the horizon, since an inadvertent magnified view of the Sun, however brief, has the potential to permanently damage your eyesight.

During the last week of October, Mercury brightens and climbs higher in the dawn skies; by the month’s end it shines at magnitude –0.4 and is a good 15 degrees high by sunrise at 8am. Telescopically, the planet’s phase is increasing while its angular diameter is shrinking (see page 43). At greatest elongation west on 1 November it is a little more than half illuminated and measures seven arcseconds in apparent diameter – still large enough for those with acute vision to see some dusky surface details, given a decent instrument (say a 125mm Maksutov–Cassegrain telescope, a 100mm apochromat or 150mm Newtonian) and good seeing conditions. Filter work in twilight will help increase contrast between planet and sky and, to some extent, improve seeing. An orange (W21), red (W23A, W25 or W29) filter will improve contrast in these conditions, while in daylight a yellow (W12) filter will reduce scattered blue light.