The opportunity exists this month for equatorial and southern hemisphere observers to catch a glimpse of the elusive planet Mercury. When viewed from Earth, Mercury never ventures far from the Sun and hence is a planet that is easily visible on only a few occasions per year. It appears as either a morning or evening star, almost always in twilight and never against a black midnight sky. On October 26th, Mercury reaches greatest eastern elongation and hence is visible for a short time in the early evening sky.
The maximum Mercury elongation possible is 28 degrees from the Sun. This month's elongation is close to the maximum at 24 degrees. However due to the ecliptic angle, the planet is not suitably placed for observers from northern temperate latitudes. However, it is an easy object for observers located at equatorial and southern hemisphere latitudes. The diagram below shows the position of Mercury as viewed from Sydney Australia (35S), 30 minutes after sunset.
Mercury evening star October 2012 - pdf format
The best time to view Mercury is a few days either side of October 26th. Start looking for Mercury about 30 minutes after sunset. Allow your eyes to relax in the evening twilight and then you should be able to pick out Mercury hovering gently above the western horizon. On October 26th, Mercury will be nearly 18 degrees above the horizon at this time. Once you locate the planet you can then watch it descend towards the horizon over the next hour or so before setting. The magnitude of Mercury is –0.2 and the illuminated phase 65%.
In fact, the brightness of Mercury changes little during October. It starts the month at magnitude –0.4 before fading to magnitude -0.1 at months end. During the same time period, the illuminated phase decreases from 91% to 53%. With a small telescope, around 100mm (4-inch) aperture, at high magnification the phase of Mercury should be noticeable.
This month, Mercury has a small apparent size between 5 and 7 arc seconds. Even with the largest of telescopes no discernable details are visible on the planets surface.
Mercury Data Table - October 2012
| Date | Right Ascension | Declination | Magnitude | Size (arc seconds) | Illum. (%) | Distance (AU) | Constellation |
|---|---|---|---|---|---|---|---|
| October 01, 2012 | 13h 25m 29s | -09d 23m 32s | -0.4 | 5.0 | 91 | 1.346 | Virgo |
| October 02, 2012 | 13h 31m 08s | -10d 04m 23s | -0.4 | 5.0 | 91 | 1.339 | Virgo |
| October 03, 2012 | 13h 36m 44s | -10d 44m 34s | -0.3 | 5.1 | 90 | 1.331 | Virgo |
| October 04, 2012 | 13h 42m 19s | -11d 24m 06s | -0.3 | 5.1 | 89 | 1.323 | Virgo |
| October 05, 2012 | 13h 47m 53s | -12d 02m 56s | -0.3 | 5.1 | 89 | 1.314 | Virgo |
| October 06, 2012 | 13h 53m 24s | -12d 41m 03s | -0.3 | 5.2 | 88 | 1.305 | Virgo |
| October 07, 2012 | 13h 58m 54s | -13d 18m 26s | -0.2 | 5.2 | 87 | 1.296 | Virgo |
| October 08, 2012 | 14h 04m 22s | -13d 55m 04s | -0.2 | 5.2 | 86 | 1.286 | Virgo |
| October 09, 2012 | 14h 09m 49s | -14d 30m 55s | -0.2 | 5.3 | 86 | 1.276 | Virgo |
| October 10, 2012 | 14h 15m 14s | -15d 05m 58s | -0.2 | 5.3 | 85 | 1.265 | Virgo |
| October 11, 2012 | 14h 20m 38s | -15d 40m 11s | -0.2 | 5.4 | 84 | 1.254 | Virgo |
| October 12, 2012 | 14h 25m 59s | -16d 13m 33s | -0.2 | 5.4 | 83 | 1.242 | Libra |
| October 13, 2012 | 14h 31m 19s | -16d 46m 03s | -0.2 | 5.5 | 82 | 1.230 | Libra |
| October 14, 2012 | 14h 36m 37s | -17d 17m 38s | -0.2 | 5.5 | 81 | 1.218 | Libra |
| October 15, 2012 | 14h 41m 53s | -17d 48m 17s | -0.2 | 5.6 | 80 | 1.205 | Libra |
| October 16, 2012 | 14h 47m 06s | -18d 17m 57s | -0.2 | 5.6 | 79 | 1.191 | Libra |
| October 17, 2012 | 14h 52m 16s | -18d 46m 38s | -0.2 | 5.7 | 78 | 1.177 | Libra |
| October 18, 2012 | 14h 57m 23s | -19d 14m 17s | -0.2 | 5.8 | 77 | 1.163 | Libra |
| October 19, 2012 | 15h 02m 27s | -19d 40m 52s | -0.2 | 5.9 | 76 | 1.148 | Libra |
| October 20, 2012 | 15h 07m 27s | -20d 06m 21s | -0.2 | 5.9 | 74 | 1.133 | Libra |
| October 21, 2012 | 15h 12m 23s | -20d 30m 40s | -0.2 | 6.0 | 73 | 1.117 | Libra |
| October 22, 2012 | 15h 17m 14s | -20d 53m 48s | -0.2 | 6.1 | 71 | 1.101 | Libra |
| October 23, 2012 | 15h 21m 58s | -21d 15m 42s | -0.2 | 6.2 | 70 | 1.084 | Libra |
| October 24, 2012 | 15h 26m 37s | -21d 36m 17s | -0.2 | 6.3 | 68 | 1.067 | Libra |
| October 25, 2012 | 15h 31m 08s | -21d 55m 33s | -0.2 | 6.4 | 66 | 1.050 | Libra |
| October 26, 2012 | 15h 35m 31s | -22d 13m 23s | -0.2 | 6.5 | 65 | 1.032 | Libra |
| October 27, 2012 | 15h 39m 44s | -22d 29m 44s | -0.2 | 6.6 | 63 | 1.013 | Libra |
| October 28, 2012 | 15h 43m 46s | -22d 44m 33s | -0.2 | 6.8 | 61 | 0.994 | Libra |
| October 29, 2012 | 15h 47m 36s | -22d 57m 45s | -0.2 | 6.9 | 58 | 0.975 | Libra |
| October 30, 2012 | 15h 51m 12s | -23d 09m 13s | -0.2 | 7.0 | 56 | 0.956 | Scorpius |
| October 31, 2012 | 15h 54m 32s | -23d 18m 53s | -0.1 | 7.2 | 53 | 0.936 | Scorpius |