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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.

Mercury as seen by the Messenger space probe (NASA)

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

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

DateRight AscensionDeclinationMagnitudeSize (arc seconds)Illum. (%)Distance (AU)Constellation
October 01, 201213h 25m 29s-09d 23m 32s-0.45.0911.346Virgo
October 02, 201213h 31m 08s-10d 04m 23s-0.45.0911.339Virgo
October 03, 201213h 36m 44s-10d 44m 34s-0.35.1901.331Virgo
October 04, 201213h 42m 19s-11d 24m 06s-0.35.1891.323Virgo
October 05, 201213h 47m 53s-12d 02m 56s-0.35.1891.314Virgo
October 06, 201213h 53m 24s-12d 41m 03s-0.35.2881.305Virgo
October 07, 201213h 58m 54s-13d 18m 26s-0.25.2871.296Virgo
October 08, 201214h 04m 22s-13d 55m 04s-0.25.2861.286Virgo
October 09, 201214h 09m 49s-14d 30m 55s-0.25.3861.276Virgo
October 10, 201214h 15m 14s-15d 05m 58s-0.25.3851.265Virgo
October 11, 201214h 20m 38s-15d 40m 11s-0.25.4841.254Virgo
October 12, 201214h 25m 59s-16d 13m 33s-0.25.4831.242Libra
October 13, 201214h 31m 19s-16d 46m 03s-0.25.5821.230Libra
October 14, 201214h 36m 37s-17d 17m 38s-0.25.5811.218Libra
October 15, 201214h 41m 53s-17d 48m 17s-0.25.6801.205Libra
October 16, 201214h 47m 06s-18d 17m 57s-0.25.6791.191Libra
October 17, 201214h 52m 16s-18d 46m 38s-0.25.7781.177Libra
October 18, 201214h 57m 23s-19d 14m 17s-0.25.8771.163Libra
October 19, 201215h 02m 27s-19d 40m 52s-0.25.9761.148Libra
October 20, 201215h 07m 27s-20d 06m 21s-0.25.9741.133Libra
October 21, 201215h 12m 23s-20d 30m 40s-0.26.0731.117Libra
October 22, 201215h 17m 14s-20d 53m 48s-0.26.1711.101Libra
October 23, 201215h 21m 58s-21d 15m 42s-0.26.2701.084Libra
October 24, 201215h 26m 37s-21d 36m 17s-0.26.3681.067Libra
October 25, 201215h 31m 08s-21d 55m 33s-0.26.4661.050Libra
October 26, 201215h 35m 31s-22d 13m 23s-0.26.5651.032Libra
October 27, 201215h 39m 44s-22d 29m 44s-0.26.6631.013Libra
October 28, 201215h 43m 46s-22d 44m 33s-0.26.8610.994Libra
October 29, 201215h 47m 36s-22d 57m 45s-0.26.9580.975Libra
October 30, 201215h 51m 12s-23d 09m 13s-0.27.0560.956Scorpius
October 31, 201215h 54m 32s-23d 18m 53s-0.17.2530.936Scorpius