Mercury remains visible during the first part of the month as an early morning object from northern and tropical latitudes. The smallest planet was at its best at the end of September when it passed through greatest elongation west (GEW). As October progresses it gradually draws into the Sun until finally lost from view later in the month.
In terms of visibility, the daily diminishing altitude of Mercury is partly offset by the planet's brightening. For example from London, Mercury shines at mag. -0.8 and appears almost 8 degrees above the horizon, 45 minutes before sunrise on October 1st. Later on the 11th, it has brightened to mag. -1.1 but now only 3 degrees above the horizon. In addition, on the same day the planet passes a degree north of Jupiter. At mag. -1.7 slightly brighter Jupiter may act as a guide to locating Mercury, although both planets will be battling against the bright twilight sky. It's worth noting despite appearing close together in the sky, Jupiter is five times more distant than Mercury. Not long after that, Mercury disappears from view as it heads towards superior conjunction on October 27th.
The chart below shows the morning apparition from latitude 51.5N (e.g. London, England). A similar view exists at other northern temperate latitudes. Those at southern latitudes are out of luck, Mercury is badly placed and not observable on this occasion.
Venus remains a brilliant object in the western sky after sunset. During October it increases slightly in brightness from mag. -3.9 to -4.0. The visibility of Venus is also continuously improving and by months end the planet can be seen for an hour after sunset from mid-northern latitudes and up to 3 hours for locations further south.
There are a handful of pleasant viewing opportunities with Venus this month. On October 3rd, the waxing crescent Moon passes 5 degrees north of the planet. Later on the 26th, Venus moves to 3 degrees north of red supergiant Antares (α Sco - mag +1.0) and then four days later it passes three degrees south of Saturn (mag. +0.6). In between on the 28th, it's positioned approx. midway between Antares and Saturn. Of course, there is no similarity in magnitude; Venus is 70x brighter than Saturn and 100x brighter than Antares!
During October, the phase of Venus decreases from 85% to 78% with its apparent size increasing marginally from 12 to 14 arc seconds.
Mars remains an evening object throughout October. The "Red planet" is now fading and well past its best but still bright enough to be easily found as it moves direct (eastwards) amongst the stars of Sagittarius. With a southern declination of 26 degrees, Mars is better seen from southern and tropical latitudes. For example from northern temperate locations, it appears low down and sets by mid-evening. From southern locations, it's much higher in the sky and sets around midnight.
During October, Mars decreases in magnitude from +0.1 to +0.4 with its apparent size reducing from 8.8 to 7.5 arc seconds. The waxing crescent Moon passes 7 degrees north of the planet on October 8th.
Mars reaches perihelion on October 29th when it will be 1.381 AU or 206.6 million km or 128.4 million miles from the Sun.
Jupiter emerges from the morning twilight during the second week of October, becoming visible above the eastern horizon just before sunrise for northern and tropical based observers. The "King of the planets" shines at mag. -1.7, brighter than any night-time star but towards the lower end of its magnitude range. From southern temperate locations, it's unlikely to been seen until near the end of month.
As previously mentioned, Mercury passes less than a degree north of Jupiter on October 11th. The thin waning crescent Moon passes 1.4 degrees north of Jupiter on October 28th.
Saturn, mag. +0.6, continues to move slowly direct in Ophiuchus a few degrees northeast of Antares. At the start of the month the planet is visible just after sunset low down above the west-southwest horizon from northern latitudes. However, it's not long before Saturn is swamped by the bright twilight sky and by months end it will be inconveniently low down and difficult to spot. From southern and tropical locations, the planet is better placed and can be seen above the western horizon throughout the month.
On October 6th, the waxing crescent Moon passes 4 degrees north of Saturn and as mentioned above, Venus moves between Saturn and Antares on October 28th.
Uranus in Pisces reaches opposition on October 15th and therefore visible all night long. The distant "Ice World" rises above the eastern horizon at sunset, reaches its highest point in the sky during the middle of the night before setting in the west as the Sun re-appears. At mag. +5.7, it's an easy binocular target and under dark skies faintly visible to the naked eye. The day after opposition the full Moon passes 3 degrees south of the planet and since the Moon is at perigee it appears larger and brighter than usual, a so-called "Super Moon".
Uranus is positioned about 15 degrees south and about 25 degrees east of the centre of the "Great Square of Pegasus", close to the mid-point of an imaginary line connecting omicron Psc (ο Psc - mag. +4.3) and epsilon Psc (ε PSc - mag. +4.3).
At opposition, Uranus is located approx. 18.951 AU (approx. 2835 million km or 1762 million miles) from Earth. The planet has an apparent diameter of 3.7 arc seconds and through a small telescope at high magnifications it appears as a small, obviously non-stellar, green disk. However, even with the largest amateur scopes it's difficult to make out any atmospheric details.
Neptune continues to move very slowly retrograde (westwards) in Aquarius during October. The most distant planet is now a month passed opposition but remains well placed for observation, visible as soon as it's dark enough and for most of the remaining night. Although beyond naked eye visibility it's relatively easy to spot with binoculars. During October, Neptune fades slightly from mag. +7.8 to +7.9.
Aquarius is the tenth largest constellation in the sky but it's faint. Its brightest star Sadalsuud (β Aqr) shines at only mag. +2.9 with most other stars notably fainter. Despite this the planet isn't difficult to find once one is familiar with the correct region of sky. To locate Neptune look about 30 degrees southwest of the centre of the Great Square of Pegasus and 2 degrees southwest of star lambda Aqr (λ Aqr - mag. +3.7). Positioned about 20 degrees or so further south is first magnitude star Fomalhaut (α Psc - mag. +1.2).
On October 13th, the waxing gibbous Moon passes 1.2 degrees north of Neptune, with an occultation visible from northwest Canada, eastern Russia (6:01 UT).
Solar System Data Table October 2016
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Oct 2016||12h 43m 58.6s||-04d 43m 32.6s||-26.7||32.0'||100||1.000||Virgo|
|Sun||15th Oct 2016||13h 20m 45.0s||-08d 30m 29.3s||-26.8||32.1'||100||0.997||Virgo|
|Sun||25th Oct 2016||13h 58m 26.6s||-12d 05m 23.2s||-26.8||32.2'||100||0.994||Virgo|
|Mercury||5th Oct 2016||11h 48m 31.0s||03d 15m 36.2s||-1.0||06.0"||75||1.124||Virgo|
|Mercury||15th Oct 2016||12h 50m 03.5s||-03d 30m 39.3s||-1.2||05.1"||95||1.320||Virgo|
|Mercury||25th Oct 2016||13h 52m 47.4s||-10d 42m 57.3s||-1.5||04.7"||100||1.417||Virgo|
|Venus||5th Oct 2016||14h 44m 12.4s||-16d 19m 54.6s||-3.9||12.3"||85||1.354||Libra|
|Venus||15th Oct 2016||15h 32m 53.1s||-20d 06m 16.3s||-3.9||12.9"||82||1.295||Libra|
|Venus||25th Oct 2016||16h 23m 27.0s||-23d 01m 03.3s||-4.0||13.5"||80||1.235||Scorpius|
|Mars||5th Oct 2016||18h 21m 31.7s||-25d 41m 46.1s||0.1||08.6"||85||1.091||Sagittarius|
|Mars||15th Oct 2016||18h 51m 43.8s||-25d 07m 11.7s||0.2||08.1"||85||1.149||Sagittarius|
|Mars||25th Oct 2016||19h 22m 24.5s||-24d 09m 33.4s||0.3||07.7"||86||1.209||Sagittarius|
|Jupiter||5th Oct 2016||12h 21m 06.0s||-01d 05m 19.4s||-1.7||30.6"||100||6.444||Virgo|
|Jupiter||15th Oct 2016||12h 28m 59.3s||-01d 55m 47.1s||-1.7||30.7"||100||6.413||Virgo|
|Jupiter||25th Oct 2016||12h 36m 45.0s||-02d 44m 55.8s||-1.7||31.0"||100||6.360||Virgo|
|Saturn||5th Oct 2016||16h 41m 28.9s||-20d 46m 12.8s||0.6||15.8"||100||10.508||Ophiuchus|
|Saturn||15th Oct 2016||16h 44m 59.4s||-20d 54m 24.1s||0.6||15.6"||100||10.644||Ophiuchus|
|Saturn||25th Oct 2016||16h 48m 56.4s||-21d 02m 52.8s||0.6||15.4"||100||10.763||Ophiuchus|
|Uranus||5th Oct 2016||01h 24m 54.7s||08d 14m 03.4s||5.7||03.7"||100||18.966||Pisces|
|Uranus||15th Oct 2016||01h 23m 23.6s||08d 05m 04.5s||5.7||03.7"||100||18.951||Pisces|
|Uranus||25th Oct 2016||01h 21m 52.4s||07d 56m 07.0s||5.7||03.7"||100||18.967||Pisces|
|Neptune||5th Oct 2016||22h 45m 49.5s||-08d 47m 54.4s||7.8||02.3"||100||29.104||Aquarius|
|Neptune||15th Oct 2016||22h 45m 03.7s||-08d 52m 25.0s||7.8||02.3"||100||29.210||Aquarius|
|Neptune||25th Oct 2016||22h 44m 26.9s||-08d 55m 58.0s||7.9||02.3"||100||29.339||Aquarius|