Mercury passed through inferior conjunction at the end of September and at the start of October remains too close to the Sun to be visible. However, the planet moves fast and just over two weeks later on October 16th it reaches greatest western elongation (18.1 degrees). As a result, Mercury is visible as an early morning object from northern temperate locations - and to a lesser extent from tropical latitudes - from about the first week of October until the last few days of the month.
This also happens to be the most favourable morning apparition of the year for observers at these latitudes. For example, from 52N (e.g. London, England) the planet will be 9 degrees above the east-southeastern horizon 45 minutes before sunrise on October 16th. Mercury's altitude then decreases gradually each subsequent day until it's lost to the morning twilight at months end. During the visibility period the planet brightens rapidly from mag. +1.3 to -1.0. In the same area of sky about 25 degrees to the northwest are Venus (mag. -4.5), Jupiter (mag. -1.8) and Mars (mag. +1.8).
On October 11th, the waning crescent Moon passes 0.9 degrees south of Mercury with an occultation visible from South S. America and the Falkland Islands. The following day the planet reaches perihelion at 0.307 AU (approx. 45.9 million kilometres or 28.5 million miles) from the Sun.
For Southern Hemisphere observers, Mercury is inconveniently placed for observation this month.
Venus like Mercury also reaches greatest western elongation this month; on October 26th it moves out to 46 degrees from the Sun. The unmistakable planet is a blazing beacon of light in the eastern morning sky, visible for over 4 hours by months end from northern temperate latitudes although only about half this time from the Southern Hemisphere.
Venus is so bright that it easily outshines any other planet in the Solar System or any night-time star. This month the dazzling planet is not alone in the early morning sky, Jupiter and Mars are nearby. Those at northern latitudes will also be able to spot a fourth planet, Mercury, low down above the horizon from the first week of the month.
On October 8th, the waning crescent Moon passes 0.7 degrees south of Venus with an occultation visible from Australia and New Zealand at 20:33 UT. The next day, Venus passes 2.5 degrees south of Regulus (α Leo - mag. +1.4) with the waning crescent Moon, Mars and Jupiter all close by. For comparison, Venus is 12 times brighter than Jupiter with Jupiter 25 times brighter than Mars!
Later on October 26th, Venus passes 1.1 degrees south of Jupiter. This month there are many wonderful viewing and photo opportunities with these planets grouped so closely together.
Mars is an early morning object in Leo during October. From northern temperate latitudes it's visible towards the east for 3 hours before sunrise at the start of the month, improving to over 4 hours by months end. However, the planet is less well placed for those living further south.
The magnitude of Mars improves slightly from +1.8 to +1.7 although it's not particularly bright. However, the planet's close proximity to Venus and Jupiter will aid significantly in locating it. With the Earth now slowly closing in on "The Red Planet" its visibility will gradually improve month by month until opposition is reached on May 22, 2016. Mars will then shine at magnitude -2.1, almost 40 times brighter than it appears now! Telescopically observers have the chance then to study or image the Martian surface in some detail, something that's currently impossible due to its small apparent size of just 4 arc seconds.
On October 9th, the waning crescent Moon passes 3 degrees south of Mars. As previously mentioned this forms a wonderful pre-dawn setting along with Venus, Jupiter and Regulus. Later on the 17th, Mars passes 0.4 degrees north of Jupiter with both planets easily fitting into the same low/medium magnification telescope eyepiece field of view.
Jupiter is now a beautiful early morning object moving direct among the stars of Leo. During October its brightness and apparent size improve slightly from magnitude -1.7 to -1.8 and 31 to 33 arc seconds respectively. What makes for wonderful pre-dawn spectacles this month is Jupiter's close proximity to even more brilliant Venus. At start of month, Venus is about 15 degrees west of Jupiter. On October 26 it overtakes the giant planet passing 1.1 degrees to the north before ending the month about 5 degrees further to the southeast.
The waxing Moon passes 3 degrees south of Jupiter on October 10th and as previously mentioned Jupiter passes less than half a degree south of Mars on October 17th.
Saturn, mag. +0.6, is an early evening object that's visible as soon as it's dark enough. On October 1st from northern temperate latitudes it appears a few degrees above the southwestern horizon, setting about 2 hours after the Sun. The visibility period of the planet continues to diminish as the month progresses and by the end of October it will be inconveniently submerged in the twilight at dusk. Saturn is somewhat easier to see further south. From southern temperate latitudes the "Ringed planet" is visible for up to twice as long.
The thin waxing crescent Moon passes 3 degrees north of the planet on October 16th. On the same day, Saturn eastward motion carries it across the constellation boundary into Scorpius from Libra. Located 10 degrees to the southeast is first magnitude star Antares (α Sco mag. +1.0).
Uranus reaches opposition on October 12th in Pisces and is therefore visible all night. 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 magnitude +5.7, the planet is faintly visible to the naked eye under dark skies. Even if you can't spot Uranus with the naked eye it's a very easy binocular target.
At opposition, Uranus is located approx. 18.984 AU (approx. 2840 million km or 1765 million miles) from Earth. The planet has an apparent diameter of 3.7 arc seconds and a small telescope at high magnification will show it as a small green disk, obviously non-stellar. However, even when viewed through the largest of amateur scopes it's difficult to make out any details.
On October 26th, the full Moon passes 0.9 degrees south of Uranus with an occultation visible from New Zealand at 10:25 UT.
Neptune continues to move very slowly retrograde in Aquarius during October. The most distant planet in the Solar System is now one month passed opposition but remains well placed for observation, visible as soon as it's dark enough and for most of the night. Neptune is too faint to be seen with the naked eye but relatively easy to spot with binoculars. During October it fades slightly from magnitude +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 magnitude +2.9 and most other stars are considerably fainter. Despite this once familiar with the surrounding area of sky locating Neptune isn't that difficult. The planet is currently positioned towards the constellation centre about three quarters along an imaginary line connecting lambda Aqr (λ - mag. +3.7) with sigma Aqr (σ Aqr - mag. +4.8).
On October 23rd, the waxing gibbous Moon passes 3 degrees north of Neptune.
Solar System Data Table October 2015
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Oct 2015||12h 41m 16.2s||-04d 26m 22.3s||-26.7||32.0'||100||1.000||Virgo|
|Sun||15th Oct 2015||13h 17m 59.8s||-08d 14m 00.0s||-26.8||32.1'||100||0.997||Virgo|
|Sun||25th Oct 2015||13h 55m 37.2s||-11d 50m 03.4s||-26.8||32.2'||100||0.995||Virgo|
|Mercury||5th Oct 2015||12h 07m 43.6s||-01d 56m 36.7s||3.1||09.6"||06||0.701||Virgo|
|Mercury||15th Oct 2015||12h 13m 58.5s||00d 12m 17.9s||-0.5||07.2"||49||0.940||Virgo|
|Mercury||25th Oct 2015||13h 02m 50.3s||-04d 31m 46.6s||-0.9||05.6"||84||1.195||Virgo|
|Venus||5th Oct 2015||09h 52m 35.4s||10d 07m 53.5s||-4.6||31.2"||38||0.534||Leo|
|Venus||15th Oct 2015||10h 26m 07.1s||08d 26m 21.3s||-4.5||27.4"||44||0.608||Leo|
|Venus||25th Oct 2015||11h 03m 04.0s||05d 56m 15.0s||-4.4||24.4"||50||0.683||Leo|
|Mars||5th Oct 2015||10h 32m 55.0s||10d 31m 20.6s||1.8||04.0"||97||2.368||Leo|
|Mars||15th Oct 2015||10h 56m 14.6s||08d 13m 33.8s||1.8||04.1"||96||2.312||Leo|
|Mars||25th Oct 2015||11h 19m 07.8s||05d 52m 38.9s||1.7||04.2"||96||2.250||Leo|
|Jupiter||5th Oct 2015||10h 52m 56.4s||08d 08m 05.3s||-1.7||31.6"||100||6.242||Leo|
|Jupiter||15th Oct 2015||11h 00m 20.2s||07d 24m 00.7s||-1.8||32.0"||100||6.154||Leo|
|Jupiter||25th Oct 2015||11h 07m 18.7s||06d 42m 14.4s||-1.8||32.6"||100||6.048||Leo|
|Saturn||5th Oct 2015||15h 57m 32.3s||-18d 40m 51.7s||0.6||15.7"||100||10.613||Libra|
|Saturn||15th Oct 2015||16h 01m 25.3s||-18d 53m 46.1s||0.6||15.5"||100||10.731||Libra|
|Saturn||25th Oct 2015||16h 05m 40.1s||-19d 07m 05.8s||0.6||15.3"||100||10.830||Scorpius|
|Uranus||5th Oct 2015||01h 09m 33.0s||06d 39m 21.9s||5.7||03.7"||100||18.991||Pisces|
|Uranus||15th Oct 2015||01h 08m 02.3s||06d 30m 09.6s||5.7||03.7"||100||18.986||Pisces|
|Uranus||25th Oct 2015||01h 06m 32.7s||06d 21m 06.9s||5.7||03.7"||100||19.012||Pisces|
|Neptune||5th Oct 2015||22h 37m 18.5s||-09d 34m 50.7s||7.8||02.3"||100||29.127||Aquarius|
|Neptune||15th Oct 2015||22h 36m 33.7s||-09d 39m 10.6s||7.8||02.3"||100||29.236||Aquarius|
|Neptune||25th Oct 2015||22h 35m 58.4s||-09d 42m 32.2s||7.9||02.3"||100||29.367||Aquarius|