For observers at northern temperate latitudes the latter part of September offers the best opportunity this year to spot Mercury in the morning sky. Before that the smallest planet in the Solar System begins the month as a challenging evening object visible very low down above the western horizon just after sunset from southern and equatorial latitudes. During the first few days of September, Venus (mag. -3.8) and Jupiter (mag. -1.7) form a compact triangle together with much fainter Mercury (mag. +1.3 on Sep 1st). Observing Mercury is made difficult due to its dimness and the bright evening twilight. Therefore viewing with binoculars is recommended but of course make sure that the Sun is hidden below the horizon before looking. On September 2nd and 3rd, the extremely thin crescent Moon is in the same region of sky but just a few days later Mercury is completely swallowed by the twilight as it heads towards inferior conjunction on September 13th.
The diagram below shows the view looking west 45 minutes after sunset between July 22nd and September 10th from latitude 35S, approx. equal to Sydney, Cape Town and Santiago. The view will be slightly different from other southerly latitudes.
After passing inferior conjunction, the fast moving planet takes just over a week before it re-appears as a morning object. Observers at northern latitudes should be able to spot it low down above the eastern horizon from about September 22nd. Mercury then remains visible until mid October, reaching its highest point in the sky on September 28th - the date of greatest elongation west (GEW). On this day, the planet is positioned 17.9 degrees west of the Sun. From mid-latitude northern locations it will appear 8 degrees above the eastern horizon, 45 minutes before sunrise (mag. -0.4). Observers should note that Mercury brightens rapidly from mag +1.5 on September 22nd to -0.6 at months end. The planet is also visible from tropical regions but those in the Southern Hemisphere are out of luck, it's badly placed and not easily observable.
On September 29th, a very thin crescent Moon passes less than a degree south of Mercury. The chart below shows Mercury's morning apparition from latitude 51.5N (e.g. London, England). A similar view exists at other northern temperate latitudes.
Venus, mag. -3.9, continues to be visible as early evening object throughout the month. The brightest of all planets can be seen towards the western horizon just after sunset. On September 1st from northern temperate locations, Venus appears just a few degrees above the horizon. Even though the planet continues to move out from the Sun it's also moving southwards in declination and as a result appears only slightly higher at months end. No such issue exists from equatorial and southern latitudes with Venus setting almost 2 hours after the Sun by months end.
On September 3rd, the thin waxing crescent Moon (4% illuminated) passes 1 degree north of Venus with an occultation visible from central Russia at 10:32 UT. Later in the month of September 17th, Venus passes 3 degrees north of Spica (α Vir - mag. +1.0) the brightest star in Virgo.
During September, the phase of Venus decreases from 92% to 86% with its apparent size increasing marginally from 11 to 12 arc seconds.
Although now fading, Mars remains a bright evening object throughout September as the famous "Red planet" continues to move swiftly eastwards (direct). It starts September in Scorpius, 4 degrees northeast of first magnitude red supergiant Antares (α Sco - mag. +1.0) and 6 degrees southwest of Saturn (mag. +0.5). The next day it passes into Ophiuchus where it remains until crossing into Sagittarius on September 22nd. As the month progresses, Mars fades from mag. -0.3 to +0.1 with its apparent size shrinking from 10.5 to 8.8 arc seconds. Consequently even the largest surface markings are now becoming challenging to spot. In addition, the planets disk appears obviously gibbous with a phase of 85 per cent. However, despite fading Mars remains unmistakable to the naked eye and is the brightest object in its surrounding sky.
At the beginning of the month from northern locations, Mars can be seen after sunset towards the south-southwest, remaining visible until about 11pm. By months end it sets an hour earlier. Those living at more southerly locations have it rather better. From Sydney, Mars appears high in the northern sky at sunset, remaining visible throughout the month until after midnight.
On September 9th, the first quarter Moon passes 8 degrees north of Mars. Later on September 28th, Mars passes just 1.5 degrees south of bright emission nebula M8 (the Lagoon Nebula).
Jupiter, mag. -1.7, passes through solar conjunction on September 26th. The planet is only likely to be seen this month from southern and tropical latitudes low down towards the west after sunset. It's visible from the beginning of September until finally lost to the bright twilight sometime during the second week. As previously mentioned, Venus is also nearby and Mercury can be spotted in the same region of sky during the first few days of the month.
For observers at mid-latitude northern temperate locations, Jupiter is not visible during September.
Saturn in Ophiuchus is visible towards the south-southwestern (Northern Hemisphere) / northern (Southern Hemisphere) sky as soon as darkness falls. By months end from mid-northern temperate latitudes the planet appears rather low down in twilight at dusk, although due to its southerly declination its much better placed for observers further south.
The beautiful "Ringed planet" fades slightly this month from mag. +0.5 to +0.6 but is easy to find 6 degrees north of Antares. As mentioned previously, Mars is also positioned nearby at the start of month but the brighter "Red planet" soon moves rapidly away to the east. On September 8th, the waxing crescent Moon will be about 4 degrees north of Saturn offering pleasant early evening viewing.
A good opportunity this month exists to spot Saturn's bizarre moon Iapetus. Known for its "two-tone" colouration where one side is much darker in colour than the other, Iapetus appears considerably brighter when positioned on the western side of Saturn (as viewed from Earth). This phenomenon occurs on September 9th when the moon reaches greatest western elongation. It then shines at magnitude +10.1 and easily within the range of small scopes.
Uranus, mag. +5.7, is now an evening object moving retrograde in Pisces as it heads towards opposition next month. At the start of September, the distant planet rises in the east less than two hours after sunset with its visibility period continuing to improve as the month progresses. By September 30th, Uranus is practically visible all night.
From a dark site, Uranus is faintly visible to the naked eye but otherwise easily spotted with binoculars and small scopes. The ice giant 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).
On September 18th, the waxing gibbous Moon passes 3 degree south of Uranus.
Neptune reaches opposition on September 2nd. Located in Aquarius the outermost planet will be 28.9454 AU (approx. 4,330 million km or 2,691 million miles) distant from Earth and visible all night long. It rises above the eastern horizon at sunset, reaches highest point in the sky during the middle of the night before setting in the west at sunrise.
At apparent mag. +7.8, Neptune is the only planet that's not bright enough to be visible to the naked eye but is relatively easy to spot with binoculars and small scopes. It's currently located about 30 degrees south of the centre of the Great Square of Pegasus and a degree southwest of star lambda Aqr (λ Aqr - mag. +3.7). Positioned about 20 degrees or so further south of Neptune is first magnitude star Fomalhaut (α Psc - mag. +1.2).
Through a small scope at high magnification, Neptune appears as a small featureless blue disk (apparent diameter 2.4 arc seconds). However, the view is not improved greatly even with large amateur scopes. It's also possible to spot Neptune's largest moon Triton (mag. +14.0) but to achieve this a scope of the order of 300mm (12 inches) aperture is recommended, especially from suburban areas.
On September 15th, the almost full Moon passes 1.2 degrees north of Neptune with an occultation visible from UK, Europe and West Russia (20:30 UT).
Solar System Data Table September 2016
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Sep 2016||10h 55m 58.5s||06d 49m 01.4s||-26.7||31.7'||100||1.008||Leo|
|Sun||15th Sep 2016||11h 31m 55.4s||03d 01m 58.3s||-26.7||31.8'||100||1.006||Leo|
|Sun||25th Sep 2016||12h 07m 48.1s||00d 50m 42.7s||-26.7||31.9'||100||1.003||Virgo|
|Mercury||5th Sep 2016||11h 43m 15.9s||-02d 57m 06.3s||2.2||10.2"||12||0.659||Virgo|
|Mercury||15th Sep 2016||11h 12m 56.1s||01d 57m 32.3s||4.9||10.2"||01||0.657||Leo|
|Mercury||25th Sep 2016||11h 06m 01.7s||06d 04m 44.8s||0.1||07.9"||32||0.849||Leo|
|Venus||5th Sep 2016||12h 27m 13.8s||-01d 56m 22.3s||-3.8||11.1"||91||1.510||Virgo|
|Venus||15th Sep 2016||13h 11m 53.0s||-07d 02m 28.7s||-3.9||11.4"||89||1.461||Virgo|
|Venus||25th Sep 2016||13h 57m 20.1s||-11d 54m 24.5s||-3.9||11.8"||87||1.409||Virgo|
|Mars||5th Sep 2016||16h 57m 50.5s||-25d 22m 45.1s||-0.2||10.2"||85||0.917||Ophiuchus|
|Mars||15th Sep 2016||17h 24m 12.1s||-25d 47m 00.2s||-0.1||09.6"||85||0.975||Ophiuchus|
|Mars||25th Sep 2016||17h 52m 13.3s||-25d 54m 21.4s||0.0||09.1"||85||1.032||Scorpius|
|Jupiter||5th Sep 2016||11h 57m 25.8s||01d 28m 21.2s||-1.7||30.8"||100||6.411||Virgo|
|Jupiter||15th Sep 2016||12h 05m 14.9s||00d 37m 17.9s||-1.7||30.6"||100||6.443||Virgo|
|Jupiter||25th Sep 2016||12h 13m 09.7s||-00d 14m 06.0s||-1.7||30.5"||100||6.454||Virgo|
|Saturn||5th Sep 2016||16h 34m 13.3s||-20d 25m 54.0s||0.5||16.6"||100||10.035||Ophiuchus|
|Saturn||15th Sep 2016||16h 36m 02.7s||-20d 31m 43.9s||0.5||16.3"||100||10.199||Ophiuchus|
|Saturn||25th Sep 2016||16h 38m 28.8s||-20d 38m 34.6s||0.5||16.0"||100||10.358||Ophiuchus|
|Uranus||5th Sep 2016||01h 28m 49.7s||08d 37m 26.5s||5.7||03.7"||100||19.181||Pisces|
|Uranus||15th Sep 2016||01h 27m 41.2s||08d 30m 34.5s||5.7||03.7"||100||19.083||Pisces|
|Uranus||25th Sep 2016||01h 26m 21.8s||08d 22m 40.7s||5.7||03.7"||100||19.010||Pisces|
|Neptune||5th Sep 2016||22h 48m 41.8s||-08d 30m 30.4s||7.8||02.4"||100||28.947||Aquarius|
|Neptune||15th Sep 2016||22h 47m 40.8s||-08d 36m 44.0s||7.8||02.4"||100||28.971||Aquarius|
|Neptune||25th Sep 2016||22h 46m 42.6s||-08d 42m 36.5s||7.8||02.4"||100||29.024||Aquarius|