Mercury is a difficult target for observation this month. The innermost planet can be seen low down in eastern twilight during the first week of September, but at best rises only about 45 minutes before the Sun.
Mercury reaches perihelion on September 2nd, when it's positioned 0.307 AU (approx. 45.9 million kilometres or 28.5 million miles) from the Sun. On September 5th, the planet (mag. -1.1) passes a degree north of Regulus (α Leo - mag. +1.4). It then moves towards superior conjunction, which it reaches on September 20th.
Venus remains a brilliant early evening object from southern and tropical locations throughout September. The unmistakable planet is located in Virgo, and appears as a beacon of light above the western horizon just after sunset. Venus increases in magnitude from -4.6 to -4.8, with its illuminated phase decreasing from 40% to 18%. As the month progresses the planet's apparent diameter increases from 29 to 46 arc seconds.
On September 2nd, Venus passes less than 1.5 degrees south of Spica, the brightest star in Virgo. However, there is no comparison in brightness between the pair. Venus, at mag. -4.6, is about 175x brighter than mag. +1.0 Spica. Later on September 12th, the thin crescent Moon passes 10 degrees north of Venus, providing pleasant early evening viewing.
Despite its brilliance, from mid-latitude northern temperate locations Venus is not so well placed. It hangs low down towards the west-southwest horizon, setting only 45 minutes after the Sun at months end.
Mars starts the month at the Sagittarius and Capricornus constellation border. The red planet continues to move direct and wanders further into Capricornus as the month progresses. It can be seen as soon its dark enough, towards the southeast from northern locations and the northeast from southern latitudes.
During September, Mars fades in magnitude from -2.1 to -1.3, with its apparent diameter decreasing from 21 to 16 ac seconds. With a small telescope, of the order of 80mm (3.1-inch) aperture, and providing recent dust storms have cleared it should be possible to spot details, such as Syrtis Major, on its pink surface.
Mars reaches perihelion on September 16th, when it's positioned 1.381 AU (approx. 206.6 million kilometres or 128.4 million miles) from the Sun. The waxing gibbous Moon passes five degrees north of Mars on September 20th.
Jupiter is moving direct in Libra. Although long past opposition and heading towards solar conjunction it can still be seen just after sunset towards the west. However, from northern temperate latitudes by months end it sets about 90 minutes after the Sun, although observers in the Southern Hemisphere have an extra couple of hours visibility time.
As the month progresses, Jupiter's brightness decreases slightly from magnitude -1.9 to -1.8 with its apparent size shrinking from 35 to 33 arc seconds.
On September 14th, the waxing crescent Moon passes 4 degrees north of the planet.
Saturn remains a well-placed evening object in Sagittarius. The "Ringed" planet begins the month moving retrograde before reaching its secondary stationary point on September 6th, after which direct motion is once again resumed. This is often regarded as signalling the end of the opposition period. To the naked eye, Saturn hardly appears to move this month.
From mid-latitude northern locations, Saturn sets around midnight by months end, but over two hours later from those located further south. During September, its brightness decreases slightly from mag. +0.4 to +0.5 with its apparent size decreasing marginally from 17.3 to 16.5 arc seconds.
The waxing gibbous Moon passes 4 degrees north of Saturn on September 17th. On September 27th, minor planet Vesta (mag. +7.3) passes 3 degrees south of Saturn.
Uranus, mag. +5.7, is now an evening object moving retrograde in Aries. The seventh planet from the Sun is currently heading towards opposition in October. At the start of the month, it rises in the east less than two hours after sunset. The visibility period continues to improve as the month progresses and by the end of September it's practically visible all night long.
From a dark site, Uranus is faintly visible to the naked eye and easily seen with binoculars and small scopes. The ice giant is currently positioned in the south-western constellation corner. By observing the planet over a number of days its slow movement relative to the "fixed" background stars can be noticed.
Uranus has an apparent diameter of 3.7 arc seconds and a small telescope at high magnifications will show a small green disk that's obviously non-stellar. However, even through the largest of amateur scopes, it's difficult to make out any detail.
On September 27th, the waning gibbous Moon passes 5 degrees south of Uranus.
Neptune, the outermost planet, reaches opposition on September 7th. On this day it will be 28.933 AU (approx. 4,328 million kilometres or 2,689 million miles) from Earth and visible all night long. The ice giant rises above the eastern horizon at sunset, reaches its highest point in the sky during the course 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 seen with the naked eye. However, it's relatively easy to spot with binoculars and small scopes. The planet can be found slowly moving retrograde in Aquarius about half way along an imaginary line connecting phi Aquarii (φ Aqr - mag. +4.2) with lambda Aquarii (λ Aqr - mag. +3.7). First magnitude star, Fomalhaut (α Psc - mag. +1.2), is about 23 degrees south of Neptune.
Through a small scope at high magnification, Neptune appears as a small featureless blue disk (apparent diameter 2.4 arc seconds). Even with large backyard scopes, the view is not greatly improved. However, it's possible to spot large moon Triton (mag. +14.0) although a scope of the order of 300mm (12 inches) aperture is recommended, especially from suburban areas.
On September 23rd, the waxing gibbous Moon passes two degree south of Neptune.
Solar System Data Table - September 2018
|Date||Right Ascension||Declination||Mag.||App. Size||Illum. (%)||Dist. (AU)||Constellation|
|Sun||Sep 01||10h 39m 39.0s||08d 28m 06.9s||-26.7||31.7'||100||1.009||Leo|
|Sun||Sep 15||11h 30m 06.0s||03d 13m 41.8s||-26.7||31.8'||100||1.006||Leo|
|Sun||Sep 30||12h 23m 59.1s||-02d 35m 34.1s||-26.7||31.9'||100||1.002||Virgo|
|Mercury||Sep 01||09h 35m 45.3s||15d 03m 44.5s||-0.8||6.4"||65||1.056||Leo|
|Mercury||Sep 15||11h 12m 52.2s||07d 00m 02.2s||-1.6||5.0"||98||1.336||Leo|
|Mercury||Sep 30||12h 50m 43.5s||-04d 41m 57.8s||-1.0||4.8"||98||1.407||Virgo|
|Venus||Sep 01||13h 21m 10.8s||-12d 00m 41.1s||-4.6||29.2"||40||0.572||Virgo|
|Venus||Sep 15||13h 57m 56.5s||-17d 21m 11.7s||-4.7||35.7"||31||0.467||Virgo|
|Venus||Sep 30||14h 21m 07.8s||-21d 10m 14.3s||-4.8||45.5"||18||0.367||Virgo|
|Mars||Sep 01||20h 07m 28.8s||-26d 00m 25.7s||-2.1||20.9"||94||0.449||Sagittarius|
|Mars||Sep 15||20h 15m 44.3s||-24d 44m 37.8s||-1.7||18.4"||91||0.509||Capricornus|
|Mars||Sep 30||20h 34m 46.7s||-22d 45m 51.6s||-1.3||16.0"||89||0.587||Capricornus|
|Jupiter||Sep 01||14h 58m 47.4s||-16d 04m 10.5s||-1.9||34.8"||99||5.668||Libra|
|Jupiter||Sep 15||15h 07m 10.2s||-16d 41m 49.0s||-1.9||33.7"||99||5.858||Libra|
|Jupiter||Sep 30||15h 17m 41.1s||-17d 25m 57.7s||-1.8||32.7"||100||6.036||Libra|
|Saturn||Sep 01||18h 10m 00.2s||-22d 41m 59.5s||0.4||17.3"||100||9.611||Sagittarius|
|Saturn||Sep 15||18h 10m 09.8s||-22d 44m 00.1s||0.5||16.9"||100||9.832||Sagittarius|
|Saturn||Sep 30||18h 11m 51.8s||-22d 45m 41.6s||0.5||16.5"||100||10.080||Sagittarius|
|Uranus||Sep 01||02h 00m 19.7s||11d 40m 05.6s||5.7||3.7"||100||19.263||Aries|
|Uranus||Sep 15||01h 59m 02.5s||11d 32m 56.2s||5.7||3.7"||100||19.092||Aries|
|Uranus||Sep 30||01h 57m 10.5s||11d 22m 40.4s||5.7||3.7"||100||18.958||Aries|
|Neptune||Sep 01||23h 06m 18.8s||-06d 49m 42.8s||7.8||2.4"||100||28.938||Aquarius|
|Neptune||Sep 15||23h 04m 53.2s||-06d 58m 45.3s||7.8||2.4"||100||28.943||Aquarius|
|Neptune||Sep 30||23h 03m 25.3s||-07d 07m 53.6s||7.8||2.4"||100||29.011||Aquarius|