For observers at northern temperate latitudes, September offers the best opportunity this year to see Mercury in the morning sky. During the second week of the month, it should be possible to spot the elusive planet low down above the eastern horizon just before sunrise. Mercury appears slightly higher in the sky each following morning until greatest elongation is reached on September 12th, when it will be 17.9 degrees west of the Sun. As it draws back towards the Sun the planet remains visible, until finally lost to the bright twilight towards the end of the month. Observers should note that Mercury increases in brightness from magnitude +0.5 to -1.3 during the visibility period.
In the same region of sky as Mercury are Venus, Mars and first magnitude star Regulus (α Leo - mag. +1.4). From about September 5th to 12th, Mercury forms a close grouping in the bright twilight with Regulus and Mars (mag. +1.8). At times, binoculars will be required to spot the trio, but some care is also needed. You should never scan the sky with optical instruments after the Sun has risen. Brilliant Venus, mag. -3.9, is positioned about 15 degrees above this grouping. On the 10th, Mercury passes just 0.6 degrees south of Regulus. Mercury then passes a mere 3.5 arc minutes north of Mars on the 16th with the waning crescent Moon passing a fraction of a degree south of Mercury on the 18th.
From southern temperate locations, the planet is unsuitably placed for observation.
Venus, mag. -3.9, remains a brilliant early morning object, visible towards the east just before sunrise. On September 1st, the planet rises about 2 hours before the Sun from northern temperate locations although considerably less for those located further south.
As previously mentioned, Mercury, Mars and Regulus are positioned in the same region of sky. On September 18th, the thin waning crescent Moon passes 0.5 degrees south of Venus and on the following day Venus passes 0.5 degrees north of Regulus.
Mars, in Leo, returns to the morning sky. The "Red planet" is faint at magnitude +1.8 and best seen towards the end of the month. It appears low down before sunrise below Venus. As stated above, Mars forms a nice trio with Mercury and Regulus from about September 5th to 12th, although binoculars at times will be required to spot all three.
On September 5th, Mars passes 0.7 degrees north of Regulus. Mercury passes 0.1 degrees north of Mars on the 16th with the thin waning crescent Moon passing a tenth of a degree north of Mars on the 18th.
Jupiter, mag. -1.7, remains visible towards the west just after sunset although observers at mid-latitude northern temperate regions will probably lose the planet to the bright twilight sometime after mid-month. However, it can be seen throughout the month from tropical and southern locations.
On September 5th, Jupiter passes 3 degrees north of Spica (α Vir - mag. +1.0). The thin crescent Moon appears close to Jupiter during the evening of the 22nd.
Saturn, mag. +0.5, remains an early evening object throughout September. It can be seen towards the southwest (northern locations) / west (southern locations) as soon darkness falls. However, the planet's visibility period is fast reducing and by month's end it will set just a couple of hours after the Sun from northern latitudes, although up to twice as long for those living further south.
Saturn is currently moving direct in eastern Ophiuchus, close to the border with Sagittarius. The teapot asterism of Sagittarius is 15 degrees southeast of the planet. To the naked eye, Saturn appears off-white or creamy coloured. Telescopically, it has an apparent size of just over 16 arc seconds with the rings inclined at 26.8 degrees to the line of sight. This is almost as open as they ever can be. Maximum tilt, at 27 degrees, will come in October.
On September 27th, the waxing crescent Moon passes 4 degrees north of the planet.
Uranus, mag. +5.7, is now an evening object moving retrograde in Pisces almost at the Aries border. 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 September 30th, it's practically visible all night long.
From a dark site, Uranus is faintly visible to the naked eye and easily visible with binoculars and small scopes. The ice giant is currently positioned about a degree northwest of omicron Psc (ο Psc - mag. +4.3). By observing the planet over a number of days its slow movement relative to the "fixed" background stars can be detected.
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 9th, the waxing gibbous Moon passes 4 degrees south of Uranus.
Neptune, the outermost planet, reaches opposition on September 5th. On this day it will be 28.939 AU (approx. 4,329 million km or 2,690 million miles) from the 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 is currently positioned in Aquarius about 30 degrees south of the centre of the Great Square of Pegasus and 2 degrees east of lambda Aqr (λ Aqr - mag. +3.7). First magnitude star, Fomalhaut (α Psc - mag. +1.2), is 22 degrees directly to the south.
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 6th, the full Moon passes a degree south of Neptune.
Solar System Data Table September 2017
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||Sep 5||10h 55m 03.7s||06d 54m 37.9s||-26.7||31.7'||100||1.008||Leo|
|Sun||Sep 15||11h 31m 00.6s||03d 07m 49.9s||-26.7||31.8'||100||1.006||Leo|
|Sun||Sep 25||12h 06m 54.5s||00d 44m 52.9s||-26.7||31.9'||100||1.003||Virgo|
|Mercury||Sep 5||09h 59m 01.9s||10d 28m 07.2s||+1.5||09.0"||16||0.748||Leo|
|Mercury||Sep 15||10h 27m 06.8s||10d 41m 52.0s||-0.7||06.7"||58||1.009||Leo|
|Mercury||Sep 25||11h 28m 28.2s||05d 25m 26.6s||-1.2||05.4"||90||1.251||Leo|
|Venus||Sep 5||08h 56m 12.4s||17d 35m 45.0s||-3.9||12.2"||85||1.366||Cancer|
|Venus||Sep 15||09h 44m 43.2s||14d 20m 24.6s||-3.9||11.8"||87||1.419||Leo|
|Venus||Sep 25||10h 32m 02.9s||10d 24m 04.3s||-3.9||11.4"||89||1.468||Leo|
|Mars||Sep 5||10h 08m 09.0s||12d 44m 14.8s||+1.8||03.6"||100||2.628||Leo|
|Mars||Sep 15||10h 32m 20.0s||10d 27m 13.6s||+1.8||03.6"||99||2.605||Leo|
|Mars||Sep 25||10h 56m 08.4s||08d 04m 14.5s||+1.8||03.6"||99||2.575||Leo|
|Jupiter||Sep 5||13h 24m 52.9s||-07d 44m 51.0s||-1.7||31.9"||100||6.179||Virgo|
|Jupiter||Sep 15||13h 32m 07.9s||-08d 28m 51.7s||-1.7||31.4"||100||6.269||Virgo|
|Jupiter||Sep 25||13h 39m 45.7s||-09d 14m 05.6s||-1.7||31.1"||100||6.341||Virgo|
|Saturn||Sep 5||17h 21m 16.4s||-21d 59m 26.0s||+0.5||16.9"||100||9.861||Ophiuchus|
|Saturn||Sep 15||17h 22m 20.6s||-22d 02m 15.3s||+0.5||16.6"||100||10.026||Ophiuchus|
|Saturn||Sep 25||17h 24m 05.2s||-22d 05m 34.5s||+0.5||16.3"||100||10.191||Ophiuchus|
|Uranus||Sep 5||01h 44m 20.2s||10d 09m 12.8s||+5.7||03.7"||100||19.195||Pisces|
|Uranus||Sep 15||01h 43m 16.6s||10d 03m 03.9s||+5.7||03.7"||100||19.087||Pisces|
|Uranus||Sep 25||01h 42m 00.6s||09d 55m 46.8s||+5.7||03.7"||100||19.002||Pisces|
|Neptune||Sep 5||22h 57m 18.8s||-07d 41m 45.7s||+7.8||02.4"||100||28.939||Aquarius|
|Neptune||Sep 15||22h 56m 17.6s||-07d 48m 06.6s||+7.8||02.4"||100||28.955||Aquarius|
|Neptune||Sep 25||22h 55m 18.6s||-07d 54m 09.7s||+7.8||02.4"||100||29.001||Aquarius|