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Mercury

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.

Mercury, Venus, Mars and Regulus in the early morning sky from September 7th to 22nd from mid-latitude northern locations (credit:- freestarcharts)

Venus

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

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

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

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.

Saturn during September 2017 (credit:- freestarcharts)

Saturn during September 2017 - pdf format (credit:- freestarcharts)

Uranus

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.

Uranus during September 2017 (credit:- freestarcharts)

Uranus during September 2017 - pdf format (credit:- freestarcharts)

Neptune

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.

Neptune as imaged by the Voyager 2 spacecraft in August 1989 (credit:- NASA/JPL)

Neptune during September 2017 (credit:- freestarcharts)

Neptune during September 2017 - pdf format (credit:- freestarcharts)

Solar System Data Table September 2017

 DateRight AscensionDeclinationApparent MagnitudeApparent SizeIllum. (%)Distance from Earth (AU)Constellation
SunSep 510h 55m 03.7s06d 54m 37.9s-26.731.7'1001.008Leo
SunSep 1511h 31m 00.6s03d 07m 49.9s-26.731.8'1001.006Leo
SunSep 2512h 06m 54.5s00d 44m 52.9s-26.731.9'1001.003Virgo
MercurySep 509h 59m 01.9s10d 28m 07.2s+1.509.0"160.748Leo
MercurySep 1510h 27m 06.8s10d 41m 52.0s-0.706.7"581.009Leo
MercurySep 2511h 28m 28.2s05d 25m 26.6s-1.205.4"901.251Leo
VenusSep 508h 56m 12.4s17d 35m 45.0s-3.912.2"851.366Cancer
VenusSep 1509h 44m 43.2s14d 20m 24.6s-3.911.8"871.419Leo
VenusSep 2510h 32m 02.9s10d 24m 04.3s-3.911.4"891.468Leo
MarsSep 510h 08m 09.0s12d 44m 14.8s+1.803.6"1002.628Leo
MarsSep 1510h 32m 20.0s10d 27m 13.6s+1.803.6"992.605Leo
MarsSep 2510h 56m 08.4s08d 04m 14.5s+1.803.6"992.575Leo
JupiterSep 513h 24m 52.9s-07d 44m 51.0s-1.731.9"1006.179Virgo
JupiterSep 1513h 32m 07.9s-08d 28m 51.7s-1.731.4"1006.269Virgo
JupiterSep 2513h 39m 45.7s-09d 14m 05.6s-1.731.1"1006.341Virgo
SaturnSep 517h 21m 16.4s-21d 59m 26.0s+0.516.9"1009.861Ophiuchus
SaturnSep 1517h 22m 20.6s-22d 02m 15.3s+0.516.6"10010.026Ophiuchus
SaturnSep 2517h 24m 05.2s-22d 05m 34.5s+0.516.3"10010.191Ophiuchus
UranusSep 501h 44m 20.2s10d 09m 12.8s+5.703.7"10019.195Pisces
UranusSep 1501h 43m 16.6s10d 03m 03.9s+5.703.7"10019.087Pisces
UranusSep 2501h 42m 00.6s09d 55m 46.8s+5.703.7"10019.002Pisces
NeptuneSep 522h 57m 18.8s-07d 41m 45.7s+7.802.4"10028.939Aquarius
NeptuneSep 1522h 56m 17.6s-07d 48m 06.6s+7.802.4"10028.955Aquarius
NeptuneSep 2522h 55m 18.6s-07d 54m 09.7s+7.802.4"10029.001Aquarius