Mercury reaches greatest elongation east on September 21st when it's positioned 26 degrees east of the Sun, close to the maximum possible obtainable. Unfortunately due to the angle of the ecliptic, the planet is unsuitably placed for observation from northern temperate latitudes this month. However for observers in equatorial and more southerly locations Mercury is superbly placed, visible above the western horizon just after sunset throughout September and into early October. From these latitudes, this also happens to be the most favourable evening apparition of the year.
For example, on September 1st from latitude 35S (approx. equal to Sydney, Cape Town and Santiago), Mercury will appear 10 degrees above the western horizon 45 minutes after sunset. The planets altitude then increases each subsequent evening, peaking on the day of greatest elongation east at 17 degrees. By months end Mercury is still 14 degrees above the horizon. The diagram below shows the changes in position of Mercury, 45 minutes after sunset from latitudes of 35 degrees south.
During September, the brightness of Mercury fades slightly from magnitude -0.2 to +0.3. On the 21st, the planet passes 0.6 degrees south of Spica (α Vir - mag. +1.0) with the thin waxing crescent Moon 4 degrees south of Mercury on the same day, forming a nice triangle.
Venus long morning apparition finally comes to an end during September. At the start of the month the planet remains a brilliant object low above the eastern horizon before dawn, shining at magnitude -3.9. However, it's not long before the brightest planet of all is lost to the glare of the Sun. For those at southern latitudes the planet will only be visible for the first week of the month, observers at northern temperate latitudes should be able to catch Venus until at least the middle of the month with those in the tropics possibly able to glimpse the planet for a few more days still.
On September 5th, Venus reaches perihelion at a distant of 0.718 AU (approx. 107 million kilometres or 66.7 million miles) from the Sun. On the same day the planet passes 0.8 degrees north of the Regulus (α Leo - mag +1.4), the brightest star in Leo.
Mars remains an evening object during September, visible after sunset above the southwestern horizon (NH) / western horizon (SH). The Earth is currently distancing itself from the planet so it continues to fade in brightness (mag. +0.6 to +0.8) and shrink in apparent size (6.8 to 6.1 arc seconds) as the month progresses.
Having overtaken Saturn at the end of last month, the "Red planet" starts September in Libra. It then continues its rapid direct movement against the "fixed" background stars, passing into Scorpius on September 13th before moving into Ophiuchus on September 26th, where it remains for the remainder of the month.
Last month it was interesting to compare the colours of deep orange-red Mars as it passed by creamish white Saturn. This month, another interesting conjunction occurs on September 27th when Mars passes 3 degrees north of red supergiant Antares (α Sco - mag. +1.0), the brightest star in Scorpius. Antares is often referred to as the "Rival of Mars" and the two should appear very similar to the naked eye, although Mars will be marginally brighter at magnitude +0.8.
On September 29th, the waxing crescent Moon passes 3 degrees north of Mars.
Jupiter. mag. +1.9, is now a brilliant morning object moving direct amongst the stars of Cancer. At the beginning of September, the dominant planet in the Solar System rises over 2 hours before the Sun increasing to over 4 hours by months end.
The planet dominates a relatively barren part of the sky although the twins of Castor (α Gem - mag. +1.6) and Pollux (β Gem - mag. +1.1) are positioned about 20 degrees northwest of Jupiter. Since moving into the morning sky, this is probably the first month when telescope users have enough observing time to study the planet in detail. As the year progresses, Jupiter will continue to brighten and increase in apparent size as it moves towards opposition early next year.
On September 20th, the waning crescent Moon passes 5 degrees south of Jupiter.
Saturn, mag. +0.6, remains an early evening object in Libra throughout the month. On September 1st, the "Ringed planet" appears rather low down above the southwestern horizon from northern temperate latitudes, setting about 2 hours after the Sun. However, it's much better placed from Southern Hemisphere locations, where it appears higher in the sky and sets much later at around midnight.
On the first day of the month, Mars (mag. +0.6) is positioned 5 degrees southeast of Saturn but as its much closer to Earth and traveling on a faster trajectory, it's not long before Mars distances itself from Saturn.
By months end, for northern-based observers Saturn will be rather low down at dusk but better placed and easier to see for observers located further south. On September 28th, the waxing crescent Moon passes 0.7 degrees north of Saturn and an occultation is visible from Hawaii, southwestern Alaska and the Pacific Ocean (4:25 UT).
Uranus, mag. +5.7, is now closing in on next month's opposition. The planet is moving slowly retrograde in Pisces and at the start of September rises in the east less than two hours after sunset. The period of visibility continues to improve as the month progresses and by month's end, Uranus is practically visible all night.
A good starting point to find Uranus is the "Great Square of Pegasus". Uranus is positioned about 25 degrees southeast of the centre of the square. The stars ε Psc (mag. +4.3) and δ Psc (mag. +4.4) are positioned 2 degrees northeast and 2 degrees northwest of Uranus respectively.
Uranus has an apparent diameter of 3.7 arc seconds and a small telescope at high magnification will show a small green disk that's obviously non-stellar. However, even when viewed through the largest amateur scopes it's difficult to make out any surface details on Uranus.
On September 11th the waning gibbous Moon passes 1 degree north of Uranus.
Neptune is located in Aquarius and has recently just past opposition (August 29th). The distant planet is moving very slowly retrograde and remains well placed for observation during September, visible as soon as it's dark enough until just before sunrise. At magnitude +7.8, Neptune is the only planet that can't be glimpsed with the naked eye (under dark skies Uranus can be seen), however it can be spotted relatively easily with binoculars or small telescopes.
The constellation Aquarius lies between the Great Square of Pegasus and the barren starfields of Capricornus and Piscis Austrinus. It contains mostly faint stars but locating Neptune is not difficult once you have identified sigma Aqr (σ Aqr -mag. +4.8). Neptune is positioned about a degree to the north with the planet being 15x fainter than the star.
On September 8th, the almost full Moon passes 5 degrees north of Neptune.
Solar System Data Table September 2014
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th Sep 2014||10h 54m 14.4s||06d 59m 41.6s||-26.7||31.7'||100||1.008||Leo|
|Sun||15th Sep 2014||11h 30m 11.3s||03d 13m 06.8s||-26.7||31.8'||100||1.006||Leo|
|Sun||25th Sep 2014||12h 06m 04.6s||-00d 39m 30.1s||-26.7||31.9'||100||1.003||Virgo|
|Mercury||5th Sep 2014||12h 13m 37.1s||-01d 58m 37.9s||-0.1||05.6"||79||1.205||Virgo|
|Mercury||15th Sep 2014||13h 00m 58.4s||-08d 29m 58.0s||0.0||06.2"||68||1.079||Virgo|
|Mercury||25th Sep 2014||13h 38m 12.3s||-13d 30m 15.6s||0.1||07.3"||53||0.926||Virgo|
|Venus||5th Sep 2014||10h 05m 54.0s||12d 57m 23.4s||-3.9||10.0"||97||1.661||Leo|
|Venus||15th Sep 2014||10h 53m 04.6s||08d 35m 05.0s||-3.9||09.9"||98||1.682||Leo|
|Venus||25th Sep 2014||11h 39m 15.0s||03d 48m 20.7s||-3.9||09.8"||99||1.698||Virgo|
|Mars||5th Sep 2014||15h 24m 47.8s||-20d 05m 21.4s||0.7||06.7"||87||1.396||Libra|
|Mars||15th Sep 2014||15h 52m 09.1s||-21d 39m 49.1s||0.7||06.5"||88||1.451||Scorpius|
|Mars||25th Sep 2014||16h 20m 56.7s||-22d 59m 21.7s||0.8||06.2"||88||1.504||Scorpius|
|Jupiter||5th Sep 2014||08h 53m 28.6s||17d 58m 47.0s||-1.8||32.2"||100||6.116||Cancer|
|Jupiter||15th Sep 2014||09h 01m 32.4s||17d 26m 46.5s||-1.9||32.7"||100||6.027||Cancer|
|Jupiter||25th Sep 2014||09h 09m 07.0s||16d 55m 39.1s||-1.9||33.3"||100||5.921||Cancer|
|Saturn||5th Sep 2014||15h 05m 00.0s||-15d 13m 31.5s||0.6||16.1"||100||10.300||Libra|
|Saturn||15th Sep 2014||15h 07m 58.9s||-15d 27m 57.8s||0.6||15.9"||100||10.446||Libra|
|Saturn||25th Sep 2014||15h 11m 25.5s||-15d 43m 49.9s||0.6||15.7"||100||10.578||Libra|
|Uranus||5th Sep 2014||00h 58m 18.7s||05d 28m 15.3s||5.7||03.7"||100||19.166||Pisces|
|Uranus||15th Sep 2014||00h 57m 03.2s||05d 20m 15.9s||5.7||03.7"||100||19.088||Pisces|
|Uranus||25th Sep 2014||00h 55m 39.2s||05d 11m 27.5s||5.7||03.7"||100||19.036||Pisces|
|Neptune||5th Sep 2014||22h 31m 28.9s||-10d 05m 03.3s||7.8||02.4"||100||28.970||Aquarius|
|Neptune||15th Sep 2014||22h 30m 28.3s||-10d 11m 00.4s||7.8||02.4"||100||29.006||Aquarius|
|Neptune||25th Sep 2014||22h 29m 31.5s||-10d 16m 32.0s||7.8||02.3"||100||29.070||Aquarius|