The Planets this Month - September 2015

Mercury

For observers at tropical and Southern Hemisphere latitudes Mercury continues to be visible after sunset in the western sky for the first two to three weeks of September. The elusive planet that never ventures far from the Sun reaches greatest elongation east on September 4th. On this day it's positioned 27 degrees east of our star just short of the maximum possible separation.

To spot the smallest planet of all you'll require a relatively unobstructed view of the western horizon and of course some clear skies. For example from latitude 35S (approx. equal to Sydney, Cape Town and Santiago), Mercury appears 17 degrees above the horizon 45 minutes after sunset at the beginning of the month. With a magnitude +0.1, it should be easy to spot the planet as the sky darkens.

Each subsequent evening after greatest elongation Mercury dims slightly and draws closer to the Sun. By the third week of the month it will be very low above the horizon down to magnitude +1.7 and effectively lost to the bright evening twilight. The planet then passes through inferior conjunction on September 30th. From northern temperate latitudes Mercury remains unsuitably placed for observation throughout the month.

On September 15th, the thin waxing crescent Moon passes 5 degrees north of Mercury. The diagram below shows the August / September evening apparition of Mercury from a latitude of 35S (approx. equal to Sydney, Cape Town and Santiago). Positions of the planet are displayed 45 minutes after sunset.

Venus

Venus becomes a magnificent morning "star" as it draws rapidly away from the Sun during September. The unmistakable planet dominates the eastern sky before dawn although the period of visibility varies considerably depending on location. From mid-latitude northern temperate regions Venus rises almost 4 hours before the Sun by months end but just half this time from the tropics and further south.

At the beginning of the month, Venus is positioned about 9 degrees south of Mars and can be used as a guide to locating the much fainter "Red planet" (mag. +1.8). At months end, Jupiter (mag -1.7) appears in the same region of sky with the three planets forming across a line before dawn. First magnitude star Regulus (α Leo - mag. +1.4) is also visible just west of Mars. The contrast in brightness of the 3 planets is stark; Venus is much more brilliant than Jupiter and Jupiter much brighter than Mars. On September 10th, the waning crescent Moon passes 3 degrees north of Venus forming a lovely morning pairing.

The phase of Venus increases slightly from 9 to 34% as the month progresses with its apparent size decreasing from 52 to 34 arc seconds. On September 21st, the planet attains its greatest brilliance at magnitude -4.7.

Venus and Mars during September 2015 - pdf format

Mars

Mars continues as an early morning object during September. However, at magnitude +1.8, the famous "Red planet" is rather faint and unremarkable but by months end it rises more than 3 hours before the Sun from northern temperate latitudes, although less from locations further south.

On September 5th, Mars moves from Cancer into Leo. It then passes 0.7 degrees north of slightly brighter Regulus (Sep. 24th). The waning crescent Moon appears 5 degrees south of Mars on September 10th. As previously noted, Mars is 9 degrees north of Venus at the beginning of the month and a lineup between Mars, Venus and Jupiter occurs at the end of September.

Jupiter

Jupiter passed through solar conjunction at the end of August. From about the middle of September it reappears above the eastern horizon shortly before dawn from northern latitudes. However, it doesn't become visible to observers in the Southern Hemisphere until the end of the month.

Jupiter the largest planet in the Solar System shines at magnitude -1.7. It's currently moving direct in Leo and although too low above the horizon for serious telescopic observation, the planet is still an impressive naked eye sight. As mentioned earlier, Jupiter will form a line of 3 planets along with much brighter Venus and much fainter Mars in the early morning twilight sky at end of September.

Saturn

Saturn, mag. +0.6, remains an early evening object throughout September visible towards the southwest (Northern Hemisphere) / west (Southern Hemisphere). However, it's period of visibility is fast diminishing and by the end of the month from mid-northern temperate latitudes the "Ringed" planet will be setting just 2 hours after the Sun, although up to twice as long for those living further south.

Saturn is currently moving direct in western Libra and appears to the naked eye as off-white or creamy "star" located about 10 degrees northwest of orange/red first magnitude red giant Antares (α Sco mag. +1.0). Telescopically Saturn has an apparent size of 16 arc seconds with the rings wide open and beautifully displayed.

On September 19th, the waxing crescent Moon passes 3 degrees north of the planet.

Saturn during September 2015 - pdf format

Uranus

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 it's visibility period continually improving 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 easily spotted with binoculars. The ice giant is positioned 15 degrees south and 20 degrees east of the centre of the "Great Square of Pegasus". Located just over 2 degrees northwest of Uranus is epsilon Psc (ε Psc - mag. +4.3).

On September 1st, the almost full Moon passes 1 degree south of Uranus with an occultation visible from New Zealand at 15:58 UT. A second Moon-Uranus occultation occurs on September 29th this time visible from South Africa (0:54 UT).

Uranus during September 2015 - pdf format

Neptune

Neptune the most distant planet in the Solar System reaches opposition in Aquarius on September 1st. On this day the planet is positioned 28.9533 AU (approx. 4331.4 million km or 2690.6 million miles) from Earth and is visible all night, rising above the eastern horizon at sunset and then setting in the west as the Sun reappears.

Despite being a considerably sized planet, Neptune is too distant and too faint (mag. +7.8) to be seen with the naked eye. However, it's visible with binoculars and a small / medium sized telescope at high magnifications will show the planet as a small bluish disk although the surface appears devoid of details.

Aquarius is faint and devoid of bright stars but locating Neptune isn't particularly difficult once one's familiarised with the surrounding area of sky. The planet is positioned towards the constellation centre about halfway along an imaginary line connecting lambda Aqr (λ - mag. +3.7) with sigma Aqr (σ Aqr - mag. +4.8). Slightly brighter star HD 214686 (mag. + 7.0) is 7.5 arc minutes southwest of the planet at the beginning of the month.

On September 26th, the almost full Moon passes 3 degrees north of Neptune.

Neptune during September 2015 - pdf format

Solar System Data Table September 2015