Our celestial neighbourhood in September
PICK OF THE MONTH
Jupiter
Best time to see: 26 September, 00:00 UT
Altitude: 37˚
Location: Pisces
Direction: South
Features: Detail of the planet’s atmosphere, Galilean moons
Recommended equipment: 75mm or larger
Jupiter will reach opposition on 26 September and, when viewed through the eyepiece, appears brightest and largest for this period of observation. Reaching an impressive mag. –2.8, it slips from Cetus into Pisces throughout September, the planet located east of the faint but distinctive Circlet asterism.
A bright full Moon lies near to Jupiter on the nights of 10 and 11 September, an impressive sight if you have clear skies. As they rise above the eastern horizon early evening on 11 September, Jupiter and the Moon will appear a little over 3˚ apart.
On opposition night, it sits 4˚ east of the First Point of Aries, one of two intersections in the sky where the celestial equator and the ecliptic cross. The First Point of Aries marks the start of the RA coordinate system (00h00m00s). Jupiter shines at mag. –2.8 on opposition night, reaching θ c elestial neighbourhood in September a peak altitude of 37˚ as seen from the centre of the UK.
Jupiter rotates quickly, the planet completing one rotation in under 10 hours. This brings atmospheric features into and out of view surprisingly quickly. A 100mm or larger telescope will show the planet’s famous Great Red Spot, the appearance of which can be determined using swirling atmosphere the freeware WinJupos application (jupos.org/gh/download.htm).
In addition to the planet’s detailed atmosphere, a small telescope will also reveal the four largest and brightest Jovian moons, the so-called Galilean moons: Io, Europa, Ganymede and Callisto.
The inner three can appear to pass in front of and behind the planet, but Jupiter’s apparent tilt from Earth has now increased such that Callisto appears to miss the disc, although its shadow can still clip Jupiter’s southern pole.
This phenomenon can be observed on 5 September between 00:30 BST (23:30 UT on 4 September) and 01:47 BST (00:47 UT).
Mercury
Best time to see: 30 September, 30 minutes before sunrise
Altitude: 5˚ (low)
Location: Virgo
Direction: East
Mercury’s position in the evening sky is poor at the start of September, appearing dim and setting shortly after sunset. Inferior conjunction is on 23 September. Mercury then reemerges into the morning sky where it becomes easier to see. By the end of the month, it shines at mag. +1.8 and rises 70 minutes before the Sun.
Venus
Best time to see: 1 September, 30 minutes before sunrise
Altitude: 7˚ (low)
Location: Leo
Direction: East-northeast
Shining at mag. –3.8 on 1 September, Venus can be seen rising above the eastnortheast horizon 90 minutes before the Sun. Telescopically, it isn’t well presented, at 10 arcseconds across and nearly fully illuminated. By the end of the month, Venus’s position degrades further and it becomes harder to see, rising 40 minutes before sunrise.
Mars
Best time to see: 30 September, 04:50 UT
Altitude: 59˚
Location: Taurus
Direction: South
Mars shows dramatic changes as it approaches opposition on 8 December. On 1 September, shining at mag. –0.1, it is located just north of the Hyades. Through a telescope the planet is 9 arcseconds across on 1 September, increasing to 11 arcseconds and mag. –0.6, a beacon between the horns of Taurus, by the month’s end. A 58%-lit waning gibbous Moon lies 3˚ north of Mars on the morning of 17 September.
Saturn
Best time to see: 1 September, 23:00 UT
Altitude: 21˚
Location: Capricornus
Direction: South Following opposition on 14 August, Saturn remains well placed all month, dropping in brightness only slightly from mag. +0.4 on 1 September to +0.6 by the end of the month. A bright waxing gibbous Moon sits nearby on the nights of 7/8 and 8/9 September.
Uranus
Best time to see: 30 September, 02:30 UT
Altitude: 54˚
Location: Aries
Direction: South
Morning planet Uranus is perfectly placed for UK observation, able to reach an altitude of around 50˚ under dark skies. Visible as a green-hued mag. +5.7 disc through the eyepiece, a rare lunar occultation of Uranus takes place on the evening of 14 September.
Neptune
Best time to see: 16 September, 00:15 UT
Altitude: 34º
Location: Aquarius
Direction: South
Neptune reaches opposition on 16 September, but at its great distance from Earth this usually favourable position has very little effect on the planet’s visual appearance. Able to reach an altitude around 30˚ under dark sky conditions all month long, you’ll need at least binoculars to spot mag. +7.8 Neptune.
The planets in September
The phase and relative sizes of the planets this month. Each planet is shown with south at the top, to show its orientation through a telescope
JUPITER’S MOONS: SEP
Using a small scope you can spot Jupiter’s biggest moons. Their positions change dramatically over the month, as shown on the diagram. The line by each date represents 01:00 BST (00:00 UT).
