The Night Sky This Week

by Mark Armstrong

Early September

What's up this week? Our weekly guide to the night sky will point you to all the planetary goings on, as well as pick out some deep sky objects for your viewing pleasure. Let's get started with a round-up of the Solar System, or follow the links above to jump to your favourite section.

Solar System Round-Up

The British summer was a bit of a non event in the latter part of August and with nearly nine hours of dark skies available by mid September from London, over seven of them astronomically dark, this signals that the short summer nights are well and truly gone. Jupiter completely dominates the scene in September but Mercury puts in a good show too.

Although better placed in the second half of September, the innermost planet is in the eastern sky shortly before sunset towards mid-month. This is view at 6am on 15 September. Graphic made using the Sky version 5 www.bisque.com.

Mercury
Mercury has its best morning apparition for the UK this month and is best placed in the second half of September. However towards mid-month it can be identified low in the eastern sky before sunrise. By 15 September at 6am it is eight degrees up with the Sun six degrees below the horizon (end of nautical twilight), so you will need quite a good horizon to get an unimpeded view. As it pulls out to greatest elongation west on 19 September it will get a little bit brighter and will be at its brightest at the end of the month. Mercury will shine at mag. +0.6 mid-month, and if you can view it through a telescope it will sport a crescent phase around 28 percent illuminated and a disc just over eight arcseconds in apparent diameter.

Jupiter
Jupiter is just a couple of weeks from opposition now and there will be much more on the gas giant next time. Jupiter is unmistakeable in the late evening sky at the start of the month; rising around 8pm it take a couple of hours to haul itself up enough for city dwellers to spot it. By midnight it is a blazing beacon in the south-east, shining at mag. +2.9 and even a small telescope at low magnification will easily show its disc - if you own a 100-mm 'scope then some of the ever-changing atmospheric detail should be well seen.

The prime feature on Jupiter is the Great Red Spot (GRS), a monstrous, long-lived storm. At the moment another great feature, Oval BA, is in conjunction with it and together make for a splendid imaging opportunity. The best time to image them is on the following dates and times; 3rd, 00.46; 5th, 02.23, 22.15; 7th, 04.01, 23.53; 10th, 01.30; 12th, 03.08, 23.00; 14th, 04.46. Uranus is less than two degrees west of Jupiter and is an easy binocular target shining at mag. +5.7. More too on Uranus next time.

Watching Jupiter's four largest moons, the Galilean satellites Io, Europa, Ganymede and Callisto, as they orbit their parent is a fascinating part of Jovian observation. Even binoculars will show them and many of the events such as occultations and eclipses (disappearing or reappearing behind Jupiter and moving in and out of its shadow) can be observed with small telescopes. The transit and shadow transits can be trickier, especially the former, but a moderate scope can bag most of the events. Here is a list of what can be seen in the first half of September:

Key: OcR occultation reappearance; EcD eclipse disappearance; EcR eclipse reappearance; TrI start of satellite transit; TrE end of transit; ShI start of shadow transit; ShE end of shadow transit.

Mars and Saturn
Mars, the red planet, is technically still visible in the evening sky but it is painfully low in the west-south-west so most observers will bypass it now for more favourable quarry. Saturn is lost to the twilight and is in solar conjunction next month.

Lunar occultation
There is quite a good grazing lunar occultation right at the start of September. Just after midnight (BST) on 1/2 September, the mag. +6.2 star 95 Tauri appears to graze the dark northern edge of an old crescent moon. Along the graze line observers will see the star blink in and out of view as it skims along the limb. To see this event you need to be under the narrow graze path which runs along the Menai Strait to Blackpool, crosses the Pennines and leaves our shores at Seaham on the coast of County Durham. Observers north of this path will see a near miss while those south will see a total occultation. In London the disappearance at the bright limb is timed for around 11.45pm, with the reappearance at the dark limb at 12.08am. The Moon is only 10 or so degrees up though.

Happenings down under: early September

It's officially spring on 1 September in Australia and New Zealand but technically it's still winter! The Sun is still north of the celestial equator and the equinox, the date when the Sun crosses the equator into the Southern Hemisphere, does not occur until just after midday on 23 September (Sydney time and latitude). Whether you prefer to call it winter or spring, there is still a healthy amount of dark time for observers to enjoy - by mid-month the Sun is more than 12 degrees below the horizon from shortly before 7pm (local Sydney time) until about 5am. True astronomical darkness lasts from just after 7pm until 4.30am.

Venus and Jupiter dominate the early and late evening sky with Mars too available for early evening viewing. Saturn sinks fast into the evening twilight so it's best to nab it early in September. Mercury is technically a morning object following inferior conjunction on 3 September, but it is still too low to observe by mid-month, although it does make a brief appearance around greatest elongation west later in the month.

Venus, Mars and Spica are in close proximity in the early evening sky at the start of September. Graphic made using the Sky version 5 www.bisque.com

Venus starts the month as a blazing evening star about 40 degrees above the western horizon as the twilight fades away. Early in September it lies close to the south of Spica (alpha Virginis, mag. +1.04) but there is no chance of confusing the two despite Spica's white light, as Venus, shining at mag. -4.4, is over 100 times brighter! Mars is right there too, almost four degrees north-west of Spica. Through a small telescope Venus displays a fat crescent with a phase of 41 percent and spans around 30 arcseconds in apparent diameter; as September progresses its phase decreases and diameter increases as it pulls away from Spica.

Mars is a pale shadow of itself earlier in the year but it will still be easy to pick out in the company of Spica, shining at mag. +1.5. It is disappointingly small now through a telescope, its disc shrunk to a mere 4.2 arcseconds by mid-month and although it maintains a healthy altitude after sunset, even the most keen imager will probably have called it a day by now. Saturn has a very short observing window, as it is as low as 20 degrees above the western horizon about 30 minutes after sunset. By mid-month it is very close to being lost in the twilight on its way to solar conjunction on 1 October.

Jupiter
Jupiter rules the roost for sure in September! At the start of September it is like a precious jewel in the sky, shining at mag. -2.9, almost as bright as it can get and through a telescope sports a disc spanning over 49 arcseconds, nearly as big as it can get. The gas giant is 20 degrees clear of the horizon from 9pm to 5.45am (8pm to 4.45am mid-month) and culminates on 1 September at 1.20am, not far off 60 degrees up. Jupiter's cloud tops are in constant motion and this ever-changing view through the eyepiece makes it the most rewarding planet to observe. Digital imagers can now capture the most intricate of detail not that long ago were the province of space probes and if you want to contribute to advancing planetary science then Jupiter should be the prime target. Indeed amateur imagers have been at it again, capturing another impact on Jupiter. On 20 August 20 at 18:22 UT, two amateur astronomers in Japan independently recorded an apparent impact on Jupiter. Masayuki Tachikawa of Kumamoto city was first to report the event. His movie of the fireball shows the fireball scintillating (twinkling) along with other features on the planet - strong evidence that this is a genuine event on Jupiter. Soon after Tokyo amateur astronomer Kazuo Aoki realised that he had recorded the fireball, too. This is the third time in little over a year that amateurs have captured an impact event! Take a look at our news story here for more info.

The Great Red Spot (GRS) and Oval BA are now in close conjunction and visible close to the centre of Jupiter's disc on the following dates and times: 1/2, 3.56am; 2/3, 11.47pm; 3/4, 5.34am; 4/5, 1.25am; 5/6, 9.16pm; 6/7, 3.03am; 7/8, 10.54am; 8/9, 4.40am; 9/10, 12.32am; 10/11, 8.23pm; 11/12, 2.10am; 12/13, 10.01pm; 13/14, 3.47am; 14/15, 11.39am.

The Galilean moon's constant dance around their parent produce phenomena that amateurs can observe; here is the list of events it will be possible to target:

Key: I Io, II Europa, III Ganymede. OcD occultation disappearance; EcR eclipse reappearance; TrI start of satellite transit; TrE end of transit; ShI start of shadow transit; ShE end of shadow transit.

Observers on South Island can enjoy a lunar occultation on 15 September. This is the view from Dunedin at 9pm as 51 Oph disappears behind the dark lunar limb. Graphic made using Guide 8 www.projectpluto.com

Lunar occultation
There is one lunar occultation worth observing, but only for observers on South Island in New Zealand. The moderately bright star 51 Ophiuchi (mag. +4.7) experiences a short duration occultation by the first quarter moon on the evening of 15 September. From Dunedin the Moon starts to move in front of the star at about 9pm with the event taking place very close to the northern cusp (position angle 16 degrees) but just inside the un-illuminated portion of the moon. From Christchurch the event starts right on the cusp but the star actually disappears behind the bright limb at about 9.16. The reappearance from behind the bright lunar limb is predicted at around 9.30pm for both cities. There will be a grazing occultation around 9.20pm for observers 30 kilometres or so north of Christchurch in the Waikulu/Amberley area.

Comet Encke should be a 10th magnitude object in the evening sky. Here is its path through the stars this month. Graphic made using Megastar http://www.willbell.com/software/megastar/index.htm.

Comet Encke
There are a couple of comets on show in September, although neither is stunningly bright. Comet 2P/Encke is in the early evening sky and predicted to be 10th magnitude as it tracks from Corvus, through southern Virgo and into Libra. 10P/Tempel is a ninth mag. object with a 6-7 arcminute coma. It is very well placed in Cetus, transiting around 2am.

Deep Sky