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Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Telescope House September Sky Guide

 

September brings the Autumnal Equinox for the Northern Hemisphere and the Vernal, or Spring Equinox for this in the Southern Hemisphere. This year these events occur on 23rd of September, where for a brief period for day and night are of nearly equal length. This equality of dark and light really depends on where you find yourself, as there are few places on Earth on the 23rd September where day and night are truly equal. However, crucially, the 23rd marks the point where the Sun crosses into the southern celestial hemisphere - which results in increasingly greater hours of darkness than light for those of us in the Northern Hemisphere; and of course increasingly less darkness for those in the Southern reaches of our planet, who conurrently experience their Vernal (Spring) Equinox. Many people for whom astronomy is of no more than at most a passing interest will bemoan the lack of daylight in the Northern Hemisphere - the same cannot (in all probability) be said of the many readers of this Sky Guide. For us astronomers, the dive towards Winter does have its perks. 

 As ever, there's a lot to see in skies above us this month...

 

The Solar System

 

The Moon

 

The Moon starts September in Taurus, at Waning Gibbous phase - rising a little before 10.30pm BST in the evening on the 1st. 

 

The Moon reaches Last Quarter on the 3rd, while still residing in Taurus.

 

The Moon reaches New as it joins the Sun in Leo on the 9th, after which it becomes an evening target. On the evening of the 12th, it may be possible to observe the very slim New Crescent Moon, sitting around 9 degrees above Venus, in Virgo, just after sundown.  The next evening, the Moon can be found 6 1/2 degrees to the north west of Jupiter, both objects in the SW at sunset. 

 

The Moon, Venus and Jupiter, sunset, 12th September.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

 

The Moon reaches First Quarter in Virgo on the 17th, in Sagittarius. This evening, the Moon can be found passing close to Saturn, sitting to the north east of the ringed planet by about 1 1/2 degrees. 

 

The Moon becomes Full on the 25th, in the non-zodiacal constellation of Cetus. Naturally, this is not the ideal time of the month for deep sky observations and imaging. 

 

The Moon ends the month at Waxing Gibbous in Taurus. 

 

 

Mercury

 

On the 1st of the month, Mercury is a morning target, -0.8 mag and is 6.3 arc seconds across, showing a 65.6% phase. It stands 14 degrees high in the dawn sky at sunrise (from latitude 51 degrees N), a resident of Leo. At the beginning of September, the planet is separated from the Sun by just under 17 degrees. 

 

Mercury, sunrise, 1st September.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Mercury has past maximum western elongation and is now heading back towards the Sun, brightening as it goes, but getting increasingly more challenging to observe. By mid-month, we find Mercury at -1.5 mag, 5 arc seconds across, some 98% illuminated. The planet stands just 4 3/4 degrees high in the east at sunrise, making it very difficult to observe, despite its relative brightness. 

 

Mercury reaches superior conjunction (the opposite side of the Sun from our perspective on Earth) in the 21st, just a little before the Sun's autumnal equinox. After which it reemerges as an evening object, though understandably, will remain unobservable for a while. 

 

Mercury ends September in the evening sky, separated from the Sun by 7 1/2 degrees, in Virgo. From a northern hemisphere perspective, this is not a great time to observe the -0.9 mag, 4.8 arc second planet, as it sits just under 2 degrees high (from latitude 51 degrees N) at sunset. Southern hemisphere observers fare considerably better with Mercury in the evenings at this time of year, as the part of the ecliptic it is in sets at a much steeper angle than it does for us northerners - increasing the planet's elevation from the horizon. 

 

 

Venus

 

By the month's beginning, Venus is found in Virgo, 1 1/3 degrees south of Spica, Alpha Virginis, shining at an impressive -4.4 mag. Sadly for observers in the temperate northern hemisphere, it will appear low in the sky at sunset - standing just over 7 degrees high (from latitude 51 degrees N). The planet displays a 29.5 arc second, 40% illuminated disk at this point in time. 

 

Venus, sunset, 1st September.  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Venus has past its greatest eastern elongation and is approaching Earth on its faster interior orbit, brightening as it does. By mid-September, the planet can be still be found in Virgo, but has brightened to a dazzling -4.5 mag, showing a 36.2 arc second disk (now larger than neighbouring Jupiter) and a 30% phase. It now stands a difficult 4 1/2 degrees high at sunset (from latitude 51 degrees N). 

 

By late September, Venus is found in the extreme west of Libra and increased brightness to a near-maximum -4.6 mag. Presenting a large 46 arc second, and a thin crescent (17.5% illuminated) disk, the planet now sets virtually in line with the Sun and stands just over 1 degree high in the west at sunset (from latitude 51 degrees N). 

 

 

Mars

 

Mars ended its apparent retrograde cycle within the ecliptic - appearing to move west within its orbital plane as it appears from Earth (rather than the more regular creep eastwards which most planets appear to make) in late August. From this point onwards, the planet will climb further north within the ecliptic - thought this will be a gradual process. On the 1st, Mars will be found in the western reaches of Capricornus, at -2.1 mag and 20.7 arc seconds across. It will now stand 12 3/4 degrees high at transit (from 51 degrees N), which will occur at 10.25pm BST.

 

Mars, late evening 1st September, showing Phobos at greatest eastern elongation.   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


By mid-month, Mars has shrunk to 18.3 arc seconds diameter and concurrently, to -1.7 mag, as we on Earth pull away from the Red Planet on our faster interior orbit. By this time, Mars will stand 14 degrees high at transit point (from 51 degrees N), which it will reach at 9.39pm BST. 

 

By the end of the month, Mars will have faded yet further to -1.3 mag and now displays a 15.8 arc second diameter disk. The planet now stands 16 degrees high at transit point (again, from 51 degrees N), which it will reach at just before 9pm BST. 

 

 

Jupiter

 

At the beginning of the month, Jupiter is a -1.8 mag target in Libra and displays a disk of 34.7 arc seconds across. The planet will stand just over 23 1/2 degrees high as it transits, which will occur at 5.17pm (BST, from 51 degrees N). The planet will set at 9.55pm, giving an indication of quite how narrow the window for Jovian observation is. 

 

Jupiter with Io and Europa dual shadow transits, September 3rd, sunset (7.42pm BST).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


As the month progresses, this window becomes smaller and smaller. Once we've hit mid-month, Jupiter transits at 4.30pm and sets at a little after 9pm - a little before current astronomical dusk. 

 

By the end of September, Jupiter is -1.8 mag, and 32.6 arc seconds in diameter. The planet now rises at just before 11.12 am and transits at 3.42pm (from 51 degrees N). As it now sets at 8.12pm, Jupiter presents only a fleeting opportunity for observation during the period after the Sun sets - and while it is perfectly possible to observe Jupiter in the daytime - the best of Jovian observation opportunities for 2018 are now behind us. 

 

 

Saturn

 

On the 1st, Saturn is a +0.4 mag and 17.3 arc seconds across target in Sagittarius. It will rise at just past 4.30pm and transits a little before 8.30, when it will stand just under 16 degrees high in the south (from 51 degrees N).

 

The first week in September sees Saturn ceasing its retrograde path through the ecliptic and starting its regular west to east motion against the background stars. 

 

The middle of the month finds Saturn still at +0.4 magnitude, but having shrunk a little to 16.9 arc seconds diameter. Saturn has begun to sink a little in the ecliptic, which began before prograde motion was resumed, but this makes little difference to its angular height at transit point, as it still stands just under 16 degrees high (from 51 degrees N).  Transit is reached at just past 7.30pm (BST) and the planet sets at 11.30pm (again, BST, from 51 degrees N).

 

Saturn and Inner Moons, early evening, 15th September.

 

By the end of the month, nothing much has changed as far as Saturn is concerned. The planet has faded to +0.5 magnitude and is now 16.5 arc seconds diameter. It reaches transit point at 6.36pm (BST), when it will stand just under 16 degrees high in the south. It will set at just past 10.30pm (again, BST, from 51 degrees N). The window for evening observation of Saturn is now beginning to close - so make the most of it while you can. 

 

 

Uranus and Neptune

 

Neptune reaches opposition on September 7th 2018. While opposition makes a big difference to the apparent brightness of the major outer planets, sadly for Neptune, the opposite is true. The planet is still much the same brightness as it has been for the past months, +7.8 magnitude - and 2.4 arc seconds diameter (a fractional improvement from last month's 2.3 arc seconds figure). On opposition evening, Neptune is 3.4 billion km from Earth and the light travel time from the solar system's furthest true planet is a staggering 4 hours. 

 

Neptune and Triton, relative positions, Opposition night (North is up).   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Neptune can be found relatively easily in reasonably powerful binoculars, but it will take a telescope at high magnification to show the planet as a disk. Indeed, under higher magnification, Neptune shows a distinct blue disk, very much reminiscent of the brighter compact planetary nebulae. 

 

Neptune isn't located in a very notable area of sky currently. It's nearest star of any note is the +3.8 magnitude Hydor, or Lambda Aquarii, which it can be found around 3 1/4 degrees west of. About 2 1/4 degrees to the east of Neptune is the fainter +4.2 mag Phi Aquarii. The three form a line in the sky, which makes up the most northerly side of the rough isosceles triangle formed between Hydor, Phi and the more southerly Psi Aquarii. 

 

Uranus and Neptune relative positions, Opposition night, 7th September.   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Uranus is found further to the east in Aries. At +5.7 magnitude and displaying a 3.7 arc second diameter disk, it will be much easier to find than Neptune. Trace a track up due north from Alrischa, Alpha Piscium (the bottom tip of Pisces' "V" feature), by about 8 3/4 degrees and you'll find the small green-gray disk of Uranus in binoculars or a small telescope. While it's Neptune's turn to hog the limelight this month, Uranus will reach opposition late next month, which we'll cover in more detail then. 

 

 

Comets

 

Periodic comet 46/P Wirtanen will be observable in telescopes for a period in September as it dips down towards the Sun.  Unfortunately, the early part of the month, when the comet is at its highest for observers in the northern hemisphere, the Moon will be in the same area of sky and will spoil the view somewhat.  The comet is still relatively faint, but will grow in brightness, towards a reasonably close (possibly naked eye) approach to Earth in mid-December.  The BAA Comet section has details online of a Pro/Am observation campaign that readers are encouraged to get involved in. It’s being run by the University of Maryland and details can be accessed here: http://wirtanen.astro.umd.edu/46P/Meetings_telecons/DPS2016_Wirtanen_workshop.pdf

 

Comet Wirtnanen path through Cetus and Fornax, during September (comet positon shown 1st Sept).   Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.


Another periodic comet, 21P/Giacobini-Zinner, will be well-placed for observation in early September, but yet again, the Moon will be in the same part of the sky as the comet during the early part of the month and won’t help viewing.  The comet passes close to Capella in Auriga in the first week of the month, making the part of sky it is found in incredibly easy to locate.  Beyond this point (7th) the comet the comet ceases to be circumpolar for observers in mid northern latitudes, but will still be visible throughout the lion’s share of the night.  After the 13th, the comet crosses into Gemini (first skirting the borders of Taurus briefly).  The comet passes close to the prominent open cluster M35 on the morning of the 15th and actually passes through the cluster shortly after, though this will be invisible from Europe, Asian and African locations as it occurs in daylight hours.  The Americas will fare better witnessing this event and great potential imaging opportunity.  

 

Comet Giacobini-Zinner's path, through September (comet positon shown 1st Sept).  Image created with SkySafari 5 for Mac OS X, ©2010-2016 Simulation Curriculum Corp., skysafariastronomy.com.

The latter part of the month sees 21P dip into Orion and then cross back into Gemini as its path follows the border between the two constellations, before crossing into Monoceros.  The comet careers through some of Monoceros’ most wonderful deep sky objects.  The early morning of the 17th finds the comet crossing the Christmas Tree Cluster/Cone Nebula conglomeration of NGC2264.  On the next few nights, 21P draws closer to the nearby Rosette Nebula, reaching closest approach of under 3 degrees on the night of the 27th.  Magnitude predictions are always a bit of a moveable feast, but the comet should peak between +5.5 and +7.5 magnitude, which should make it easy in binoculars from a dark location.

 

Meteors

 

There are no major meteor showers which peak in September.  Two minor showers reach their maxima during the month: the Aurigids (which peak on September 1st), and the September Epsilon Perseids (which peak on September 9th).  These showers are very sparse, with Zenithal Hourly Rates of around 5 meteors per hour.  The sometimes-mentioned Piscids (which peak on 21st September) are also supposedly active during this period, but opinions seem to differ as to whether or not this shower is truly active any longer.  There are another couple of minor showers associated with Pisces - the Delta and Pi Piscids, which are active (if that’s the right word) during June and July, but these have ZHRs of less than 2 meteors.  Certainly not worth braving the wee small hours looking out for.

 

The positive identification of the source of a particular meteor can be tricky with minor showers, especially at this time of year when the Perseids are still reasonably active until late August (and occasionally beyond) and the next major shower to peak, the Orionids (more of which in next month’s Sky Guide) run from late August to mid-November.  Taking photos or sketching the direction of meteors is the only reliable way of estimating which shower they belong to.  Tracing the direction of the meteor back to a radiant will allow an observer to make an educated assumption.  Of course, there’s always the possibility of catching a sporadic meteor, which can come from any direction at any moment.

 

 

Deep Sky Delights in in Aquarius and Capricornus 

 

After climbing the Milky Way through the rich area of Cygnus and surrounding constellations, last month, we now drop down our galaxy's Eastern flank, headed through a much less rich, but in many ways, just as interesting area of sky.

 

Capricornus and Aquarius

 

To the naked eye, the constellations of Aquarius and Capricorn are particularly unspectacular, but they contain some very interesting objects for those with telescopes and binoculars to observe.  The most northerly of these is the fine globular cluster M2.  At +6.46 mag, it is amongst the brighter of these interesting objects, lying 37,500 light years away from us and about 175 light years in diameter.  From Earth, it appears 2.1 arc minutes in diameter, M2 is about the same relative size and brightness of the neighbouring M15 (more of which next month) and the second of Hercules' well-known globulars, M92.  Discovered by Comet Hunter Jean-Dominique Maraldi in 1746, it languished in relative obscurity until Messier added it to his list in 1760, describing it as a "Nebula without stars".  Modern instruments show it as most definitely "with stars", indeed there are several beautiful star chains visible through telescopes, as well as some deep, dark lanes and patches, adding to the "three-dimensionality" of the object, particularly in larger telescopes.  There are quite a mix of older orange and newer blue stars within M2, making it a particularly pretty telescopic sight.

 

M2 - HST Image.  Public Domain.

 

Moving south west from M2, we arrive at three objects in quick succession: NGC 7009, The Saturn Nebula, the asterism M73 and another globular, M72.  The Saturn Nebula is a fascination Planetary Nebula, well worth seeking out in any telescope, as it is reasonably bright, at +7.8 mag, yet compact at 0.5 arc minutes across.  Telescopes of 6-8-inch aperture will be needed in order to see the two extended lobes that give the object its popular name.  Lord Rosse, observing NGC 7009 in 1850, described two lobes or projections sitting either side of the nebula, making it appear very similar to Saturn, when its rings are edge on to us.  Although the object has a distinctly un-Saturn-like green-blue hue, which is most easily seen in long duration photographs.  The Saturn Nebula, in common with some other Planetaries - including the Blinking Planetary, mentioned in last month's Observing Highlights - can appear to blink on and off when looking at it for prolonged periods.  This is of course a trick of the eye, caused by NGC 7009's reasonably bright central star overwhelming a dark-adapted observers eye.  When the observer averts their vision slightly, the Saturn Nebula returns to view.  Although the Blinking Planetary is the most well-known object that exhibits this phenomenon, to the writer's mind, the Saturn Nebula is actually the best example of a "Blinking" Planetary Nebula.  As ever, aperture helps in resolving the finer details of NGC 7009 (especially the projections), but the Saturn Nebula should be sought out by all those with telescopes - it's certainly bright enough to be seen in even the smallest scopes.

 

The Saturn Nebula - HST image.  Public Domain.

 

The next object is an interesting one.  When is a star cluster not a star cluster?  Answer: when it's an Asterism like M73. Lying less than 2 degrees SW of the Saturn Nebula, M73 has been the subject of some controversy over the years since its discovery.  Charles Messier first noted it in 1780 as a "cluster of four stars with nebulosity", although this nebulosity has never been picked up by any other observers.  John Herschel, whilst including it in his General Catalogue, was suspicious of its definition as a true cluster.  Debate raged on throughout the 20th century as to the true nature of the Y-shaped M73, with evidence of a relationship between the members of the group being published for and against.  The matter was finally and conclusively put to bed in 2002, when spectral signatures of each of the constituent members, gathered in high resolution, concluded that they were all moving in different directions and the cluster was not, it fact, a cluster.  M73 is not unique amongst the Messier list for controversial description, but remains interesting for the fact that it took so long to finally work out its true nature.

 

1.5 degrees to the west of M73 is the slightly less controversial Globular Cluster M72.  At +9.27 mag, it is considerably fainter than M2, despite being not much smaller.  M72 is considerably further away from us than M2 - it lies 55,000 light years distance from Earth.  As it is fainter and further away, M72 requires a larger telescope to resolve individual stars.  It is a pleasing sight in a 10-inch reflector and above, though William Herschel in his observing notes of 1783, noted that a power of 150x was needed to resolve the individual stars "fairly".

 

M72 - HST image.  Public Domain.

 

Moving east across the constellation boundaries into Capricorn, we come to another Globular, M30.  Bigger at 2.1 arc minutes across and brighter at +7.19 mag than M72, M30 was discovered by Charles Messier in 1764.  Bright at the core with numerous tendrils of stars (some appearing to us from our perspective on Earth as remarkably straight) and dark lanes bisecting these bright areas, M30 is an attractive object.  Though reasonably compact for an object of its type at only 100 light years across, M30 is also reasonably close at 26,000 light years distance.  Unlike M72, M30 is a much more rewarding target for owners of smaller telescopes, as its bright, compact core and surrounding halo of star appears well resolved in any instrument.

 

M30 - HST image. Public Domain.

 

Further to the east, back across the border into Aquarius, is the interesting galaxy - or galaxies - NGC 7252.   Although this target is faint at +11.39 mag, it can still be sen as a fuzzy blur in larger instruments.  Long duration exposures of this object reveal great loops of stars streaming from its core.  This give rise to its popular name, the "Atoms For Peace" Galaxy - named after President Eisenhower's 1953 speech on the peaceful use of atomic technology.  The galaxy does indeed resemble an atom is structure, with the great star loops appearing to describe the motions of an electron around an atomic nucleus.  It appears that these loops have been caused by the central galaxy colliding with another.  The core galaxy being a very compact spiral, this collision is of great interest to Astronomers attempting to predict the outcome of our own Milky Way's collision with the M31 Andromeda Spiral in 4.5 Billion years time.  Eventually, it is postulated the Atoms For Peace Galaxy will become an elliptical galaxy, much as the Milky Way-Andromeda pairing is predicted to do.  Further tidal observations of regions in this fascinating object will hopefully bear this out.

 

NGC 7252 - HST Image.  Public Domain.

 

Lastly, we journey 4 degrees NE of NGC 7252, to rendezvous with the closest Planetary Nebula to Earth, NGC 7293 - The Helix Nebula.  Overlooked by experienced observers, such as Messier and William Herschel, it is not difficult to understand why.  Though intrinsically quite bright at +7.59 mag, the Helix is half the diameter of the Full Moon, which spreads its surface brightness out considerably.  Observation or the Helix requires either large binoculars and a very dark site, or a wide field low power eyepiece and as much telescopic aperture as you can throw at it!  Large Dobs are the ideal instrument for observing the Helix, particularly when coupled with an OIII filter.  From our perspective on Earth, we see the Helix like looking down a tube.  Its prolate spheroid shape is almost aligned on axis with us, at a distance of 650 light years.  2.5 Light years across, the Helix appears 14.7 arc minutes across at its widest point.  A magnificent object, it will take the right conditions to see it well - if the Moon's up, you'll have to wait until it has set before attempting to locate the Helix.  It will be well worth the wait though.

 

 The Helix Nebula - HST image. Public Domain.

 

Text: Kerin Smith