[ANS 2016] - ANS 2017 [Jan][Feb][Mar][Apr][May][Jun][Jul][Aug][Sep]
Click here to download the September 2017 PDF version

 

Spring has arrived, celebrate the warmer weather with views of the night sky aided by:

The Australian Night Sky - September 2017

By Wayne Roberts

What's In The Sky This Month

The Planets

Our Monthly Feature

 

[Click here to show or hide the explanatory notes]
Top of page

What's in the sky this month; September 2017:

 1st Alpha [α] Aurigids meteor shower peaks.
 5th Mercury stationary;
     Neptune at opposition.
 6th Neptune occulted by Moon (not from Australia);
     Full Moon.
 9th September Perseids meteor shower peaks.
12th Mercury at greatest elongation west (of the Sun);
     Aldebaran (Alpha Tauri, magnitude 0.8) occulted by Moon (not from Australia).
13th Last quarter Moon.
14th Moon at perigee (closest to Earth, 369,860 km);
     Saturn at eastern quadrature.
15th Cassini’s fiery dive into Saturn’s disk;
     Mercury at perihelion (closest to Sun, 46.00 million km / 0.3075 au).
18th Venus occulted by Moon;
     Regulus (Alpha Leonis, magnitude 1.3) occulted by Moon (not from southern Australia).
19th Mars occulted by Moon (not from Australia);
     Mercury occulted by Moon (not from Australia).
20th New Moon.
21st Kappa [κ] Aquariids meteor shower peaks (date of maximum varies).
23rd Earth at spring equinox (autumnal equinox in northern hemisphere).
27th Moon at apogee (farthest from Earth, 404,348 km).
28th First quarter Moon;
     Pluto stationary (ends retrograde motion).

N.B.: When reading the following, refer back to the explanatory notes at the beginning of this article (click on the above link to expand) for information on terminology, angular separation approximations and adjustment of latitude & longitude.

The Moon throws its weight around this month, occulting, from various places on Earth, no fewer than four planets and two stars. First comes Neptune, on the 6th, the occultation being visible from Antarctica, SE Sth America and South Georgia, but not, unfortunately, from Australia; while Neptune is occulted from our location, the event plays out with the bodies almost 40° below our eastern horizon, from 1:48 pm to 2:14 pm.

Next comes Aldebaran on the 12th, the occultation of which can be witnessed by viewers in Hawaii and the Azores. Locally, the Moon’s limb passes just over ½° from the planet at 9:11 pm, more than three hours before the pair rise.

Brilliant Venus is next, on the 18th, and this time we are favoured, albeit to witness a daytime event, with our natural satellite passing in front of the brightest of all planets between 10:47 am and 12:02 pm, at altitudes of 39° N & 35° NNW respectively. As neither the waning crescent Moon, at a very thin phase of less than 6%, nor Venus will be visible to the naked eye (the planet can be seen in broad daylight if you know where to look, but it’s difficult), viewing the occultation will require scanning the appropriate area of sky with binoculars or a telescope; if you do so, be aware that the Sun is a mere 28° away – hide it behind a suitable obstruction to prevent instant and irreparable eye damage should it enter the field of view. New Zealand and SE Asia can also join in the fun.

Just a few hours later, Regulus steps up to the plate, but misses from our perspective; the Moon’s limb is never closer to the star than 12' (' denotes arc-minute, 1/60th of a degree, and itself divisible into 60 arc-seconds, symbol "), at 4:36 pm, around ½ hour after the pair have set. The Moon does cover the star, albeit in daylight, from locations in the northern half of Australia, but even so, near or below the horizon in the NE quadrant of the continent (the far NW fares best). NE Africa, the Middle East and SE Asia are favoured.

The Moon’s encounters with Mars and Mercury on the following morning also disappoint from our point of view. Mars, occulted from Hawaii, the Galapagos Islands, NW Sth America and NE Micronesia, is not occulted from our point of view, closest approach of the Moon’s limb being 38' (just under ⅔°) at 4:38 am, with the planet 12° below the morning horizon. Mercury’s encounter, while still not an occultation from our perspective (Micronesia, Polynesia and far east Asia are the places to be), at least sees closest approach of ½° occur above the horizon, at 9:07 am, 34° high in the NE.

Only two meteor showers peak this month, and neither is worth writing home about. First, on the 9th, is the September Perseids (do not confuse with last month’s Perseids), with a ZHR of just five; although outbursts were recorded in 2008 and 2013, none is expected this year. The 93% illuminated waning gibbous Moon conspires to further diminish the event, rising at 8:07 pm on the 8th and remaining above the horizon all night. The following chart shows the position of the radiant as it transits at an altitude of 12° at 4:21 am (on the morning of the 9th). The chart also plots Kappa Aquarii, magnitude 5.0, as my software does not show the shower associated with this star; the Kappa Aquariids peak on the 21st (or thereabouts, the peak is said to wander a little). While this shower is not Moon affected, its ZHR of three does not cry out for attention; a few other prominent stars are plotted for reference.

Those who have been following the Cassini spacecraft’s daring dives between Saturn and its rings will be aware of its upcoming grand finale on the 15th, when it will dive head first into Saturn itself, providing data about the atmosphere before meeting its fiery end – see the feature section for more.

Top of page

THE PLANETS

Mercury

Having spent the latter half of July and the first half of August in its best evening apparition of the year, September sees Mercury’s worst morning show of 2017.

Having swung in between Earth and the Sun on the 27th of last month, it rises, on Sept 1st, at 6:07 am, not much more than ½ hour before sunrise, 6:42 am, at which time the planet sits only 6° above the ENE horizon. The innermost planet’s 10" disk, just 5% illuminated, shines rather feebly at magnitude 3.1 and is not a viable target, totally swamped by morning twilight.

This apparition being as poor as it is, matters don’t improve a great deal in the days to come. On the 5th, its westerly motion relative to the stars ceases (it is said to be stationary), and although angular separation from the Sun in our morning sky continues to increase until the 12th, it is nevertheless rising less than ¾ hour before sunrise (5:42 am vs 6:26 am) on that morning, and under 8° clear of the horizon as the Sun makes its daily debut.

Mercury reaches perihelion on the 15th, its closest point to the Sun in its orbit, at a distance of 46.00 million km, or 0.3075 au (astronomical unit, the average Earth-Sun distance).

Mercury and Mars have an extremely close conjunction in our morning sky on the 17th, the latter sitting just 4' – 1/15th° – above its sibling when Mercury rises at 5:43 am. With the Sun rising only 35 minutes later (6:18 am), the pair are heavily ensconced in morning twilight, making observation difficult. Your best bet, with the aid of a flat eastern horizon, is to try and spot Mercury, shining strongly at magnitude -0.8, as soon as possible after it rises, then look for Mars, much fainter at magnitude 1.8, immediately above it. As stated in the earlier section relating to Venus’ occultation by the Moon, exercise extreme caution not to allow the Sun to enter the field of view of your viewing instrument lest your eyesight be damaged.

New Moon this month falls mid-afternoon on Wed 20th; consequently the evening of Saturday 23rd will serve as our designated viewing night this month, prompting the provision of the following Mercury data for the morning of Sunday 24th: rise 5:47 am; altitude at sunrise (6:07 am) 3½°; span of disk 5.5", phase 88%; magnitude -1.2.

By month’s end, Mercury is only nine days short of rounding the far side of the Sun, rising less than 10 minutes before Sol, 5:49 am vs 5:58 am; its disk spans just 5", 97% illuminated, and is totally overwhelmed by the brightening eastern sky despite shining at magnitude -1.3.

Mercury describes a sweeping loop throughout Leo this month, and transitions to Virgo on the 26th.

Venus

On September 1st Venus rises at 5:14 am, still almost 1½ hours before sunrise, 6:42 am, when the planet sits 14° clear of the NE horizon; it displays a 12.4" disk, 84% illuminated, blazing at magnitude -3.95. The planet enters morning twilight on the 2nd, the same morning that it passes under 1½° from the core of M44, the Beehive cluster. Here’s a chart configured for 5:41 am, one hour before sunrise, with Venus at an altitude of a little under 4½°.

Venus will descend deeper into twilight throughout the rest of the year, dipping closer to the sunrise horizon with each passing day; its inherent brilliance will render it a naked eye target until mid-December, albeit very close to the horizon by then, and perhaps even until the end of the year. Having risen a little later daily since May 20th, Venus reverses this trend on Sept 5th, but continues to drop back towards the morning horizon courtesy of the progressively earlier rise time of the Sun. Come our viewing night of the 23rd, or morning of the 24th in this instance, the planet rises an hour before sunrise (5:07 am as against 6:07 am); it’s at an altitude of 11° as the Sun breaches the horizon, spans 11.4", 89% lit, and effortlessly pierces the twilight at magnitude -3.94 (refer to the earlier notes following the monthly summary for details of Venus’ occultation by the Moon six days earlier, on the 18th).

At the end of the month, Venus rises at 5:03 am, and is just 10° high when the Sun rises at 5:58 am; spanning 11.2" at a phase of 90%, it remains prominent in the pre-dawn sky at magnitude -3.94.

Beginning the month in Cancer, Venus crosses the border into Leo on the 10th, and remains in that constellation for the remainder of the month.

Earth

Our home world gets a brief mention this month, the spring equinox arriving on the 23rd; the Sun rises in the east and sets in the west, with day and night of approximately equal length (north of the equator, it’s the autumnal equinox).

Mars

Mars, which drifts at a leisurely pace against the starry backdrop from our perspective, is slowly clawing its way into our morning skies. On the 1st, it rises at 6:18 am, and is at an altitude of 4° ENE at sunrise, 6:42 am. See the Mercury notes for details of Mars’ tantalizingly close brush with that planet, albeit in heavily light polluted skies close to the horizon. On the morning of the 24th, following on from our viewing night of Sat 23rd, rise time has improved to 5:26 am, and Mars sits at an altitude of 7° at sunrise, 6:07 am, almost exactly halfway between Venus and Mercury.

Month’s end sees the Red Planet 8° clear of the ENE horizon at sunrise, 5:58 am, after having itself risen at 5:12 am. All in all, not a good time to observe Mars; such an exercise requires patience, as the planet will rise away from the morning horizon in its own good time, not clearing the twilight zone until November 20th.

Mars spends the month of September within the constellation of Leo, being overtaken on its easterly trek against the stars by Mercury on the 17th, with Venus closing at month’s end.

Jupiter

This apparition of mighty Jupiter is nearing an end, the planet setting in twilight from October 2nd and being in conjunction with the Sun late in that month. As September begins, the King of the Planets sets at 9:27 pm, having been at an altitude of 23° WNW as evening twilight faded at 7:26 pm; its disk spans 32.2" and shines at magnitude -1.74.

On the 23rd, the situation has deteriorated markedly – Jupiter is a mere 7° clear of the western horizon as the sky fully darkens (7:45 pm), setting at 8:22 pm; the span of its disk is down to 31.2" and its brightness to magnitude -1.68. Here’s a chart configured for the cessation of twilight, showing the King’s somewhat compromised position; the star at far left of the chart is Spica (Alpha Virginis, mag 1.0).

Although viewing the planet at such a low altitude requires looking through a good deal of the Earth’s distorting atmosphere, here’s a magnification of the above chart for those who want one last peek:

West of Jupiter (below, on the chart), Callisto is moving away from its parent, while to the east, Ganymede is receding, with Io and Europa approaching; respectively, the moons shine at mag’s 6.6, 5.6, 6.0 and 6.3. The brightest star in the field of view, HIP66535, is labelled with its magnitude of 9.06 to emphasize the fact that none can be confused with the moons.

On September 30th, Jupiter’s particulars are further degraded, the planet sitting a paltry 1½° above the horizon at the end of evening twilight, 7:52 pm, then setting at 8:01 pm; the disk spans 31.0", still shining at magnitude -1.68. Last call for the Jovian express folks; get your views in now .

Jupiter will remain within Virgo until mid-November.

Saturn

Although well past opposition, which it reached back in mid-June, beautiful Saturn and its captivating ring system remain well positioned for viewing throughout September. At the start of the month, Saturn transits at 6:59 pm, and remains 73° clear of the northern horizon at the expiration of evening twilight, 7:26 pm, before setting at 2:15 am. The planet’s disk spans 17.0", the marvellous ring system 38.5" at an inclination of 26.90°; disk and rings together shine at magnitude 0.41.

Saturn is at eastern quadrature on the 14th – the Sun-Earth-Saturn angle is 90° and the disk casts its maximum shadow onto the back of the rings. On the following day, the signal from the Cassini spacecraft is predicted to be lost at 8:45 pm as the craft burns up in Saturn’s atmosphere; with Saturn still well positioned in our sky, you could do worse than have your ‘scope trained on it at that time to imagine the fiery descent in real time and share in a piece of history.

Come our designated viewing night of the 23rd, Saturn transits considerably earlier, at 5:36 pm, but is still at a very healthy altitude of 58° in the NW as twilight ends at 7:45 pm; it subsequently sets at 12:51 am. The span of the disk is down a little, to 16.3", as is that of the rings, 37.1", inclined at 26.96°; visual magnitude is just a little dimmer, at 0.49. The following chart is configured for 7:45 pm, to permit viewing under a dark sky at the earliest opportunity; Pluto is plotted for later reference.

Saturn is readily identifiable as the brightest star anywhere near the zenith[directly overhead] (at 7:45 pm), and positioned between and below the constellation figures of Scorpius – anchored by Antares, aka Alpha Scorpii, mag 1.0 – and Sagittarius. When you have Saturn in your sights, refer to the following magnification, configured for the same time.

Before turning your attention to Saturn’s seven brightest moons, as labelled on the chart, spend some time drinking in the splendour of the ring system, which is face on from our perspective to the greatest extent in almost 15 years, and due to reach a maximum inclination of 26.98° next month. The Cassini division will be obvious as a dark line scribed roughly around the centre of the ring plane, while careful examination of the rings beyond the disk should reveal the shadow of the latter on the former.

The labelled moons shine at the following visual magnitudes: Iapetus 11.0, Enceladus 12.2, Mimas 13.4, Rhea 10.2, Tethys 10.7, Dione 10.9 and Titan 8.8. The labelled star, TYC6246-205-1, shines at magnitude 10.6; as such you must be careful not to mistake it for a moon, four of which are of similar brightness. All other stars in the field of view of the chart are 15th magnitude or, in the vast majority of cases, even dimmer, and so can not be confused with the moons. Any telescope will pick up Titan, the brightest moon, a six incher will show the three 10th magnitude satellites and Iapetus, while Enceladus will probably require an eight incher; tiny faint Mimas, close to the bright ring system, may require a ten or even a twelve incher.

Although transiting at 5:10 pm – almost 1¼ hours before sunset – on the last day of the month, Saturn is still 52° high in the NW as twilight fades; as the evening unfolds, it arcs across the sky towards the WSW horizon, where it sets at 12:25 am. The disk spans 16.2", the rings 36.7", inclined at 26.97°; the planet as a whole shines at magnitude 0.51.

Saturn inhabits the constellation of Ophiuchus throughout September, October and more than half of November.

Uranus

Uranus continues its march into our late evening and early morning skies, rising at 9:52 pm as the month of September begins; catching it at a decent altitude still requires late nights/early mornings – it doesn’t transit until 3:21 am (on the morning of the 2nd). Uranus’ disk spans 3.7" (all month) and shines at magnitude 5.73.

Those wishing to turn their telescopes toward it on our viewing night will find it rising at 8:22 pm before transiting at 1:53 am, and shining marginally brighter, at magnitude 5.70. Here’s a wide field chart configured for time of transit, showing where in the sky it is to be found; Neptune is also plotted, as are a number of stars as reference points. The chart approximates a naked eye view, plotting only stars brighter than magnitude 5.5.

Begin searching for Uranus by first identifying the Great Square of Pegasus, in particular Algenib (Gamma [γ] Pegasi, mag 2.8) at its top right corner. Look then for Eta [η] Piscium, mag 3.6, 19° to Algenib’s upper right (as a rough guide, one finger held at arm’s length spans a little over 1°, a closed fist 10°, an open hand, tip of little finger to thumb tip, 20°). Although very comfortably naked eye, Eta Psc is not bright and prominent; it should be readily identifiable however, for three reasons – firstly, you know where to look in relation to Algenib; secondly, it is, as the chart shows, in a barren part of the sky where naked eye stars are concerned; thirdly the two brighter stars to its lower left point at it.

With Eta under your belt, look 7° (¾ of a closed fist) above and a little to the right for Omicron [ο] Psc – still comfortably naked eye at mag 4.25, but somewhat faint; there are no naked eye stars between Eta and Omicron, save for the marginal mag 5.5 Pi [π] Piscium, extremely faint (if you can see it at all) and very nearly at the midpoint of a line joining Eta & Omicron. Confirm that you have correctly identified Omicron by noting its position directly on a line joining Algenib and Kaffalijidhma (Gamma Ceti), mag 3.5, circled to the left of Menkar (Alpha Ceti), which at mag 2.5 itself stands out as easily the brightest star in its part of the sky – brighter, in fact, than any other within 20° in any direction.

As the final magnification below (which labels stars with their visual magnitudes) shows, once you’ve found Omicron Psc, Uranus itself (which, at mag 5.7, may be glimpsed by the eagle eyed under dark skies) has nowhere to hide.

This chart is intended for use in conjunction with your finder ‘scope, and as such plots stars down to magnitude 9.5, a little beyond the reach of a typical such instrument; it is delimited by Eta and Omicron Psc, labelled at the bottom and top of the chart respectively, with the previously referred to Pi Psc at centre. As you can see from the chart, Uranus is easily the brightest point of light between Omicron and Pi; the only star anywhere near it of comparable brightness is the relatively remote HIP8588, labelled with its brightness rating of 5.9. The planet will also stand out courtesy of both its steady shine relative to the twinkling stars and its subtle blue-green hue; Uranus’ position at the start and finish of September is indicated by white crosses respectively right and left of its plotted location on our viewing night. When you think you’ve identified your target, confirm capture by switching to the main eyepiece at a magnification of 150x or more to resolve the planet’s small (3.7") disk, the colouration of which will be much more noticeable and aesthetically pleasing at the increased magnification.

At the end of September, Uranus is rising at 7:53 pm, after which it transits at 1:24 am; its brightness is unchanged to all intents and purposes, at mag 5.69. The planet will remain within the constellation of Pisces until 2018/19.

Neptune

Neptune is at opposition early this month (on the 5th), and as such will be ideally positioned for viewing in the coming months. As September gets underway, Neptune rises in the east at 6:09 pm, ten minutes after the 5:59 pm sunset, and is 15° high in the east as twilight fades at 7:26 pm; its disk shines at magnitude 7.81, spanning 2.4" throughout the month.

Come our viewing night, visual magnitude has regressed incrementally, to 7.82. The planet is rising considerably earlier, at 4:40 pm, and sits 36° high in the ENE at the end of twilight, 7:45 pm; it transits at 11:06 pm, then sets at 5:32 am, not quite an hour after morning twilight commences at 4:39 am. The first of the two charts in the Uranus notes shows where Neptune sits in relation to the Great Square of Pegasus and two other asterisms, the Circlet in Pisces and the ‘Y’ of Aquarius; these latter two are composed primarily of 3rd and 4th magnitude stars (mostly 4th) and so are clearly, if a little faintly, visible to the naked eye. Here’s another chart depicting the relationship at 10:00 pm, with Neptune at an altitude of 56° NNE.

The chart, with a mag 5.5 cut-off, approximates a naked eye view, and is intended to aid in identification of Lambda [λ] Aquarii, magnitude 3.7, the star just to Neptune’s lower left on the chart, using the ‘Y’ as a starting point. Note that there is only one naked eye star between the ‘Y’ and Lambda, the very faint magnitude 5.0 Kappa Aquarii; this fact along with Lambda’s domination of its immediate locale should facilitate easy capture. Having bagged Lambda, refer to the final magnification below to pinpoint your planetary target using a finder ‘scope.

Stars are plotted down to magnitude 9.5, so your finder is unlikely to show any others; white crosses are included to plot the planet’s position at the start and end of the month. Centre Lambda (‘3.71’), and examine the star field just to the ESE (upper right on the chart – remember your optics will likely alter the view). Note that the chart as a whole spans less than 2° and Neptune is only ¾° (around ⅔ the width of a finger at arm’s length) from Lambda; as the view through your finder will span perhaps 4°, Lambda, Neptune and the rest of the labelled stars will be concentrated towards the centre of the field of view. Arguably then, this exercise is best conducted through the main eyepiece at very low magnification, in order to spread out the star field a little, but that approach has the disadvantage of displaying a good deal more stars from amongst which you must tease out the planet.

Persisting with use of the finder, as I suggest you do, look in the appropriate direction from centred Lambda for two ‘stars’ extremely close together (separated by 7'), and of very nearly equal brightness. I refer to the star of mag 8.00 (HIP113231), and Neptune itself, mag 7.82. One of the pair will tend to reveal its planetary status via a relatively steady shine compared to the twinkling stars, along with its subtle blue-grey hue.

When you think you’ve got your quarry, switch to the main eyepiece at high magnification (I suggest 250x or more) and look closely to resolve Neptune’s tiny 2.3" disc.

As the month of September concludes, Neptune crests the eastern horizon earlier in the afternoon, 4:12 pm, is 42° clear of the NE horizon at the end of twilight, 7:52 pm, transits at 10:38 pm, then sets in morning twilight at 5:04 am, a little under 40 minutes after the impending dawn starts to brighten the sky.

Moving at a leisurely pace in the outer solar system, Neptune will not exit Aquarius until 2022/23.

Pluto

Prime viewing time for Pluto continues throughout September, the planet (some would disagree ) having been at opposition in July. On the first day of the month, Pluto is already at an altitude of 66° in the NNE as evening twilight subsides at 7:26 pm. It transits less than 1½ hours later, 8:50 pm, then sets at 4:04 am; the tiny frozen orb spans 0.097", far too small to be resolvable in amateur instruments, and glows meekly at magnitude 14.24.

By the time our viewing night comes around, it is transiting at 7:23 pm, just over 20 minutes before all light drains from the sky (7:45 pm), and setting at 2:37 am; span and brightness sit at 0.096" & mag 14.27. The wide field chart for Saturn shows where Pluto lurks near the three naked eye stars below the handle of the Teapot asterism of Sagittarius (as we in the south view the Teapot upside down, it may be more accurate to say above the handle). Here’s an initial magnification, configured for 8:30 pm – as will be the others to follow – with Pluto 68° clear of the NNW horizon; it labels the three stars referred to above: Albaldah (Pi Sagittarii), mag 2.9, Omicron Sag, mag 3.75 and Xi22] Sag, mag 3.50.

Now for a further magnification relating Pluto to the closest two of the above reference stars, as well as two more of 6th and 7th magnitude – HIP94372, mag 6.4, and HIP94338, mag 7.8 – which lie much closer to the planet; the chart includes markers, in the form of white crosses, for Pluto’s position on the first and last Saturdays of September (the 2nd & 30th).

This chart plots only stars brighter than mag 15.5, as will the two additional charts to follow. It spans approximately 2½° diagonally and is best used in conjunction with a low-medium power eyepiece – such a view won’t capture the fainter stars shown here, but should be ideal for identifying the two HIP designates which lie near the edges of the further magnification below:

This penultimate view labels only those stars brighter than mag 12.1 (it shows a portion of the star field to the right, on the chart, of the mag 6.37 star; this is somewhat removed from the area of interest and is included only to avoid constructing a boundary which bisects labels). The chart contains a number of double stars, each pair shining as one as follows: 12.09 + 11.85 → 11.3; 12.00 + 11.81 → 11.2; 11.75 + 11.70 → 11.0; 12.05 + 11.59 → 11.1.

While Pluto’s position on the 23rd is labelled in blue, white crosses plot all other Saturdays in September, from the 2nd (rightmost cross) to the 30th; as the chart’s diagonal span is a little over ⅓°, it is best suited for use in conjunction with medium-high power magnification. Note Pluto’s curved track, courtesy of the fact that it reverses direction on September 28th as its period of retrograde motion (see the explanatory text at the beginning of these viewing notes) comes to an end. Use the chart to identify, by reference to the two relatively bright HIP designates at top and bottom of the chart, the 10th and 11th magnitude stars closer in to Pluto, in particular that of magnitude 10.43 (TYC6295-1510-1); this star will serve as the launching point for identification of Pluto with the aid of the final magnification below.

All star charts courtesy of StarryNight®ProTM Version 7.6.1.1327/Simulation Curriculum Corp.

This view labels all stars, with their magnitudes, to facilitate a final assault on Pluto (note that the double star of magnitudes 13.6 & 13.7 will shine as one of mag 13.0). Use the highest magnification that equipment and conditions permit – apart from the faint glow of both Pluto and most of the stars shown here, the entire chart only spans around 4' diagonally, and unless you have a very large ‘scope all the stars shown will occupy a fairly small area of the field of view through the eyepiece (for example, a twelve inch ‘scope fitted with a 5mm eyepiece with an apparent field of view of 80° will give a true field of view of 16' – look back at the July notes for more information on the field of view of a given ‘scope/eyepiece combination).

In view of the small expanse of the area of sky under consideration, you may find it easier to just go with the second last chart, but the final magnification was necessary to provide space for labelling the stars. In either case, begin the final hunt from the mag 10.43 TYC designate – as both charts show, it forms a straight line with two other stars to its upper right (on the chart), with Pluto extending the line on a slight angle (the fuzzy plot of the planet in the final view is due to the software beginning to show its five moons).

Because Pluto is far too small to allow confirmation of capture by means of resolving the disk, you may wish to sketch or image the star field and return the following night to see which ‘star’ has moved; with this in mind, the final chart incorporates a white cross marking the planet’s position on the following night of the 24th (the cross toward the top of the chart is for the 30th, that at far right shows its position on the 16th).

On the final day of September, Pluto transits at 6:56 pm, is at an altitude of 70° NNW when twilight wraps up at 7:52 pm, and subsequently sets at 2:10 am; span and brightness stand at 0.095" & mag 14.28.

Pluto is in no hurry to exit Sagittarius, and will not do so until 2023/24.

Top of page

Our Monthly Feature

While composing this edition of The Australian Night Sky, I was unable to decide on a feature article; a call for help on my Facebook page resulted in a number of suggestions, a coverage of the Cassini mission being one. As Cassini is due to end its mission on the 15th of this month, with a fiery dive into Saturn’s disk to prevent any possibility of the craft impacting possibly life bearing Saturnian moons in the future, now is certainly an opportune time to look at the journey and achievements of this marvellous spacecraft.

There are many sites on the ‘web covering the mission, and it soon became obvious that, rather than simply reproduce the information contained therein, the best approach was to present a few basic facts accompanied by pertinent links, and that will be the method employed below.

On October 15th, 1997 (our time; October 14th at the launch site), the Cassini orbiter and its Huygens probe were dispatched on their way to Saturn; Huygens was destined to descend to the surface of Titan, Saturn’s largest moon, and the second biggest in the solar system, after Jupiter’s Ganymede.

The spacecraft was blasted into orbit atop a Titan IVB/Centaur rocket, launched from Cape Canaveral, on a tortuous journey which would involve a number of gravity assist fly-bys, two of Venus and one of Earth itself. While the maximum separation between Earth and Saturn is only 1.7 billion km, the gravity assists needed to achieve the required velocity, which reached 19 km/sec (68,400 km/hour), increased the total distance traversed to 3.5 billion km.

The craft measures 6.7 metre by 4.0 metre, and weighs 5712 kg, the Huygens probe 2.7 metre in diameter, with a mass of 320 kg; because the destination was so far from the Sun, it was decided that it would be nuclear powered, fuelled by 33 kg of plutonium-238 (in the form of plutonium dioxide). The total cost of the mission has been put at $3.27 billion, approximately 80% of which was provided by the US.

Here is the first link, providing a detailed timeline of the journey; clicking on any of the events shown in the timeline will bring up a graphic and dialogue relating to that event:

https://saturn.jpl.nasa.gov/the-journey/timeline/

The second link below provides details of each of the 22½ final orbits of Saturn – those christened The Grand Finale – between the rings and the disk (the ½ represents the dive into Saturn itself), which were preceded by 270 orbits gathering all manner of information about the Saturnian system. As these notes are being composed, Cassini is beginning its fifth last full dive between the disk and rings:

https://saturn.jpl.nasa.gov/mission/grand-finale/grand-finale-orbit-guide/

A link to images, taken by Cassini itself, relating to the mission

Finally, here’s an excellent interactive tool which allows you to view the spacecraft and its assortment of instruments from different angles; click and drag to rotate the image, scroll the mouse wheel to zoom in and out. There is also a facility to zoom in on any given instrument; clicking ‘EXPLORE’ will then give detailed information on that instrument (choose Spacecraft>Spacecraft at top right to change instruments).

https://saturn.jpl.nasa.gov/the-journey/the-spacecraft/

That’s all we have for this month folks, tune in next month when The Australian Night Sky returns with more of the fascinating science and hobby that is astronomy.

As always, any questions, comments or suggestions are welcome and may be directed to: waynerobertsau@yahoo.com.au

Until next month:

CLEAR SKIES and HAPPY HUNTING