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As Autumn concludes and the nights grow longer, explore the night sky with:

The Australian Night Sky - May 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]
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What's in the sky this month; May 2017:

 2nd Mercury stationary.
 3rd First Quarter Moon.
 4th Regulus (Alpha [α] Leonis, magnitude 1.3) occulted by Moon.
 6th Eta [η] Aquariid meteor shower peaks;
     Mercury at aphelion (farthest from Sun, 69.82 million km/0.4667 au).
 9th Eta Lyrid meteor shower peaks.
11th Full Moon.
13th Moon at apogee (farthest from Earth, 406,210 km).
18th Mercury at greatest elongation west, 26°.
19th Last Quarter Moon.
20th Neptune occulted by Moon (not from Australia).
26th New Moon;
     Moon at perigee (closest to Earth, 357,207 km).

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.
Note also that Mercury’s aphelion shown above relates to the physical separation of planet and Sun, and is unrelated to their angular separation in our sky.

The occultation of Regulus by the Moon on the 4th is tailor made for locations in SE Australia. The star is swallowed by the dark limb of the 64% illuminated waxing gibbous Moon at 8:08 pm, with Regulus at an altitude of 40° in the north; it reappears at 9:20 pm, 35° clear of the NNW horizon. Central Australia is also favoured by the occultation, while the event begins at sunset in Darwin and a little over 20 minutes before sunset from Perth.

Neptune is not so obliging on the 20th, the event only being visible from the Maldives, Madagascar and the southern portion of Africa; locally it’s a non-event, being only a close occultation, the Moon’s limb passing just over 4' (' denotes arc-minute, 1/60th of a degree) from the planet at 4:24 pm, 27° below our SW horizon.

Two meteor showers peak in May, the prolific Eta Aquariids on the 6th and the minor Eta Lyrids on the 9th. The Eta Aquariids, the parent body of which is none other than Halley’s Comet (comet 1/P Halley), is one of the more prolific and reliable meteor showers in the southern hemisphere. The ZHR is 40, and unlike some showers, the peak is broad, often displaying a number of sub maxima, with hourly rates of 30 or more often seen between the 3rd and 10th – this is fortunate because the main peak is not expected until midday on the 6th. While the waxing gibbous Moon (see the notes at the beginning of this article) is 75% illuminated in the early hours of the 6th, it sets at 2:16 am, not long after the radiant rises at 1:52 am. This leaves the viewing hours before dawn, when an observer’s location is at the forefront of the planet’s charge through space (giving a front row seat to dust particles being swept up), Moon free. The following chart shows the location of the radiant as the Moon sets, with the radiant low in the east at an altitude of 4°.

The Eta Lyrid radiant is also shown ( just 2½° high in the NNE). This shower has a very modest ZHR of 3 and is adversely affected on its expected peak three days later on the 9th (when the location of the radiant at 2:15 am will be barely improved, 3½°) by a 95% waxing gibbous Moon which doesn’t set until 5:09 am, just under ½ hour before twilight begins to brighten the morning sky at 5:38 am.

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THE PLANETS

Mercury

The innermost planet comes out of hiding for its best morning apparition of 2017 during May, riding clear of morning twilight for all but the first couple of days of the month.

Even on the 1st, Mercury rises only nine minutes after twilight commences, 5:41 am as against 5:32 am; its 10% illuminated disk, spanning 11" (1", or one arc second, is 1/60th of an arc minute, itself 1/60th of a degree) will be difficult to spot at this stage, shining modestly at magnitude 2.5.

The planet quickly moves up and away from the morning horizon, reaching its greatest angular separation from the Sun (‘greatest elongation west’) of 26° on the 18th. On this morning, it rises at 5:04 am, almost ¾ hour before twilight begins to paint the sky at 5:45 am, at which time it has attained an altitude of 7°; its phase has fattened to 40%, with the disk down to 8" and shining much more vigorously at magnitude 0.55, rendering it visible in the east until the advancing twilight overwhelms it.

As New Moon falls on Friday 26th, we’ll nominate Saturday 27th as our viewing night, and so present data here for the morning of the 28th. Mercury crests the eastern horizon at 5:24 am, still almost ½ hour before the commencement of twilight at 5:51 am, when it’s at an altitude of 4½°; it spans 7", is 58% illuminated and shines vigorously at magnitude 0.0.

Three days later at month’s end, the above read 5:35 am, 17 minutes, 5:52 am, 3°, 6", 64% and -0.2, with the planet’s increasing brightness working to counteract its later rise time and consequent greater susceptibility to twilight.

The data throughout the month graphically illustrates just how quickly the phase, span and brightness of Mercury’s disk changes (due to its tight orbit around the Sun). The brightening of the disk as Mercury pulls away from Earth on its inner orbital track, thus increasing its distance from us is also laid bare – if you are unable to reconcile this counter-intuitive behaviour, feel free to contact me via the link given at the end of these viewing notes.

The following chart shows where Mercury sits at 5:51 am on the 28th (the commencement of twilight). Brilliant Venus is shown to aid in identification; Uranus and Neptune are also labelled for later reference.

Speedy Mercury, known in earlier times as The Messenger of the Gods due to its relatively rapid movement against the starry backdrop, begins the month in Pisces, crosses into Cetus on the 19th, and then Aries on the 22nd, seeing out the month in that constellation.

Venus

Brilliant Venus is at its best in the morning sky this month. Having shone at its brightest on the closing day of last month, it still shines vigorously, even by its stellar standards (excuse the pun ), and stands even higher in the morning sky – its altitude above the sunrise horizon reaches a maximum for the year, approaching 39°, on May 20th. Despite this, its greatest angular separation from the Sun in our sky doesn’t occur until a couple of weeks later, June 3rd; I invite you to resolve this apparent contradiction, feel free to contact me if you are unable to do so.

As May begins, the scintillating orb rises at 3:49 am, and has already attained an altitude of 35° in the NE at sunrise, 7:01 am; its disk spans 38", displays an attractive crescent phase of 27%, and shines at magnitude -4.5. On the date of maximum altitude at sunrise as mentioned above, the 20th, Venus breaches the eastern horizon at 3:34 am, almost 3¾ hours before the Sun (7:17 am); its disk spans 29", is 41% illuminated and shines at magnitude -4.4. On the morning of the 28th (the 27th being our nominated viewing night), it rises at 3:36 am; the Sun rises at 7:24 am, at which time Venus sits 38¼° high in the NNE. The phase of the disk is out to 46% but has shrunk noticeably to span 26"; it still commands attention in the morning sky at magnitude -4.35.

Statistics at the end of the month are a 25" span, 48% illuminated and shining at magnitude -4.33; rise time is 3:37 am and altitude at sunrise (7:26 am) is just under 38°.

Enjoy views of Venus at your leisure; while at its best this month and next, it won’t re-enter morning twilight until early September, shining in the eastern sky for the rest of the year, albeit very close to the horizon as 2017 winds up (rising only 10 minutes before the Sun on December 31st).

Venus spends the month in the constellation of Pisces, apart from a brief flirtation with a corner of Cetus from shortly after midday on the 11th until just prior to 2 am on the 12th.

Mars

Having descended into evening twilight last month, Mars continues its slow march towards the western horizon – no other planet can linger in the west as does Mars, its longevity attributable to the fact that it’s our relatively very close outer neighbour, whereby we pull away from it on our inner orbital track only slowly.

At the start of May, it sits at an altitude of 12° in the NW when the Sun sets at 5:33 pm, itself setting at 6:52 pm. Its tiny 3.9" disk is nominally out of round, at a visually unresolvable phase of 98%, and struggles to be seen through the twilight at magnitude 1.6.

On our viewing night of the 27th, setting time is 6:17 pm, having been 10° clear of the NW horizon at sunset a little over an hour earlier (5:11 pm); the disk is 99% illuminated, spans 3.7" and fades into the twilight at magnitude 1.7. The following chart shows where Mars sits in our sky at 5:45 pm, approximately half way between its setting time and that of the Sun. As both the chart and the statistics show, it is not an inviting target; this apparition, while dying a slow death, is over in all but name.

Come the 31st, the Red Planet sets at 6:13 pm (the outer planets tend to set around four minutes earlier each day; Mars takes four days to do the same this month, testifying to its leisurely descent), just over an hour after the 5:10 pm sunset, when its altitude is 9½°; phase, span and brightness remain as above.

Mars remains within the constellation of Taurus throughout the month of May.

Jupiter

Having been at opposition last month, prime viewing time for mighty Jupiter continues throughout the month of May.

On the first of the month, Jupiter has already attained an altitude of 30° in the ENE when evening twilight fades at 7:02 pm. It subsequently transits at 10:41 pm before setting at 4:57 am; the disk of the King of the Planets spans 43.5" and hangs prominently in the sky at magnitude -2.41.

On the 27th, it’s fully 45° high in the NE when the sky fully darkens (6:44 pm), transits at 8:51 pm, then sets at 3:05 am. The span of the disk has reduced a little, to 41.3" – reflecting its increasing distance from Earth – but still dominates the night sky at magnitude -2.27. No wide field chart is needed to identify brilliant Jupiter, which shines far more brightly than any star in the night sky; the following magnified view shows the position of its four Galilean moons at 8:51 pm (time of transit). Io and Ganymede sit to the east (right, on the chart), moving away from and towards Jupiter respectively, while on the other side of the planet, Europa is approaching while Callisto is receding. The respective visual magnitudes of the four are 5.4, 5.0, 5.7 and 6.0; as all stars in the portion of sky covered by the chart are 14th magnitude or fainter, they are not depicted. The barely visible dot to Jupiter’s lower left is the tiny moon Amalthea which, itself shining faintly at magnitude 14.5, will remain unseen through all but large ‘scopes, unless time lapse imaging is employed.

Four days hence on the 31st, Jupiter’s altitude at the end of twilight (6:43 pm) is 48°; it transits at 8:35 pm before setting at 2:49 am. The disk’s span is further reduced, marginally, to 40.9" and its brightness to magnitude -2.25.

Jupiter will remain in the constellation of Virgo throughout the month.

Saturn

Due to be at opposition on the 15th of next month, the marvellous Ringed Planet is well established in our evening skies in May, especially towards month’s end when our viewing night arises.

On the first day of May, Saturn rises at 8:14 pm, still almost 2¾ hours after sunset, and doesn’t transit until 3:29 am the following morning. Its disk spans 17.8", the ring system 40.4", inclined at 26.45°; the overall visual magnitude is 0.25.

The situation is much improved on our viewing night of the 27th – rise time is 6:26 pm, only 1¼ hours after the 5:11 pm sunset, and the planet transits commensurately earlier, at 1:41 am. The span of the disk is out to 18.3", and that of the rings to 41.45", at an inclination of 26.53°; the system also shines brighter, at magnitude 0.10. The wide field chart below shows where Saturn sits, 34° clear of the eastern horizon, at 9:30 pm (Pluto is also plotted for later reference).

The next chart, configured for the same time, zooms in to show Saturn’s seven brightest moons; left to right on the chart they shine at the following magnitudes: 10.7, 9.9, 11.9, 10.4, 13.1, 10.6 and 8.5.

The stars in this area of the sky have been depicted (they are barely visible) on this chart with a brightness roughly in proportion to that of the faintest moon shown, Mimas – the star catalogued as USNO J1741090-215948, just to the moon’s lower right (you should be able to make it out, just a little closer to Mimas than is Dione) shines at magnitude 14.7. While the other six moons are unfortunately all depicted at the same size by the software (which as a rule does a good job of plotting brighter objects commensurately larger, but fails here), it is obvious that no stars can be confused for moons.

All the visual magnitudes given here have been taken from Starry Night software except for that relating to Iapetus. The stated magnitude is my (confident) estimate, necessitated by the fact that the software does not take into account the stark variation in the albedo of the moon’s opposing hemispheres, significantly varying its brightness as seen from Earth over its 79 day orbital period around Saturn.

As a guide to visibility, Titan will be visible in any telescope, probably even a finder ‘scope or binoculars, while Rhea, Tethys, Dione and Iapetus will show in a six incher; an eight incher may be necessary for Enceladus, and Mimas may call for a twelve incher.

As the month draws to a close, Saturn is rising at 6:09 pm, just under an hour after sunset (5:10 pm), then transiting at 1:24 am; the disk spans 18.3", the rings 41.5", inclined at 26.55°, and the system as a whole shines at magnitude 0.07. With the ring inclination nearing its maximum – just shy of 27°, in October – and with Saturn at a southerly declination, thus placing it higher in our skies, views of this beauty will be memorable in the months to come.

Saturn will cross the border from Sagittarius into Ophiuchus on the 19th, leaving proponents of that quaint practice known as astrology high and dry .

Uranus

Having been in conjunction with the Sun last month, Uranus is poorly positioned for viewing in May, albeit starting to gain a little altitude in the pre-dawn skies late in the month.

On the 1st, it crests the eastern horizon at 5:46 am, 1¼ hours before sunrise and still within twilight; its disk spans 3.4" (throughout the month) and shines at magnitude 5.91. The chart in the notes on Mercury shows where the two planets sit in relation to one another, just over 1° apart.

Come the morning of the 28th, Uranus, incrementally brighter at magnitude 5.89, is better positioned for viewing, both by virtue of a greater altitude and its proximity to a first rate signpost – brilliant Venus, less than 6° away. The chart below shows the view at 5:51 am, the commencement of morning twilight; having risen at 4:06 am, Uranus sits at an altitude of 19° in the NE. Apart from the three stars with their magnitudes labelled in green (top to bottom, they are Mu [μ], Nu [ν] and Omicron [ο] Piscium), Uranus is the brightest point of light in the area of sky shown; accordingly the chart should facilitate capture through your finder ‘scope, with identification aided by the planet’s subtle blue green hue and steady glow relative to the twinkling stars. White crosses mark Uranus’ position at the start and end of the month, and Venus’ at month’s end (Venus sits far off the top left [west] of the chart on the 1st) .

When you think you’ve identified the target, switch to the main eyepiece at a magnification of 150x or more (250x is better) to resolve its tiny disk, the colour of which will be pleasingly vivid at the increased magnification.

On the 31st, rise time is 3:55 am; the planet’s brightness has improved imperceptibly to magnitude 5.88. Uranus will remain within Pisces until 2018/19.

Neptune

Neptune rides ahead of its inner brethren in our skies, but requires viewing in the hours well past midnight to catch it at a respectable altitude. At the beginning of May, the outer planet – since Pluto’s questionable demise – doesn’t rise until 2:21 am and is still almost 1¾ hours short of transiting at sunrise (7:01 am); its disk spans 2.2" and shines at magnitude 7.93.

The planet’s circumstances are somewhat better on the morning of the 28th, although still far from optimal. Rising at 12:37 am, it transits more than 20 minutes before sunrise, albeit an hour and ten minutes after the sky begins to brighten; the span is out to 2.3" and visual magnitude stands at 7.90. The Mercury chart, which is configured for 5:51 am, the beginning of morning twilight, will give you an idea of where Neptune sits in relation to two nearby asterisms, the Circlet in Pisces and the ‘Y’ of Aquarius, each composed of faint naked eye stars.

The chart below, configured for the same time and intended for naked eye use, magnifies this area, labelling stars with their magnitudes and plotting only stars brighter than magnitude 5.5. Note that the central star of the ‘Y’ has been left unlabelled; this has been done because there are two superimposed stars at this position – in some forums only the brighter member, Zeta11] Aquarii, magnitude 4.3, will be labelled, but its light output, when combined with the glow of Zeta 2 Aqr, mag 4.4, is rated at mag 3.6.

Use the chart to identify the two fairly faint but comfortable naked eye stars, of magnitudes 3.71 and 4.21, which flank Neptune as shown – they are Lambda [λ] and Phi [φ] Aqr respectively. Such identification should be straightforward because, as the chart shows, only one star brighter than magnitude 5.5 – around the limit of naked eye visibility for most folk under a dark sky – sits between them and the ‘Y’ (the mag 5.03 star is Kappa [κ] Aqr). Proceed then to the final magnification, which will facilitate capture of your planetary target through a finder ‘scope:

This chart is delimited by Lambda & Phi Aqr (which are circled), plots only stars brighter than magnitude 9.2, around the limit for a typical finder ‘scope, and labels only those stars brighter than Neptune, mag 7.9 (no stars below the ecliptic – the green line – are labelled). Star hopping from either of the two reference stars and noting that Neptune is very nearly in line with them, a little closer to ‘3.71’ (Lambda), should enable you to zero in on your target. As with Uranus, Neptune’s colour – a subdued blue-grey – and steady shine should be of assistance.

Switch to the main eyepiece at high magnification – I suggest 250x or more if conditions and equipment permit – to confirm capture by looking very closely to resolve the planet’s miniscule disk.

Neptune will cease its eastward trek against the stars next month as it begins retrograde motion; consequently, it is slowing this month and doesn’t move a great deal, as witnessed by the marker for its position at the start of the month.

As the month of May concludes, Neptune rises at 12:25 am, transits at 6:49 am, a little under an hour after twilight begins, and shines at magnitude 7.89; due to its leisurely outer orbit, it will continue to inhabit Aquarius until 2022/23.

Pluto

The outermost planet will reach opposition in July; as such, and in line with previous practice, May will be the last month when only general directions are given here, with a chart of intermediate magnification being introduced next month, followed by a detailed finder chart in July and the following months.

On May 1st, Pluto is still a little reluctant to enter the evening stage, not rising until 9:52 pm, more than 4¼ hours after sunset, and nearly three hours after the cessation of evening twilight. Its disk spans 0.096" (far too small to be resolved through amateur instruments) and shines at magnitude 14.25; the tiny orb doesn’t transit until 5:04 am on the morning of the 2nd, just under ½ hour prior to the commencement of morning twilight at 5:33 am.

It is far more accommodating on the night of the 27th/28th, rising at 8:08 pm, short of 1½ hours after twilight fades, then transiting at 3:20 am; the span of the disk has increased ever so slightly, to 0.097", as has its brightness, to magnitude 14.21. Its location in our sky is plotted on the first chart in the notes on Saturn; here’s a better view:

The teapot constellation, with Kaus Borealis (Lambda Sagittarii, mag 2.8) labelled at the tip of the lid, is shown for reference, as is Saturn and the trio of naked eye stars Xi22] Sag, Omicron Sag and Albaldah (Pi [π] Sag), magnitudes 3.5, 3.75 and 2.9 respectively.

At the end of May, Pluto rises in the ESE at 7:52 pm, before transiting at 3:04 am; its disk spans .098" and magnitude is unchanged from our viewing night.

Pluto will remain within the constellation of Sagittarius until early January 2024, save for a little over two months spent in Capricornus in 2023.

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Our Monthly Feature

This month’s feature is a double act, using arguably one of the night sky’s most prominent and spectacular sights to locate a far larger – in an absolute sense – but fainter and much more distant object.

The ‘finder’ is Omega [ω] Centauri, aka NGC 5139, the granddaddy of all globular clusters, and the ‘findee’ Centaurus A, aka NGC 5128, sometimes characterised as a ‘train wreck’ galaxy. First the globular, Omega Centauri:

Omega Centauri is quite easily visible to the naked eye as a faint ‘star’; newcomers to astronomy are invariably flabbergasted when, after having this ‘star’ pointed out to them, they are then shown it through a telescope – visually exploding, as it does, into a huge ball of stars. Even intermediate views, through a finder ‘scope or binoculars are impressive, the naked eye point of light transforming into a prominent fuzzball.

The first mention of the globular is attributed to Ptolemy, who listed it as a star in his catalogue Almagest in the year 150 CE (CE denotes ‘Common Era’ and replaces AD; similarly BCE – Before Common Era is increasingly used in preference to BC). This ‘star’ received its designation of Omega Centauri by the German Johann Bayer in his Uranometria of 1603; Edmond Halley was the first, in 1677, to recognise its non-stellar nature and James Dunlop, a Scottish astronomer residing in Parramatta, Sydney, completed the journey of discovery in 1826, declaring it to be a globular cluster. There has been one further chapter in categorizing it – modern conjecture has it that it is the remnant core of a small galaxy cannibalised by the Milky Way long ago.

Omega Cen lies 15,800 light years away and contains perhaps ten million stars; its total mass is put at four million solar masses (s.m.) and it may have an intermediate mass black hole (larger than that resulting from the collapse of a single star but smaller than the super massive black holes residing at the core of most galaxies) of around 40,000 s.m.

The globular is very easy to find – refer to the below chart, configured for 9:00 pm on our viewing night, which shows our ultimate destination, Centaurus A, as well; the chart approximates a naked eye view under a dark sky, showing stars down to magnitude 5.5:

All star charts courtesy of StarryNight®ProTM Version 7.5.6.1208/Simulation Curriculum Corp.

Referring to the chart, and relating it to what you see in the sky, first locate the constellation Crux (the Southern Cross), and the Pointers to it, Alpha Cen, magnitude -0.3 and Hadar (Beta [β] Cen), mag 0.6 (it’s perhaps worth mentioning here that Alpha Cen is a double star with components shining at magnitude 1.34 and -0.04, together they shine at magnitude -0.3).

Construct, in your mind’s eye, a line from Alpha Cen to Hadar, then veer off to the left at an angle of about 45° and look for a fainter star, Epsilon [ε] Cen, mag 2.3, a bit over 1½ times as far from Hadar as Hadar is from Alpha Cen. Extend this line an intermediate distance between the two previous measures, and you come to a quite faint ‘star’, which is Omega Centauri (ignore the haze shown on the chart just short of the globular, it appears to be merely an artefact of the software, entreaties to remove same having thus far fallen on deaf ears). If you’re new to the pursuit of astronomy and haven’t seen this beauty before, train a telescope on it and prepare to have your socks knocked off! Here’s an image, courtesy of ESO (the European Southern Observatory), that closely matches the view through my twelve inch reflector:

Now for our final destination, Centaurus A. This galaxy is thought to be the result of a collision of an elliptical galaxy with a smaller spiral galaxy (hence the ‘train wreck’ descriptor); when viewed through the eyepiece, it presents as an oval haze bisected by a prominent dust lane (its appearance has also led to the nickname ‘Hamburger Galaxy’). The galaxy was discovered on 29/4/1826 by James Dunlop from his Parramatta home (the same James Dunlop referred to in relation to Omega Cen). Part of the Virgo supercluster, as is the Milky Way, it lies about 13 million light years away; it is thought to have a mass of about one trillion s.m. and to contain a central black hole of 55 million s.m. Two supernovae have been seen in the galaxy, in 1986 and 2016.

To locate Centaurus A, refer back to the chart and note that reaching it requires a further extension of the line that bought us to Omega Cen by about the distance between the Pointers, while veering slightly, about 15°, to the left. The globular and the galaxy are separated in our sky by 4½°, serendipitously the field of view of a 10x finder ‘scope, so if you place the globular at one extreme through your finder, the galaxy will sit, in the appropriate direction, on the other side. Centaurus A will appear smaller than Omega Cen and far fainter. I have seen, in print, a claim that the galaxy can be glimpsed with the naked eye under ideal conditions, but in my experience this is – excuse the pun – pie in the sky. Views of all except the brightest of objects suffer heavily when light pollution is present, and this is certainly the case with Centaurus A; I strongly advise viewing it under dark skies to avoid disappointment!

Having located the galaxy through your finder, switch to the main eyepiece and see how much magnification it will take before dimming detracts from the spectacle. The following view, again from ESO, gives an idea of the view, albeit enhanced considerably, afforded by a twelve inch reflector – you can see how the ‘Hamburger’ tag came about.

That wraps up The Australian Night Sky for the month of May;

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