[ANS 2016] - ANS 2017 [Jan][Feb][Mar]

 

Explore the final month of Summer skies with:

The Australian Night Sky - February 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: Feb 2017

 4th First Quarter Moon.
 6th Aldebaran (Alpha [α] Tauri, magnitude 0.8), occulted by Moon (not from Australia).
 7th Moon at perigee (closest to Earth, 368,816 km);
     Jupiter stationary, begins retrograde motion.
 8th Mercury at aphelion (farthest from Sun, 0.4667 au / 69.82 million km);
     Alpha Centaurid meteor shower peaks.
11th Full Moon;
     Penumbral eclipse of Moon (not from Australia).
12th Regulus (Alpha Leonis, magnitude 1.4) occulted by Moon (see below).
17th Jupiter at aphelion, 5.4565 au / 816.3 million km;
     Venus greatest brilliancy (aka greatest illuminated extent), magnitude -4.63.
19th Last quarter Moon;
     Moon at apogee (farthest from Earth, 404,376 km).
21st Venus at perihelion (closest to Sun, 0.7185 au / 107.5 million km).
24th Delta [δ] Leonid meteor shower peaks.
27th New Moon;
     Annular eclipse of the Sun (not from Australia).

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 the occurrences of aphelion and perihelion shown above relate to the physical separation of the Sun and the relevant planet, and are unrelated to their angular separation in our sky.

The occultation of Aldebaran by the Moon on the 6th is only visible from Central America, the Caribbean and Southern Europe. For us, it’s a non-event as only a close conjunction results, with the star just over ¼° from the Moon’s limb, and closest approach, at 8:37 am, taking place almost 70° below our northern horizon. The circumstances of the Regulus occultation on the 12th are much kinder – the star is occulted, high in our northern sky, from 12:28 am until 1:50 am.

Neither of the eclipses mentioned above – a penumbral eclipse of the Moon on the 11th and an annular eclipse of the Sun on the 27th – are visible in Australian skies. The lunar eclipse can be seen from locations in South America, Europe, Africa and western Asia; from our location, having set at 6:38 am, the Moon is below the horizon by 36° when the eclipse begins and 65° when it ends. The solar eclipse of the 27th can be viewed from Chile, Argentina, a path across the Atlantic Ocean and parts of Africa. The Moon’s shadow doesn’t touch Australia; maximum eclipse occurs over the Atlantic Ocean at 1:58 am our time.

The Alpha Centaurid meteor shower peaks on the 8th, at the unfortunate time of 11:30 am. This shower, which under ideal conditions usually results in around six meteors per hour streaking across our skies, is very heavily impacted by the 85% illuminated waxing gibbous Moon, which doesn’t set until 3:30 am, leaving just a 1½ hour dark sky window before morning twilight commences at 5:02 am. As the radiant is circumpolar, never setting, and will be at an altitude of 57° at the beginning of the window of opportunity, improving to 66° at the end, some worthwhile activity may be forthcoming during this brief period. Accordingly, the following chart shows the location of the radiant as the Moon sets; also marked is the radiant of the almost incidental Delta Leonids, which peak on the 24th, with an unspectacular ZHR of 1.

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THE PLANETS

Mercury

As February begins, Mercury is in the declining half of a morning apparition in which it just barely manages to break free of twilight, between January 20th and February 5th (my apologies for indicating last month that it wouldn’t quite do so, it actually beats twilight to the punch by a maximum of 6½ minutes on Jan 27th). It rises (on Feb 1st) at 4:47 am, and is at an altitude of 19° when the Sun breaches the eastern horizon at 6:34 am. The innermost planet’s disk spans 5.6" [" denotes arc-second = 1/60th of an arc-minute (symbol ') or 1/3600th of a degree (symbol °)] and is 81% illuminated; at magnitude -0.2, the planet will be clearly visible to the naked eye before twilight washes it out. Because it will be an unviable target later in the month, the following chart shows where it sits, 8° clear of the horizon, at 5:30 am on the 1st; the constellation figures of Scorpius and Sagittarius are plotted, as are Saturn and Pluto for later reference.

As the month progresses, Mercury descends towards the eastern horizon at an ever increasing pace. By our viewing night, which we’ll nominate as Saturday 25th (New Moon falling on the following Monday), it’s effectively run its race, rising less than ¾ hour before the Sun (6:16 am vs 7:00 am) and attaining an altitude of just 8° come sunrise. While its disk, 97% lit and spanning 4.9" as it increases its distance from Earth, shines more brightly at magnitude -0.9, it will be swamped by twilight (note that Mercury, alone amongst all the planets, is brightest when far from Earth and dimmest when near us, due to…what? Feel free to contact me via the address at the end of these viewing notes if you’re unable to resolve this).

Come the end of the month, the data of the 25th read: a little over ½ hour, 6:31 am, 7:03 am, 6°, 98%, 4.9" and mag -1.1.

Beginning February in the constellation of Sagittarius, fleet footed Mercury moves into Capricornus on the 8th and Aquarius on the 25th.

Venus

February is an exciting time for those of us who view Venus through a telescope; while it loses a little altitude nightly, its disk grows in size and shrinks in phase quite dramatically as it blazes fiercely in the western sky. It begins the month 21° clear of the horizon at sunset, 8:33 pm, and doesn’t itself set until 10:23 pm; its disk, spanning an already impressive 31", presents as a pleasing 39% lit crescent, shining as only it can (none bar the Sun and Moon, and occasional Iridium flash, are brighter; it would easily outshine the Moon were it of comparable size in our sky) at a punishing magnitude -4.56. Venus reaches its greatest brilliancy – known as greatest illuminated extent – at a staggering magnitude -4.63 on the 17th. Come our viewing night of the 25th, it is setting considerably earlier, at 9:05 pm, having been at an altitude of 11° when the Sun set just under one hour earlier, at 8:06 pm. The planet’s earlier setting time is, however, more than compensated for by the evolving circumstances of its disk – it then spans a whopping 44", as a beautiful 20% illuminated crescent, still shining brilliantly at magnitude -4.61. By month’s end, the above read 8:52 pm, 9°, 8:01 pm, 46" and mag -4.59. Be sure to follow the scintillating orb throughout February and the first week or so of March as its span increases to in excess of 50" and the crescent phase becomes ever slimmer – it really is a fantastic sight, just be sure to wait until the Sun is below the horizon to protect your eyesight. Venus remains within Pisces throughout February.

Mars

While the Red Planet will adorn our western skies for some months to come, not reaching conjunction with the Sun until the last week of July, its days of doing so in a fully dark sky are effectively over even at the start of February. No other planet lingers in the west so tenaciously however; Mars does so for two reasons: it is our relatively very close outer neighbour, whereby we, on our inner and faster orbital track, pull away from it only slowly, and also because, for this apparition, Mars’ retreat towards the western horizon coincides with the time of the year when the Sun is setting earlier each night, causing the sky to progressively darken earlier while Mars is still some way above the horizon.

On Feb 1st, Mars is only 25° above the WNW horizon at sunset (8:33 pm), less than 5° when the sky fully darkens at 10:14 pm, and sets at 10:45 pm; its 92% illuminated disk spans just 5.1" (too small to see detail, and probably also to detect the less than full phase) and shines at magnitude 1.11. Wait until the sky darkens sufficiently to see Venus, Mars and the 21% illuminated waxing crescent Moon forming a straight line spanning just 9°.

On the 25th, our designated viewing night, the situation has deteriorated somewhat – the planet sets at 9:53 pm after having been at an altitude of 20° when the Sun set at 8:06 pm, and less than 2½° when twilight faded at 9:38 pm; its 94% illuminated disk is both smaller, 4.7", and fainter, magnitude 1.27. The following chart, configured for 8:51 pm, ¾ hour after sunset and well within twilight, gives an indication of just how low Mars sits before the sky darkens sufficiently to invite viewing. Note also the position of Uranus, the closeness of which to Mars will be further discussed in the notes on the gas giant.

On the 28th, Mars sets at 9:46 pm, less than ¼ hour after the cessation of twilight (9:33 pm). Still at a phase of 94%, its 4.6" disk has further dimmed to magnitude 1.29.

Beginning the month swimming with the fishes in Pisces, Mars just barely cuts through a corner of Cetus (the whale or sea monster) on the 7th, before resuming its accompaniment of the small fry for the rest of the month.

Jupiter

The King of the Planets is now rising before midnight, and so becoming a fixture in the evening, as well as the morning, sky. At the start of the month it rises less than ½ hour before midnight, 11:33 pm, and transits at 5:59 am, a little over ½ hour before sunrise (on the 2nd), its disk spanning 39.0" and dominating the sky at magnitude -2.15. Jupiter is stationary on the 7th, as it begins four months of retrograde motion, moving east to west relative to the stars. Rise and transit times have improved markedly by the 25th, to 9:58 pm and 4:23 am (on the 26th, almost an hour before twilight begins at 5:28 am) respectively; its disk is also bigger and brighter at 41.8" and mag -2.30.

Due to Jupiter’s brilliance, no help is needed to pinpoint it – just look for the brightest ‘star’ in the sky (it rises in the east, don’t mistake it for brilliant Venus in the west). The following magnified view is configured for 12:30 am on the morning of the 26th, with Jupiter 29° high in the ENE. The time has been chosen to catch Europa and Callisto just 24" apart, as the former reaches its maximum eastern elongation from Jupiter, thereafter moving west from our perspective, back towards Jupiter, while Callisto, with a wider orbit, continues to move away from its parent. If you plan to view late into the morning, you’ll see the gap between the two moons widen noticeably. Ganymede and Io are to Jupiter’s west, with the former moving away from its parent while Io is about to begin moving back to the east from our perspective. Even the brightest of the stars shown shines faintly at magnitude 13.7 (probably remaining unseen through the eyepiece in other than large amateur ‘scopes) and so none can be mistaken for the 5th and 6th magnitude moons.

As February concludes, Jupiter’s rising time stands at 9:46 pm, its disk spanning 42.1" and shining at magnitude -2.32; the giant planet will glide among the stars of Virgo until November.

Saturn

Saturn and its marvellous ring system have now become a fixture in our morning skies, affording fascinating views to those who view through into the wee hours.

Before delving into its circumstances this month, I’ll briefly address the problem raised last month in relation to Saturn increasing in brightness in the lead up to conjunction and decreasing afterwards, contrary to the minimum at conjunction that conventional wisdom would dictate.

Determined to come to grips with this counter-intuitive behaviour, I examined the data relating to all the outer planets, Mars through Pluto, for the period 2014-2017, and found what I can only describe as a dog’s breakfast (with apologies to man’s best friend); in a nutshell, here are my findings:
MARS – dims up until conjunction, levels out around conjunction, with a propensity to brighten marginally (by a factor of magnitude 0.01) for a few days, then resumes dimming for 2-3 months afterwards.
JUPITER – at its dimmest anywhere between a week before conjunction and a couple of weeks after.
SATURN – brightens for 50-60 days in lead up to conjunction, then dims for a similar period after conjunction.
URANUS and NEPTUNE – dimmest at conjunction.
PLUTO – brightens for 35-55 days in lead up to conjunction, dims for similar period after conjunction.

As the above shows, the planets exhibit a range of behaviours in the period before and after conjunction – I remain unable to reconcile this, other than to say that Saturn & Pluto appear to behave in a like manner, as do Uranus & Neptune, while both Mars and Jupiter follow their own unique regimes. All I can do at this stage is flag the particular trends as each plays out, and commit to reporting back in future editions of these viewing notes if clarification ensues. If any readers can help to resolve this, please do so.

At the beginning of February, Saturn is rising at 2:56 am, and is 21° (vertically) clear of the horizon when morning twilight commences at 4:53 am. Its disk spans 15.6", the rings 35.3" at an angle of 26.7°; disk and rings together shine at magnitude 0.54.

When viewing on the 25th, you’ll find the planet rising in the east at 1:29 am and attaining a much more pleasing altitude of 45° at the commencement of twilight (5:27 am). The span of the disk and rings will have improved to 16.1" and 36.5" respectively, with the latter inclined at 26.6°; at the end of the month, the above figures stand at 1:18 am, 48°, 16.1", 36.6" and 26.5°.

Refer to the meteor and/or Mercury charts to determine where Saturn sits in the sky. While those charts are configured for the 8th and 1st respectively, Saturn will be at very nearly the same position relative to the stars on our viewing night. At 4:00 am on the 25th it will actually be within about 1° of the altitude depicted in the Mercury map, and this is the time – just under ½ hour before twilight starts to paint the sky – for which the following magnified view of Saturn and six of its seven brightest satellites is configured. Saturn is at an altitude of 28°, very near the 30° this column recommends in order to totally avoid the murk found near the horizon:

While Saturn is currently traversing the rich star fields of Sagittarius, the brightest stars in the field of view afforded by the chart are 15th magnitude, far dimmer than, and not able to be mistaken for, the moons, and have consequently been omitted for the sake of clarity.

The magnitude rating of the brightness of the moons is as follows: Titan 8.8, Enceladus 12.2, Dione 10.9, Mimas 13.4, Tethys 10.7 and Rhea 10.2. Titan will be visible through any telescope, even finder ‘scopes and most binoculars, although faint; Rhea, Tethys and Dione (as well as Iapetus, see below) should show up in a four to six incher, while Enceladus may call for an eight to ten incher and tiny Mimas a twelve incher.

The following chart, configured for the same time, includes Iapetus, which has a much wider orbit than the others:

Enigmatic Iapetus has one hemisphere much brighter than the other, whereby its brightness in our sky is considerably more affected by its 79 day orbit around Saturn – being brightest when to the west of the planet and dimmest when to the east – than the Earth-Saturn separation, which cycles over roughly 12 months. Accordingly, whereas the brightness of the other moons as given above is taken from Starry Night software, which unfortunately does not account for the moon’s orbital position, that for Iapetus, mag 10.5, is my own (confident) estimate.

This latter chart retains the star field as an aid in identifying the outlying moon. The brightest stars between Saturn and Iapetus are 14th magnitude, and the closest stars to the moon 15th-17th magnitude, as are stars beyond Iapetus for well past the extremes of the chart, the one exception being the unlabelled relatively bright star at top left (which, even so, is only magnitude 14.4, far dimmer than the moon and unseen through all but large ‘scopes). Identification should therefore not be too difficult – just look in the direction indicated, at the appropriate distance from Saturn, for the lone relatively bright ‘star’. Iapetus is only 7' – just a little over 0.1° – from Saturn and thus both will be in the same field of view even at high magnification. Note that whereas the software normally does a good job of depicting brighter objects as larger, it fails in this case – all the moons except Mimas are plotted at the same size, even though there is considerable variation in brightness. Mimas itself is too small as compared to the others, and the stars, being as faint as they are, are shown as being too big in relation to the moons.

Saturn will transition from Ophiuchus to Sagittarius on the 24th; retrograde motion will actually return it to Ophiuchus in the second half of May, before it again crosses the border on November 19th after prograde motion (west to east relative to the stars) resumes.

Uranus

Uranus will be in conjunction with the Sun in mid-April, and as such it is even now becoming too low in the west for decent viewing once darkness has descended. Its brightness declines ever so slightly from 5.85 to 5.88 over the course of February, the span of the disk from 3.5" to 3.4".

It is still accessible on the 1st, setting almost three hours after the Sun (11:30 pm vs 8:33 pm); however it only sits at an altitude of 14° once twilight fully fades at 10:14 pm (and with a 21% waxing crescent Moon lingering). By the 25th, it’s becoming quite marginal – setting time is 9:58 pm as compared to 8:06 pm for Sol, and the planet sits just 3½° clear of the horizon at 9:38 pm, the cessation of twilight.

The following chart is configured for the 25th at 9:06 pm, one hour after sundown, with Uranus and Mars at an altitude of 9½° and 8½° respectively, some 1¼° (around the width of a finger at arm’s length) separating them. As indicated earlier, Uranus has a close conjunction with Mars this month, closest approach of a little over ½° occurring on the night of the 27th; green crosses show the position of the two on that night. The brightest stars in the vicinity are labelled with their magnitudes to emphasise the fact that magnitude 5.88 Uranus is easily identified through a finder ‘scope by reference to Mars, barring too much atmospheric interference at the low altitude. Once you’ve spotted it through your finder, switch to the main eyepiece at a magnification of 150x or more to resolve its wee disk.

At the end of February, Uranus sets at 9:46 pm, being just 2° high when twilight wraps up at 9:33 pm; the gas giant is in Pisces until 2018/19.

Neptune

Neptune is even further advanced in its current apparition than its inner brethren, viewing being affected, or even essentially disqualified, by twilight all month; the planet reaches conjunction on the 2nd of next month.

Even as February gets underway it sets within twilight, a little over 1¼ hours after the Sun sets (9:52 pm/8:33 pm), when it’s at an altitude of just 15°. The situation is truly hopeless on our viewing night of the 25th, with the planet setting at 8:20 pm, less than ¼ hour after the 8:06 pm sunset, while at month’s end, the above data read 8:08 pm, 7 minutes and 8:01 pm.

Neptune’s brightness declines from magnitude 7.95 to 7.96 throughout February, its disk spanning 2.2" all month. Views of the outermost planet proper (although Pluto is held in planetary status by this column) are best held over until it reappears in the morning sky and gains a little altitude.

Neptune, which chugs along at a gentle pace amongst the stars due to its wide and slow orbital track, will remain within the constellation of Aquarius until 2022/23.

Pluto

Pluto is a little better situated than Neptune, albeit on the other side of conjunction, having reached that point early last month. The Mercury chart provides an overview of where Pluto sits at 5:30 am on the morning of the 1st, and at the resolution provided by the chart it won’t move noticeably throughout February thanks to its sluggish movement relative to the stars, slower even than Neptune.

On the first of the month, it rises at 4:39 am, almost two hours before the Sun breaches the eastern horizon at 6:36 am, but less than ¼ hour before twilight commences (4:53 am). By the 25th it has gained altitude, rising at 3:08 am (sunrise 7:00 am), but is still just 26° high as the Sun’s light begins to filter across the sky at 5:27 am; three days later at the end of the month, these figures read 2:57 am, 7:03 am, 29° and 5:31 am.

The span of this tiny frozen orb increases incrementally over the course of the month from 0.093" to 0.094" (far too small to be resolved by amateur instruments), while its visual magnitude actually drops (as related in the earlier discourse on counter intuitive planetary brightness around conjunction) from magnitude 14.29 to 14.30.

As Pluto is difficult to locate at the best of times, these viewing notes only give detailed directions for doing so in the months around and after opposition, when it can be found high in the sky at a convenient hour. For now, here’s a magnified view showing its approximate position at 5:00 am on the 25th, near the trio of naked eye stars – Xi2 [ξ2] Sagittarii, mag 3.5, Omicron [ο] Sag, mag 3.75, and Albaldah (Pi [π] Sag), mag 2.9 – below the handle of the teapot asterism:

Pluto will remain in Sagittarius until 2023/24 (the first three days only of the latter year).

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Our Monthly Feature

Asteroid 4 Vesta, the ‘4’ denoting that it was the 4th asteroid to be found, after Ceres, Pallas and Juno – the first of which has since been reclassified as a dwarf planet – was discovered on 29/3/1807 by the German astronomer Heinrich Olbers; in doing so he became the first person to discover two asteroids, having previously bagged Pallas (or 2 Pallas).

Named after the Roman goddess of home and hearth, Vesta is both the second largest and second heaviest (after Ceres, which is considerably more massive, Vesta only having 28% its mass) body circulating in the asteroid belt between Mars and Jupiter, and accounts for 9% of the total mass of bodies in the belt.

It is, on average, the brightest of all asteroids – brighter even than Ceres, thanks in part to its slightly smaller orbit and to its greater albedo. Albedo is the term used to describe the percentage of light reflected by a body, 0 for a body which reflects no light (and therefore appears totally black) and 1 for a body which reflects all light incident upon it. Vesta’s albedo is 0.43; by comparison the albedos of a few other bodies are: Ceres 0.1, Earth 0.31, our Moon 0.11, Mars 0.25 and brilliant Venus, which is covered with a thick layer of highly reflective white cloud, 0.77. Vesta is, at its brightest, dimly visible to the naked eye, and always visible in binoculars or a finder ‘scope, its brightness varying from magnitude 5.1 to 8.5. At the time of compilation of these viewing notes (mid-January) it shines at mag 6.1; come our viewing night of Feb 25th, this will have reduced slightly to 6.3, reflecting (excuse the pun) the fact that it reaches opposition, and therefore peak brightness, on Jan 18th. This places it out of reach of the naked eye for all but those with very keen eyesight under the darkest of skies, but it’s a prominent object viewed at any degree of magnification.

Vesta orbits at a distance from the Sun which varies from 2.15 au - 2.57 au (322 million km - 384 million km) and can approach to within 1.14 au (171 million km) of Earth. It has an orbital period of 3.63 of our years, orbiting at an average velocity of 19.3 km/sec (as a comparison, Earth orbits at 29.8 km/sec) but a very short ‘day’, rotating once on its axis in 5.34 hours. Lacking an atmosphere to retain heat, Vesta’s surface temperature varies from -3°C to -188°C.

Vesta is not quite massive enough for its gravity to have pulled it into a spherical shape; it measures 569 km x 555 km x 453 km – it is thought to have a nickel-iron core 214-226 km in diameter. With a mass of 2.59*1017 tonne, objects dropped near its surface accelerate downwards under the force of gravity at a leisurely 0.25 m/sec2 (as compared to 9.8m/sec2 on Earth and 1.6 m/sec2 on the Moon).

NASA’s Dawn spacecraft orbited Vesta from 16-7-2011 until departing for Ceres on 5-9-2012 (it was originally intended that Dawn would depart from Ceres on July 1st last year en-route to another asteroid, 145 Adeona, but the decision was taken to leave it orbiting Ceres, where it remains today).

Vesta has two comparatively huge impact craters dominating its southern hemisphere. The largest, dubbed Rheasilvia, is around 500 km wide, almost as wide as the asteroid itself, 19 km deep at its lowest point, and contains a central peak 23 km high; the rim of the crater formed by the impact reaches up to 31 km above the floor. There is another crater not far behind in terms of size, Veneneia, some 400 km across; Rheasilvia is younger and overlays its smaller brother. Many relatively small bodies, which along with Vesta are collectively known as the Vestian asteroids, were probably blasted off the surface by the impact which created Rheasilvia, as were the HED (howardite-eucrite-diogenite) meteorites, which make up around 5% of all meteorites striking Earth. Here’s an image of Vesta taken by Dawn:

Vesta’s circumstances for the month of February, for the 1st, 25th and 28th respectively in each case, are:
Rise 7:41pm, 5:56 pm, 5:43 pm;
Transit 12:20 am (on the 2nd), 10:29 pm, 10:17 pm; transit altitude 28°, 26°, 26°;
Twilight ends 10:14 pm, 9:38 pm, 9:33 pm; at altitude 21°, 25°, 25°;

Disk span and visual magnitude 0.42" & 6.13, 0.38" & 6.33, 0.38" & 6.36. Note that Vesta’s span is too small to allow it to be resolved by amateur instruments.

The wide field chart below shows where Vesta sits in the sky on the 25th at time of transit (10:29 pm), followed by a magnified view, in which all stars brighter than magnitude 8.0 are labelled with their magnitudes.

All star charts courtesy of StarryNight®ProTM Version 7.5.5.1109/Simulation Curriculum Corp.

Relate the wide field chart to the magnification using the naked eye stars Iota [ι] Gem, mag 3.75 & Upsilon [υ] Gem, mag 4.03, which are comfortably the brightest stars in their immediate locale – they are clearly visible on the wide field chart just beneath the marker for Vesta, to the upper left of Pollux (Beta Geminorum, mag 1.15).

Having identified Iota & Upsilon Gem, train your finder ‘scope on the area above them, as indicated by Vesta’s position on the chart. As the asteroid is the next brightest ‘star’ on the chart (barring the stars of mag 5.0 and 5.75 near the bottom, with ‘6.34’ being essentially equal in brightness), identification should not be difficult – the star of magnitude 7.31 (HIP36262), ½° to Vesta’s lower left and the brightest star close to the asteroid, may help to confirm its identity. If you’re still not sure, sketch or image the brightest stars in the field and return the next night to see which one has moved.

That’s all we have for this month folks, tune in next month when The Australian Night Sky returns with another tour of the heavens.

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