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It’s summertime; spend balmy nights under the stars with:

The Australian Night Sky - December 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; December 2017:

 3rd Mercury stationary;
     Aldebaran (Alpha [α] Tauri) occulted by Moon (not from Australia).
 4th Full Moon (2:47 am, closest Full Moon this year, 357,983 km);
     Moon at perigee (7:42 pm, 357,492 km).
 9th Regulus (Alpha Leonis) occulted by Moon (not from Australia).
10th Last quarter Moon.
12th Mercury at perihelion (closest to Sun, 46.00 million km/0.3075 au).
13th Mercury in inferior conjunction (passes between Earth and Sun).
14/15th Geminids meteor shower peaks (view late evening of the 14th, early morning of the 15th).
15th Asteroid 4 Vesta occulted by Moon (not from Australia).
16th ASV CHRISTMAS STAR-B-Q.
18th New Moon.
19th Moon at apogee (farthest from Earth, 406,603 km).
22nd Summer solstice in the southern hemisphere;
     Saturn in conjunction with Sun.
23rd Mercury stationary.
25th MERRY CHRISTMAS.
26th First quarter Moon.
31st Aldebaran occulted by Moon.

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.

Of minor interest is the fact that the Moon reaches full phase on the same day as its closest approach to Earth, whereby keen eyed observers may notice that it appears a little larger than normal in our skies; don’t get too excited though, the difference is not stark. Of more import, our satellite occults Aldebaran (twice, 3rd & 31st), Regulus and the asteroid 4 Vesta during the course of December, although none of these events, unfortunately, can be witnessed from Australia.

On the 3rd, Aldebaran is never closer than a little over 1¼° south (above right) of the limb of the Full Moon, at 10:55 pm, 27° above the NNE horizon. Regulus, on the 9th, is even less accommodating; again the event is only a close conjunction from our viewpoint, and closest approach at 11:54 am – which sees the star just over ¾° SW (left) of the limb of the 65% waning gibbous Moon – occurs 2¾° below our WNW horizon, eleven minutes after the star has set. The event of the 15th involving Vesta is the closest of the local conjunctions discussed here, with the asteroid barely more than ¼° from the west (upper right) limb of the 11% waning crescent Moon at 4:26 am, just 4° above the eastern horizon. Aldebaran’s re-run of the 31st is the most removed from our southern experience – while the limb of the 93% waxing gibbous Moon comes to within a little over ¾° from the star at 11:30 am, it does so almost 70° below our southern horizon.

The earlier Aldebaran encounter can be witnessed as an occultation from NW North America, N Greenland and Central & N Asia, the later event from Europe, other than the south and west of Russia, Greenland and most of N America. Regulus is occulted from some northerly parts of Micronesia, N Asia, N Greenland and NE & Central Europe, while viewers in Central Polynesia and parts of Chile & Argentina are favoured re Vesta.

The yearbook of the ASV lists no fewer than seven meteor showers peaking in December, but only one will be canvassed in any depth in these notes – the Geminids, peaking on the 14th/15th. Of the others, the Phoenicids, Puppid Velids and Monocerotids, peaking on the 2nd, 7th (date varies) and 9th respectively are all heavily Moon affected in the early morning hours favoured for meteor observation. The Sigma [σ] Hydrids on the 12th, with a ZHR of only three, don’t escape the attention of the Moon either, as it rises at 2:29 am, while the Coma Berenicids of the 16th, which escape interference by the Moon (it rises in morning twilight), have the same unspectacular ZHR of three. The last of the also rans, the Ursids, peak on the 23rd with a ZHR of 10, but as the radiant is always more than 24° below our northern horizon, this shower is best left to viewers north of the equator.

The Geminids, predicted by the IMO (International Meteor Organization) to peak at 5:30 pm on the 14th, are a very reliable shower with a mammoth ZHR of 120; because the radiant is in the northern skies, however, hourly counts of 25-30 can be expected from Down Under. The Moon rises at 4:03 am on the morning of the 15th, six minutes after the commencement of morning twilight, and so is not a factor; additionally, this shower has a broad maximum spread over almost 24 hours, whereby the timing of the peak in late afternoon is of reduced consequence. The following chart is configured for midnight of the 14th/15th; the shower’s radiant rises some ¾ hour earlier, at 11:13 pm, and transits (reaches its maximum altitude in the north, a modest 19°) at 3:15 am.


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THE PLANETS

Mercury

The innermost planet spends the first couple of weeks of December in the final stages of an evening apparition and the rest of the month in our morning skies. On the first day of December, the planet sets, in twilight, at 10:02 pm, having been at an altitude of 17° WSW at sunset, 8:27 pm; its disk spans 8", is 39% illuminated and shines at magnitude 0.1.

Mercury is said to be stationary on the 3rd, when it ceases to move eastwards with respect to the stars, before accelerating back down towards the evening horizon. As it does so it passes by Saturn, which is descending at a lesser pace; the two are closest on the evening of the 7th, when they are separated by 1¼° (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°). Here’s a twilight image configured for 9:00 pm on the 7th, just under ½ hour after the 8:32 pm sunset, with Mercury and Saturn respectively 4⅓° and 4¼° clear of the horizon. At magnitude 1.7, you’ll need binoculars to see Mercury, but keen eyes may be able to pick up mag 0.5 Saturn unaided; Pluto is labelled for later reference.

In the days following its brush with Saturn, Mercury loses altitude rapidly and passes between the Sun and Earth, a configuration referred to as inferior conjunction, on the 13th. New Moon falls on Monday 18th this month, suggesting Saturday 16th as our preferred viewing night. Mercury is extremely poorly positioned for viewing on the following morning of the 17th; nevertheless, its circumstances follow. The planet rises at 5:31 am, just 22 minutes before the Sun; its disk is 6% lit, spans 10" and shines at magnitude 2.7. If you wish to catch a glimpse of it through binoculars or a telescope, look about 4° to the left of Venus as shown in the chart below, which is configured for 5:41 pm, just twelve minutes before sunrise, with Mercury and Venus (magnitude -3.9) at altitudes of 1½° and 1⅓° respectively. The 2% illuminated Moon is also in the vicinity as shown; at around 1½ days short of New Moon, binoculars will pick it up with careful examination. Ensure that these activities are conducted with the Sun below the horizon to prevent instant and irreparable eye damage should it enter the field of view.

By month’s end, Mercury has risen away from the morning horizon, but still emerges well after twilight commences, 4:35 am vs 4:06 am. Sitting 15° above the horizon by sunrise, 6:01 am, it spans 7", 59% lit, and shines at magnitude -0.2.

The Messenger of the Gods passes from Sagittarius into Ophiuchus on the 9th, remaining in that constellation for the remainder of the year.

Venus

Despite its brilliance, Venus is all but lost in the solar glare as it nears superior conjunction (rounding the far side of the Sun), but if you have a perfectly flat eastern horizon and an absence of cloud and murk near the horizon, you should still be able to pick it up all the way through to the end of the year without optical assistance, just above and to the left of where you expect the Sun to rise.

Other than the aforementioned exercises of following Venus down to the horizon and using it to find Mercury, it has practically run its race this time around, with even its brilliant glow (magnitude -3.9 throughout December) all but totally overwhelmed by twilight. For the record, its vital statistics for the 1st, 17th and 31st are respectively: rise 5:24 am (Sun 5:52 am, planet altitude 5°), Phase 99%, disk span 9.9"; 5:32 am, 5:53 am, 3°, 100%, 9.8"; 5:50 am, 6:01 am, 1½°, 100%, 9.8".

Venus races through the constellations this month; beginning in Libra, it crosses into Scorpius on the 4th, Ophiuchus on the 8th and Sagittarius, where it sees out the month, on the 22nd.

Earth

Our oft-neglected home base receives a plug this month, with summer solstice occurring on the 22nd (at 3:28 am), with the longest day and shortest night. Naturally, those north of the equator experience the winter solstice, with short days and long nights.

Mars

Having first risen before the commencement of morning twilight last month (on the 20th), Mars becomes more prominent with each passing day, with the best views since 2003 on offer in the middle of next year.

As the month of December begins, Mars is rising at 3:44 am, and has attained an altitude of 3° when morning twilight commences at 4:01 am; its disk spans 4.2", is 95% illuminated and shines at magnitude 1.7.

Come our viewing night (morning, in this case) of the 17th, rise time has come forward to 3:07 am, and altitude at the commencement of twilight (3:57 am) has improved to 9°; the Red Planet’s 4.5" disk, 94% lit, has brightened just a little, to magnitude 1.6. The following chart is configured for 3:52 am on the 17th, five minutes before the first vestiges of light begin to invade the eastern sky.

Mars is brighter than all stars shown apart from Spica (Alpha Virginis), which shines at magnitude 1.0; both star and planet should be readily identifiable by reference to Jupiter (which rises at 3:35 am), blazing fiercely at mag -1.7 and 10° (a fist at arm’s length) from its sibling.

As 2017 comes to a close, Mars rises at 2:37 am and is 17° clear of the horizon as the sky begins to brighten at 4:06 am; the disk, now 93% illuminated, continues to brighten slowly, reaching magnitude 1.5.

Mars transitions from Virgo into Libra on the 22nd.

Jupiter

Having been in conjunction with the Sun in late October, Jupiter is still struggling to pull away from the morning horizon. On December 1st the King of the Planets rises at 4:28 am, almost ½ hour after morning twilight commences (4:01 am), and is a mere 16° high at sunrise, 5:52 am; its disk spans 31" and punches through the twilight at magnitude -1.70.

While matters have improved somewhat by the morning of the 17th, they are still far from optimal. Rising at 3:35 am, it sits at a virtually unusable altitude of less than 4° when the sky starts to brighten at 3:57 am; spanning 32", the disk shines a touch brighter, at magnitude -1.74. See the notes on Mars for a visual guide to Jupiter’s place in the sky; the usual chart for, and discussion of, Jupiter’s Galilean moons will be omitted this month due to the low altitude.

Come the end of the month, the King is better placed again, rising at 2:47 am, but still fails to attain a decent altitude, just 15°, before the sky begins to brighten at 4:06 am. As Earth continues to catch up to Jupiter on our inner, faster orbit, the span of its disk has grown, to 33", and it shines a little brighter, magnitude -1.80.

Jupiter currently resides in the constellation of Libra, and will remain there until late next year.

Saturn

Saturn reaches conjunction with the Sun on the 22nd, just six days after our nominated viewing night, when it is obviously not a candidate for viewing. While you can still observe it low in the west early in the month, such views will be heavily compromised by atmospheric turbulence and murk. On the 1st, it is only 14° above the horizon at sunset, 8:27 pm, before itself setting at 9:47 pm; its disk spans 15.1" and shines at magnitude 0.49, while the rings span 34.3", inclined at 26.8°. On the 16th, these figures read 2½°, 8.39 pm, 8:56 pm, 15.1", mag 0.45, 34.2" and 26.7°. At month’s end, having transitioned to our morning skies, Saturn rises at 5:30 am and is 5° clear of the ESE horizon at sunrise, 6:01 am; visual magnitude is 0.47, the span of disk and rings is as for the 16th, and the inclination of the rings is 26.6°. Saturn’s visual magnitude has been given to two decimal points here to highlight a puzzling anomaly whereby it counter-intuitively becomes a little brighter at conjunction (reaching 0.44 on the 22nd); see the November 2016 edition of these viewing notes, archived on the ASV’s website at www.asv.org.au/night-sky for a discussion of possible causes.

Don’t spoil the memory of the magnificent apparition we have witnessed in the past few months; wait for this beauty to rise well away from the morning horizon before turning your ‘scope on it again. Saturn is in Sagittarius, where it will remain for the rest of this year, all of 2018 & 2019, and parts of 2020.

Uranus

Having been at opposition in the second half of October, Uranus is now well positioned for views in the early evening. As December gets underway, the planet transits at 10:12 pm, a mere six minutes before the sky fully darkens at 10:18 pm (when it’s at an altitude of 43°), then sets at 3:45 am; span and magnitude stand at 3.7" and 5.72.

On our viewing night, transit time has come forward to 9:11 pm; as twilight wraps up, the planet’s altitude has decreased, as compared to the first of the month, by only 4°, to 39°. The span of Uranus’ tiny disc is incrementally smaller, at 3.6", and its brightness is similarly affected, standing at magnitude 5.74; it sets at 2:45 am (on the morning of the 17th). Here’s the scene at 11:00 pm, with the planet at an altitude of 37° in the NW; this particular viewing time, 26 minutes after the western horizon darkens, has been chosen in order to reserve the earlier timeslot for viewing Neptune, which is further to the west and consequently lower in the sky. M45, the Pleaides, is labelled to aid in orientation.

Use the above chart to determine where to look for Uranus (if your sky is dark and your eyesight good, you may be able to spot it unaided) in the following (suggested) manner. First identify the Great Square of Pegasus as shown, with Algenib (Gamma [γ] Pegasi), magnitude 2.8, at its top right corner. From Algenib, look to the upper right (east) for the fainter Eta [η] Piscium, magnitude 3.6; extending a line from Markab (Alpha Peg, mag 2.5) to Algenib – these two being separated by 16½° – by 19°, while veering slightly to the right, will bring you to Eta Psc (remember that an open hand at arm’s length spans around 20°). Eta should be readily identifiable, as there are no other naked eye stars in its immediate vicinity; confirm that you have correctly identified it by noting that the two brighter stars to Eta’s lower right point toward it.

From Eta, look then for Omicron [ο] Psc, fainter again, but still comfortably naked eye at magnitude 4.25. Omicron is 7° from Eta and once again the only naked eye star in the target area. Identification of Omicron can be confirmed by noting that it lies directly on a line from Algenib to Kaffalijidhma (Gamma Ceti), mag 3.5, the unlabelled but circled star near Menkar (Alpha Ceti), which, at mag 2.5, outshines all stars around it for more than 20° in every direction. With Omicron in your sights, refer to the following enlargement to hone in on your planetary target.

This chart labels stars with their visual magnitudes and is delimited by Eta Psc (‘3.59’) and Omicron Psc (‘4.25’) at the bottom and top respectively; it plots only those stars brighter than magnitude 9.5, and so is best suited for use in conjunction with views through a finder ‘scope, which will struggle to show the dimmest stars plotted. Uranus is circled but unlabelled in order to show the white crosses marking its position at the start and end of the month, respectively right and left of its plotted position of the 16th. Note the planet’s sluggish progress relative to the stars, especially in the second half of the month – this is symptomatic of the fact that it is stationary on the 3rd of next month as it ceases retrograde motion and reverts to an easterly trek against the starry background. Note also that, at magnitude 5.7, Uranus is brighter than any star in its vicinity, but that each member of the line of 6th magnitude stars as labelled is almost its equal; identify Uranus by reference to these three stars, then cross reference by noting that the planet forms a near perfectly equilateral triangle with Omicron and Pi [π] Psc, mag 5.53. Uranus will tend to advertise its identity courtesy of a subtle blue-green hue and a lack of the twinkle often associated with the stars (which are point sources due to their extreme distance from us; planets span a measurable portion of sky and are thus less affected by our turbulent atmosphere).

Confirm capture by switching to the main eyepiece at a magnification of 150x or more (the higher the better, conditions and equipment permitting) to resolve the planet’s disk; the colouration will be much more noticeable, and – to my eye – aesthetically pleasing.

At the end of December, Uranus transits more than ½ hour before sunset (8:12 pm vs 8:45 pm), before setting at 1:46 am; span and brightness are 3.6" and mag 5.77; it is to be found drifting among the stars of Pisces until 2018/19.

Neptune

The outermost planet (apart from Pluto) is now well advanced in this apparition. Even at the start of the month, it transits almost an hour before sunset (7:32 pm vs 8:27 pm), and more than 2¾ hours before the end of evening twilight (10:18 pm), then setting at 1:58 am. Neptune’s disk spans 2.3" and shines at magnitude 7.88.

On the night of the 16th, the planet’s status vis-a-vis viewing has deteriorated further, as it transits (at 6:33 pm) in excess of four hours before twilight fades, 10:34 pm, at which time its altitude has decreased to 28°; span and brightness stand at 2.3" and mag 7.90.

As the wide field chart in the earlier notes on Uranus shows, Neptune is very close to the naked eye star Lambda [λ] Aquarii, magnitude 3.7. Additionally, having been stationary at the completion of retrograde motion last month, Neptune is now moving eastwards relative to the stars and so will stay close to Lambda all through December. The best way to identify Lambda is to begin with the Great Square of Pegasus; from there, look above and to the left for the Circlet in Pisces and the ‘Y’ of Aquarius as shown (in the earlier chart).

The ‘Y’ now becomes your reference point; as the chart – which approximates a naked eye view – shows, there is a paucity of stars visible without optical assistance between the asterism and Lambda, which is a little under 10° away. The one intervening star shown, Kappa Aqr (about halfway from the ‘Y’ to Lambda) shines gently at magnitude 5.0 and so will itself be very faint. Lambda should therefore be easy to find; having done so, refer to the following chart to capture Neptune. Stars are labelled with their visual magnitudes (Lambda is ‘3.71’), and the two white crosses show Neptune’s position on the 1st (near the left hand edge of the chart) and 31st of the month. It’s interesting to compare Neptune’s motion against the starry backdrop with that of Uranus – Neptune was only recently stationary and is picking up speed as evidenced by its greater progress in the second half of the month, whereas Uranus is slowing as it approaches stationary status.

The easiest way to find Neptune is probably to just centre Lambda in your finder ‘scope. As these instruments typically have a field of view of around 4°, and Neptune is only a touch over ½° from the star, the planet will be obvious as the ‘star’ next to Lambda. The dimmest stars your finder will display are around magnitude 9.5; as such, all stars in the area of concern bar the two of 9th magnitude will go unseen, and even these two will be extremely dim, if not invisible. Just be sure you are looking in the correct direction (remember your optics may invert the view) from Lambda to avoid confusing Neptune with the stars labelled at right on the chart. You should be able to detect the subtle blue-grey hue of the planet as compared to the whites, yellows and oranges of the stars, as well as the lesser tendency to twinkle. Confirm that you have located your quarry by switching to the main eyepiece at the highest practical magnification (I suggest 250x or more), to resolve, with careful examination, its miniscule disk.

On December 31st, Neptune is just 16° high as twilight wraps up at 10:40 pm; the span of its disk is nominally reduced to 2.2", while its brightness registers magnitude 7.92; the planet can be found within Aquarius until 2022/23.

Pluto

Pluto’s apparition is effectively done, with the planet reaching conjunction with the Sun on January 9th. The first chart in the Mercury notes shows where it sits in relation to the three stars near the handle of the Teapot asterism of Sagittarius; here’s a table outlining its circumstances for the month of December, followed by a magnified view (intended to give just an idea of where it sits) configured for 9:39 pm on the 16th, one hour after sunset.


Pluto will continue to plough slowly through the dense star fields of Sagittarius until 2023/24.

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Our Monthly Feature

This month’s feature is the Seyfert galaxy, Messier 77, aka NGC 1068, the bright nucleus of which is designated Cetus A or 3C71; it is receding from us at a speed of 1137 km/sec. The galaxy was discovered by the French astronomer Pierre Méchain on 29/10/1780; Charles Messier added it to his catalogue of ‘non-comets’ on December 17th of the same year. Méchain described it as a nebula, Messier as a star cluster with nebulosity. M77 is the dominant member of a group of at least seven other galaxies, loosely called the M77 group. Located 47 million light years away, M77 contains a central supermassive black hole of some 15 million solar masses (s.m., the mass of our Sun); the total mass of the galaxy is estimated at one billion s.m.

My personal experience of viewing M77 has yielded less substantial views than that indicated in reviews I have read – I find it just barely visible as a very faint ‘star’ through my telescope’s finder, which jells well with its stated magnitude of 9.6, and can discern nothing of the galaxy’s structure through the eyepiece, just a bright core surrounded by a small glowing haze. I will have to make a point of viewing this object from a location with extremely dark skies to better appreciate it and look for structure.

Locating M77 is, as with most objects in the sky, fairly straight forward if approached in the correct manner; if you have read the earlier notes on the planet Uranus, you’re halfway there already. Referring back to those notes, in conjunction with the wide field chart accompanying them, first identify Kaffalijidhma, the circled but unlabelled star on that chart. Look three degree (typically a touch over the width of two fingers held at arm’s length) above it for the fainter Delta [δ] Ceti, magnitude 4.1, still easily seen with the naked eye and on the afore-mentioned chart. Centre Delta in your finder ‘scope, then refer to the following magnification, configured for around 10:30 pm (on the 16th), and labelling stars with their visual magnitudes:

In relating this chart to what you see through your finder ‘scope, remember that the optics will probably invert the view – left becomes right and up becomes down. Look at the chart and imagine pinning it down at the centre then rotating it 180°, that’s the view you can expect to see. Centre Delta Ceti (‘4.06’) and look for the two labelled 9th magnitude stars 1° away. These stars will be very faint in your finder, but the angle at which they sit in relation to one another and the fact that there will be no stars visible between them and Delta should make them stand out. As the chart shows, the location of M77 is easily determined by reference to the two; I can actually make out M77 itself, as mentioned earlier, as an extremely faint ‘star’. Centre M77 in your finder, or centre the appropriate spot if you can’t see the galaxy, then switch to the main eyepiece at medium power (100x is a good starting magnification) and you’ll see the small but bright core surrounded by a (still quite small) glow. Try pumping up the magnification to see which affords the best view – around 150x seems most satisfactory through my instrument.

While observing in preparation for this article, I came across two larger but fainter galaxies past M77; it’s probable that they are UGC 2245 and UGC 2247 (UGC< >Uppsala General Catalogue), but Starry Night plots these farther from M77 than I recall seeing them.

Dropping the magnification back to around 100x will allow M77 and its two 9th magnitude companions to share the same field of view as the stars of magnitudes 6.68 & 7.56 and the edge on spiral galaxy NGC 1005; look carefully to spot this galaxy, it’s quite faint.

That’s all we have for this month folks, ANS will return in 2018 for another year of exploration 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