It’s mid-spring, the chill is off and the nights are still long; look up with the aid of:
[Click here to show or hide the explanatory notes]
1st Daylight saving commences, 2:00 am becomes 3:00 am.
3rd Neptune occulted by Moon (in Australia, visible only from Tasmania and S/E Victoria);
Venus at perihelion (closest to Sun, 107.5 million km / 0.7184 au).
6th Full Moon;
Close conjunction of Mars and Venus (see planetary notes below).
8th Mars at aphelion (farthest from Sun, 249.2 million km / 1.666 au).
9th Mercury in superior conjunction (passes behind the Sun from our perspective);
Moon at perigee (closest to Earth, 366,855 km);
Draconids meteor shower peaks.
10th Aldebaran occulted by Moon (not from Australia);
Taurids South meteor shower peaks.
12th Last quarter Moon.
15th Regulus occulted by Moon (not from Australia).
20th Uranus at opposition;
21st Orionids meteor shower peaks.
24th Leo Minorids meteor shower peaks.
25th Moon at apogee (farthest from Earth, 405,154 km).
27th Jupiter in conjunction with Sun.
28th First quarter Moon;
29th Mercury at aphelion (farthest from Sun, 69.82 million km / 0.4667 au).
31st Neptune occulted by Moon (for the second time this month; not visible 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 that daylight saving commences on the morning of October 1st, when 2:00 am instantly becomes 3:00 am. As there are no references to times between midnight and 3:00 am of the 1st in these viewing notes, all times given are AEDT, Australian eastern daylight time.
The Moon occults Neptune twice this month, on the 3rd and 31st. The earlier event, while not quite an occultation from our perspective (occultation occurs south of a line from the Victorian-NSW coastal border to Cape Otway), almost goes one better – Neptune very nearly grazes the lower limb of the Moon, with Starry Night software indicating the planet is a mere 8" [" denotes arc second = 1/60th of an arc minute (symbol ') or 1/3600th of a degree (symbol °)] from the lunar disk at 11:28 pm. The planet is fully occulted from the far SE of the mainland (as detailed above), Tasmania, New Zealand and SW Polynesia.
On the 31st, Neptune is occulted from our location, but well below the SSW horizon – the Moon slips in front of the planet between 8:06 am and 8:43 am, at altitudes of -40° and -42° respectively. Viewers at the southern tip of Africa and most of Antarctica will be able to view the occultation.
Aldebaran and Regulus are also covered by the Moon, on the 10th and 15th respectively, although not from our location in either case. Aldebaran is never closer than a little less than 1½° above the Moon; this occurs at 5:03 am, with the star at an altitude of 35° in the north. Closest approach to Regulus is a little under ⅔° at 10:22 pm, 63° below the NNW horizon. The event involving Aldebaran is accessible to those in central and NE Asia, Alaska and NW Canada, while Cape Verde, W. Africa, most of the Caribbean and North America south of Canada are favoured re Regulus.
Four meteor showers peak this month – the Draconids, Taurids South, Orionids and Leo Minorids, climaxing on the 9th, 10th, 21st and 24th respectively. The Draconids have a variable ZHR (the peak hourly rate under ideal conditions, with the radiant – the point from which the meteors appear to emanate – at the zenith and a clear dark sky), with no significant activity predicted for this year. The radiant never crests the northern horizon, approaching within about 2° at best; as it does so mid-late afternoon, and is some 40° to 70° below the horizon during the post-midnight hours – favoured for meteor observing – it only receives a mention here courtesy of being included in the year book of the ASV, for which these notes are primarily compiled. The situation is further compromised by the 90% illuminated waning gibbous Moon, which rises at 10:14 pm on the 8th and floods the sky with light for the remainder of the night.
On the night of the 9th/10th, the spectacle of the Taurids South, which result from debris shed by Comet Encke, is also adversely impacted by the 82% lit Moon rising at 11:23 pm, a little over an hour after the radiant, 10:15 pm, and thus positioned to interfere with post-midnight viewing. While the shower’s ZHR is a modest 5, some can be expected to be bright, even attaining fireball status (loosely, as bright or brighter than Venus), and so visible despite the Moon’s presence in the sky. The chart below shows the scene at 1:00 am on the 9th; the horizon has been rendered translucent in order to depict the radiant of the Draconids, deep in the north.
The Orionids, peaking on the 21st, are the pick of this month’s crop; not only are they free from interference by the Moon, peaking as they do one day after New Moon, they also boast a ZHR of 15-20. Additionally, the peak is broad, and substantial activity can be expected for a few days either side of the peak. The radiant rises at 12:33 am, and is at an altitude of 35°, only 50 minutes short of transiting, when morning twilight first starts to filter across the sky at 4:53 am. The following chart is configured for 4:00 am, with the radiant at an altitude of 31° NNE. The radiant of the minor Leo Minorids shower is also depicted (at its peak on the 24th, it will be 2° higher than the -24° shown here); while this shower also benefits from a dark sky, a miserly ZHR of 2 is somewhat unappealing.
As October begins, Mercury is in the final throes of what was, even at best, a somewhat lamentable morning apparition, the worst of the year. It rises at 6:49 am, just seven minutes before sunrise; despite shining strongly, at magnitude -1.35, its 5.0" disk, 97% illuminated, is far too close to the Sun to even consider trying to view it.
After superior conjunction on the 9th, swinging around the far side of the Sun from our perspective, the innermost planet re-emerges in the evening sky to begin a reasonable, but by no means outstanding, evening apparition. As New Moon falls on Friday 20th this month, Saturday 21st will serve as our preferred viewing night; on this date the planet is still heavily ensconced in twilight, being only 7° clear of the western horizon at sunset, 7:43 pm, and setting itself less than ¾ hour later, 8:24 pm. The span of Mercury’s disk is illuminated to the same extent, 97%, as at the start of the month, but of course it’s now the eastern, rather than the western, edge of the disk which escapes the Sun’s attention; the disk’s span is also similar, 4.7", but it shines less strongly, at magnitude -0.7. If you have an unobstructed western horizon, binoculars will pick it up close to the horizon after sunset – don’t mistake it for Jupiter, which may also show up, just barely above the horizon, but intrinsically brighter at magnitude -1.7. An extremely important piece of advice here: do not attempt to view either planet until the Sun is totally beneath the horizon if you value your eyesight.
Here’s a chart depicting the view to the west ten minutes after sunset (note that Mercury and Jupiter were very close – less than 1° apart – three nights earlier, on the 18th). This chart is probably best utilised in attempting to pick up, through your binoculars, the 1½ day old, 2.5% illuminated waxing crescent Moon at an altitude of 12° (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°). Again, you must wait for the Sun to set.
By the end of the month, Mercury is better positioned, but still well within twilight; it sets at 9:05 pm, having been at an altitude of 13° when the Sun set at 7:54 pm. The planet’s disk is incrementally larger, 4.9", and illuminated to a lesser extent, 93%, as it begins to approach Earth, and shines at magnitude -0.4 (for those new to the pursuit of astronomy, who may at this point be wondering why Mercury’s brightness is decreasing at a time when its distance from us is also decreasing, the reason is that the increase in brightness due to the lesser separation is overpowered by the decrease due to the lesser phase – Mercury, with its tight orbit around the Sun, is the only planet which exhibits this apparent anomaly).
Beginning the month in Virgo, Mercury crosses into Libra on the 22nd, and remains in that constellation for the remainder of the month.
Venus is now far removed from the dizzying heights in the morning sky that it displayed earlier in the year; it is also both intrinsically less bright (by more than half a magnitude) and well into morning twilight. This trifecta would knock out any other planet, but as it has descended from a truly lofty height, as regards both altitude and brightness, the Morning Star still commands attention in the east, effortlessly punching through the brightening twilight.
On the 1st, Venus rises at 6:03 am, and has attained an altitude of 10° by the time the Sun rises, 6:56 am; its disk spans 11.2", is 91% illuminated, and shines at magnitude -3.94, modest by its standards, but far brighter than any of its planetary siblings. Venus is at perihelion, its closest point to the Sun in its orbit – note that this is unrelated to the angular separation of Sun & planet in our sky – on the 3rd, at a distance of 107.5 million km (just under 72% of the average Earth-Sun span). Three days later, on the morning of the 6th, Venus, which is dropping a little lower with each successive day, meets Mars, which is increasing its altitude daily, with the latter sitting a little over 0.2° above its sibling (they’ll be within ½° - ¾° on the 5th & 7th, and around 1¼° a day further out either side). While Venus will be easily visible, Mars, shining at magnitude 1.8 in morning twilight, will be very faint to the naked eye; its proximity to Venus will, however, serve as an excellent guide, and the pair should be a riveting sight at any magnification from a binocular view to that afforded by a telescope at high power. Here’s the view, on the 6th, at 6:18 am, ½ hour before sunrise, with Venus at an altitude of not quite 3½° in the east – obviously viewing this conjunction will require a flat eastern horizon. Heed again the caution given in the notes on Mercury, and ensure the Sun remains below the horizon while viewing the planets through any degree of magnification. As a matter of interest, Mars sits 379 million km from Earth at this time, and Venus 227 million km.
Come the 22nd, the morning following our designated viewing night, Venus rises earlier, at 5:45 am, but is outpaced by the Sun in this regard, which breaches the eastern horizon at 6:26 am. Consequently, the planet’s altitude at sunrise is down to 8°; the span of its disk has reduced to 10.6" as it moves farther away from Earth, at an increased phase of 94%, and it shines incrementally less fiercely, at magnitude -3.93.
On the 31st, the trend continues – it rises eight minutes earlier, 5:37 am, but the Sun again outperforms it, rising eleven minutes earlier, at 6:15 am. Sunrise altitude is 7°, the span of the disk 10.3", 96% illuminated, and visual magnitude is essentially unchanged, remaining at -3.93. Early risers can continue to follow Venus throughout the year as it sinks ever lower to the morning horizon; its brilliance will certainly render it visible to the naked eye into December, and even through to the end of the year, if you know where to look.
Venus transitions from Leo into Virgo on the 9th, where it remains for the rest of October.
Although Mars remains within morning twilight throughout October, it is distancing itself from the horizon daily (rising in a dark sky from the 18th of next month) and thus becoming a more inviting target. On October 1st, the Red Planet crests the eastern horizon at 6:10 am, just over ¾ hour before sunrise (6:56 am), by which time it’s 9° high in the NNE; its disk spans just 3.7", is nominally out of round at a phase of 99%, and shines at magnitude 1.83. Refer to the above notes on Venus for Mars’ conjunction with that planet. Note that Mars reaches aphelion, the farthest point in its orbit around the Sun, on the 8th – 249.2 million km from Sol, which is equivalent to 1⅔ times the Earth’s average distance from the Sun.
On the morning of the 22nd, matters have improved considerably, with Mars rising at 5:19 am, in excess of an hour before the Sun (6:26 am), by which time the planet’s altitude is 13°, a little under 10° to the upper left of prominent Venus; the phase is down to 98%, and span & brightness have increased ever so slightly to 3.8" and magnitude 1.81. Here’s the scene ½ hour before sunrise, with Mars and Venus at altitudes of 7° and 2° respectively (the 3rd, 4th and 5th magnitude stars shown here won’t pierce the twilight without optical enhancement).Come month’s end, Mars is rising at 4:58 am, and is 15° high at sunrise, 6:15 am; phase, span and brightness stand at 97%, 3.9" and mag 1.79. Mars leaves Leo for Virgo on the 13th.
It’s all over bar the shouting for the King this month, as it reaches conjunction with the Sun (on the far side of Sol from our perspective, rising and setting with it) on the 27th.
Even at the start of the month, it is a mere 18° clear of the western horizon as the Sun sets at 7:24 pm, and less than 1° at the cessation of evening twilight, 8:53 pm. Setting at 8:58 pm, Jupiter’s disk spans 31.0"; it shines at magnitude -1.7 (throughout the month).
Its circumstances on our viewing night are truly forgettable: it is 3° above the sunset (7:43 pm) horizon, and sets itself at 8:01 pm; the disk spans 30.6". Reappearing in the morning sky after the 27th, the planet is rising at 6:12 am on the 31st, just three minutes before the Sun; the span of the disk is 30.7", as its distance from Earth begins to decrease.
Next month, Jupiter will move from Virgo, in which constellation it has resided since last August, into Libra.
Although lagging behind Jupiter in their mutual march toward the western horizon, Saturn’s days are also numbered, as it descends into evening twilight late next month. Don’t miss the chance to turn your ‘scope on it this month, however, as it still maintains a respectable altitude as the sky darkens, and the captivating ring system is face on to us to the greatest extent in almost 15 years.
On the first day of the month, Saturn is at an altitude of 51° WNW at 8:53 pm, the end of evening twilight, and doesn’t subsequently set until well after midnight, at 1:22 am. The disk and rings span 16.2" and 36.7" respectively, the latter inclined at 26.97°; overall, disk & rings shine at magnitude 0.51.
Come our viewing night of the 21st, Saturn is still well up in the sky as twilight fades – 32°W @ 9:17 pm – and remains above the western horizon until 12:10 am; the span of the disk and rings is slightly reduced, to 15.7" & 35.7", as is the system’s brightness, at magnitude 0.54, but the inclination of the ring system is at its maximum of 26.98°. Here’s where to find Saturn, on the 21st, as the sky fully darkens – it should be obvious as the brightest ‘star’ bar none between and below the constellation figures of Scorpius and Sagittarius (the brightest within almost 50° in any direction, in fact), 15° NE (upper left) of Antares (Alpha [α] Scorpii, mag 1.0). Pluto is labelled for later reference.
The Cassini division, roughly half way out in the ring plane, will be obvious under a dark sky (bar excessive turbulence in our atmosphere), and careful examination should reveal the shadow of the disk cast on the back of the rings, which was at a maximum last month. If you can tear yourself away from the rings, consult the following magnification, configured for 9:30 pm, for assistance in identifying six of Saturn’s seven brightest moons (the wide orbit of Iapetus necessitates a separate chart).
Top to bottom on the chart, the labelled moons shine at the following visual magnitudes: Titan 8.85, Rhea 10.2, Mimas 13.4, Dione 10.9, Enceladus 12.2 and Tethys 10.7 .For those with large apertures and an appetite for challenge, unlabelled Janus, magnitude 15.1, shows faintly just to the mid-left (on the chart) of the outer edge of the ring system, with Hyperion, mag 14.8, at far bottom right; as all stars in the field of view are fainter than mag 16.6, none are depicted. Any telescope will pick up Titan, the brightest moon, a six incher will show the four 10th magnitude satellites, 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. The following chart zooms out to pick up Iapetus:
Iapetus shines at magnitude 10.6; this is my own (confident) brightness rating as no resource at my disposal, including Starry Night software from which the mags of the other moons are taken, takes into account the critical variation in the brightness of Iapetus arising from the stark difference in albedo (reflectiveness) of its opposing hemispheres. Any readers wishing to examine the approach taken in arriving at the figure given are invited to contact me via the link at the end of these viewing notes.
The star field has been retained in this chart, as it aids in identifying the far flung moon. The size at which the stars in the field are depicted has been adjusted to depict the labelled mag 9.0 star (TYC6247-444-1) as a little larger than mag 10.6 Iapetus (bright Titan should obviously be depicted as a little larger than the other moons; whereas the software does this well in relation to stars and planets, the depiction of moons is a little wanting in this regard). As the chart shows, no star near Iapetus (either on the chart or for a considerable distance beyond it in any direction) is of comparable brightness. To emphasise this, note that the star just to the lower left, on the chart, of the TYC designate shines at mag 13.4, and the brightest stars near Iapetus, the pair to the moon’s upper left on the chart, are mag’s 14.4 and 15.9. Iapetus should be a fairly easy capture by noting that it lies on an extension of a line from Rhea to Enceladus, and is twice as far from Saturn as the mag 9.0 star (for the benefit of those familiar with the field of view afforded by their scope-eyepiece combinations, Iapetus is a little under 7' from Saturn and 4½' from the TYC designate).
At the end of October, Saturn sets in the WSW at 11:34 pm, having been 22° high in the west when evening twilight faded at 9:31 pm. The disk spans 15.5", the rings 35.2", at an inclination of 26.97°; brightness remains at magnitude 0.54.
Saturn will remain within Ophiuchus until mid-November.
Although Uranus is now establishing itself in our evening skies, October 1st sees it not rising until 8:49 pm, still close to 1½ hours after sunset, and transiting well after midnight, 2:20 am (on the 2nd). The ice giant – a term applied to Uranus & Neptune, whereas Jupiter and Saturn, which are closer to the Sun, are referred to as gas giants – displays a disk spanning 3.7" (which it does all month), shining at magnitude 5.69.
Uranus reaches opposition – the Sun, Earth and Uranus form a (very nearly) straight line with Earth in the middle, whereby Uranus rises in our sky as the Sun sets – on the 20th, and this once again raises an issue which I have grappled with unsuccessfully for some time now. The problem is that although Uranus is at opposition on the 20th, as confirmed by a number of resources, the 20th is not the date on which Uranus rises as the Sun sets! On the 20th, Uranus rises at 7:30:45 pm, 11½ minutes before sunset, which is at 7:42:12 pm; best fit is actually two days earlier on the 18th, when the respective rise and set times are 7:39:01 pm and 7:40:13 pm. Being currently still unable to explain this apparent contradiction, I’ll take it no further at this stage, other than to invite input from readers who may be able to assist.
On the 21st, our viewing night, Uranus rises and transits (reaches its highest point in the sky) at 7:27 pm and 12:58 am (on the 22nd) respectively; the planet’s visual magnitude has peaked (just a day after opposition, when the Earth-Uranus distance is at a minimum), at mag 5.68. The following chart shows where to look for Uranus as it transits; Neptune is labelled for later reference.
The chart shows stars down to magnitude 5.5, thus approximating a naked eye view under a dark sky; use it to determine where Uranus sits in the sky in the following manner. Begin by identifying the Great Square of Pegasus, with Algenib (Gamma [γ] Pegasi), mag 2.8, and Markab (Alpha Pegasi), mag 2.5, at its top right and left corners respectively. Use these two stars to identify the fainter star (still easily visible to the naked eye) Eta [η] Piscium, mag 3.59 – extending a line from Markab to Algenib to a little over double its length will bring you close to Eta Psc (Markab and Algenib are separated by 16½°, Algenib and Eta by just under 19°). Identifying Eta should be straight forward, as there are no other naked eye stars in its immediate locale, and the two brighter stars to its lower right point at it. Now target Omicron [ο] Psc, fainter again but still comfortably naked eye (under a dark sky), at magnitude 4.25; as a guide to finding Omicron, note that it lies almost directly on a line joining Algenib and Kaffalijidhma (Gamma Ceti), mag 3.5, the unlabelled star to the upper left of Menkar (Alpha Ceti, mag 2.5) in the constellation figure of Cetus. Having located Omicron, proceed to the final magnification below to nail Uranus.
This chart, which displays stars down to magnitude 9.5, just a little beyond the reach of a typical finder ‘scope, should easily facilitate capture of Uranus through such an instrument. It labels Eta Psc and Omicron Psc with their visual magnitudes (3.59 & 4.25 respectively), as well as Pi [π] Psc, mag 5.53, and the star of mag 6.31 (HIP7819). The HIP designate is labelled to illustrate the fact that, bar Omicron, Uranus is considerably brighter than any star in its vicinity (far brighter, other than HIP7819; at magnitude 5.68, you may even have glimpsed Uranus with the naked eye when locating Omicron). In addition to its brightness, Uranus will advertise its identity courtesy of a subtle blue-green hue and a steady shine as compared to the twinkling stars (planets, which span a measureable portion of sky are less affected by turbulence in the atmosphere than are the pinpoint stars). Note that white crosses indicate the planet’s position on the first (labelled “F”) and last (“L”) days of the month.
When you have Uranus in your sights, switch to the main eyepiece at a magnification of 150x or more (the higher the better, equipment and conditions permitting) to resolve the planet’s disk, virtually the same size, this month, as Mars, at 3.7" (despite the huge difference in their distance from us, 2.83 billion km vs 369 million km). The colouration of the disk will be considerably more noticeable and aesthetically pleasing than the view afforded earlier through the finder ‘scope.
Here’s a question posed before, at appropriate times, in these viewing notes – the above charts show Uranus transiting, at its highest altitude for the evening, so why isn’t it at the crest of the ecliptic, the green line tracing the path of the Sun and (very nearly) the planets across the sky? Feel free to e-mail me if you’re unable to resolve this apparent contradiction.
On October 31st Uranus’ rise time has come forward to 6:45 pm, and that of transit to 12:17 am (on Nov 1st), while visual magnitude is back out imperceptibly to 5.69; it will reside within Pisces until 2018/19.
Having been at opposition on September 5th, Neptune is beautifully positioned for viewing this month. As October commences, it is already 43° high as evening twilight fades at 8:53 pm, and transits at 11:34 pm; its 2.4" disk shines at magnitude 7.82.
The outermost planet proper (since Pluto’s demotion) is even better positioned on our viewing night of the 21st, being at an altitude of 58° in the north at the end of twilight (9:17 pm), and transiting at 10:14 pm; span and brightness have regressed incrementally to 2.3" and magnitude 7.84. The wide field chart relating to Uranus shows Neptune’s location relative to the Great Square of Pegasus, the Circlet in Pisces and, in particular, the ‘Y’ of Aquarius – the Circlet and ‘Y’ are composed primarily of 3rd & 4th magnitude stars and so are clearly visible to the naked eye under a dark sky, albeit somewhat faintly so. Having identified the ‘Y’, use the following chart, which plots only naked eye stars – defined here as those brighter than magnitude 5.5 – to assist in locating Lambda [λ] Aquarii, the magnitude 3.71 star which lies close to Neptune. To aid in orientation, the chart also plots the westernmost three stars of the circlet; all labelled stars are clearly visible on the Uranus wide field chart referred to earlier, with the exception of ‘5.43’. With only one star visible to the unaided eye between the ‘Y’ and Lambda – that of magnitude 5.03, Kappa [κ] Aquarii, itself very faint – and Lambda unmatched in brightness by any star near it, an easy capture is assured.
Having identified Lambda, refer to the final magnification below to find your planetary target, just 33' – a touch over ½° – away.
This chart plots stars down to around 14th magnitude, but only labels those brighter than magnitude 11.0; it may be used in conjunction with either your finder ‘scope or at moderate magnification through the main eyepiece (around 180x works for me). The finder route has the advantage that, with a limiting magnitude of 9.5 or less, no stars will be visible between Lambda and Neptune, which will appear very close together in the typically 4° field of view. The advantage of viewing through the main eyepiece, on the other hand, is that the view is spread out and more easily related to the chart. This does not incur a disadvantage of making it difficult to pick out Neptune from the star field, as the planet is far brighter than any star in its immediate vicinity (the unlabelled star just to its left on the chart shines at mag 9.34). As with Uranus, Neptune’s colouration – blue-grey in this case – and steady shine will help to distinguish it from the stars. Note that in addition to plotting Neptune’s position on the 21st, the chart also contains markers for where the planet sits on the 1st and 31st (labelled “F” and “L” respectively).
Having found the planet, employ the highest practical magnification and close examination (I suggest 250x or more) to resolve its tiny disk.
On October’s final evening, Neptune transits at 9:34 pm, three minutes after the cessation of evening twilight; its disk spans 2.3" and shines at magnitude 7.85. The planet resides within Aquarius until 2022/23.
Having reached opposition back in July, Pluto has by now lost considerable altitude by the time the sky fully darkens. As locating dim star-like Pluto, especially in the rich star fields of Sagittarius, is an exercise best conducted with the planet well elevated in a dark sky, this will be the last month when detailed directions are provided.
Even at the beginning of October, Pluto transits at 7:52 pm, already more than an hour before evening twilight is extinguished (8:53 pm); its miniscule disk, far too small to be resolved in amateur instruments, spans 0.095" and shines faintly at magnitude 14.28.
Come the 21st, it is transiting at 6:34 pm, almost 2¾ hours before the sky fully darkens at 9:17 pm, at which time it sits 51° clear of the WNW horizon; span and visual magnitude are down to 0.094" and 14.31. The wide field view in the Saturn notes shows Pluto’s position just to the east of the three naked eye stars below the handle of the Teapot asterism (above the handle actually, we view it upside down). Here’s a magnification of the scene at 9:17 pm, labelling the three stars in question, the visual magnitudes of which are as follows: Albaldah (Pi Sagittarii) 2.87, Omicron Sag 3.75 and Xi2 [ξ2] Sag mag 3.50.
The next chart, which shows stars down to around magnitude 13.0 (still brighter than Pluto), and labels them with their brightness ratings rather than names or designations, is delimited by Omicron Sag and Albaldah at bottom left and right respectively, and Pluto at top right. It includes markers for Pluto’s position at the start and end of October, respectively below left and above right of where Pluto sits on the 21st; also labelled are two more stars which are beyond naked eye range but easily seen in a finder ‘scope – HIP94372, mag 6.37 and HIP94338, mag 7.81 (both visible in the previous chart):
As we continue to zero in on our planetary target, the two HIP designates delimit the final chart below; as these stars are separated by 17', this chart is suited for use with an eyepiece delivering high magnification. As an example, a 6 mm eyepiece with an apparent field of view of 80° used in a telescope with a focal length of 1500mm would show a patch of sky 19' across (6*80/1500 degree, multiplied by 60 to convert to arc-minutes = 19.2; refer back to the July edition of ANS for more on calculating the actual field of view of a given eyepiece/telescope combination). The next brightest star in the field, TYC6308-944-1, is also labelled with its magnitude of 9.81 for reference.
All star charts courtesy of StarryNight®ProTM Version 220.127.116.117/Simulation Curriculum Corp.
This chart plots stars of magnitude 15.5 or brighter; as such, unless you have a large ‘scope, you will not see any which do not appear on the chart, and may well not see the faintest shown here. Ideally, you will have chosen an eyepiece which places the two HIP designates near the edges of the field of view; as shown, Pluto sits a little over half way from the brighter to the dimmer of the two. As a guide to determining which of the points of light is the planet, note the two circled stars; the one above Pluto – on the chart, remember your telescopes optics may invert the view – shines at magnitude 14.2 (it’s catalogued as USNO J1912127-215106), virtually on a par with the planet’s 14.3, while the lower star (USNO J1912240-214732) shines at mag 13.25, and so will be noticeably brighter.
The chart includes white crosses marking Pluto’s location on the preceding Saturdays of the 7th & 14th (to the lower left of its labelled location of the 21st) and on the 22nd and 28th (respectively one day and one week after our viewing night). The indicator for the 22nd is included to facilitate verification of capture by means of imaging or sketching the star field to see which point of light has moved from one night to the next, although the small displacement would probably indicate returning two nights later rather than one as a better strategy.
As the month concludes, Pluto’s altitude at the cessation of twilight (9:31 pm) is down to 41°; span and visual magnitude are unchanged from the 21st.
The tiny frozen orb, revealed so strikingly by New Horizons, will traverse the star fields of Sagittarius until 2023/24.
This month’s feature arose and evolved from a Facebook friend’s suggestion which was in turn a response to a request for ideas for future ANS feature articles.
We all, for now, live on planet Earth (I’m sure that will change in the not too distant future), and our orientation on this (roughly) spherical planet depends on what part of it we inhabit – whereas we perceive ourselves to be standing upright, we’re doing a headstand from the perspective of those directly opposite us on the globe (The North Atlantic, midway between Florida and NW Africa, is a good fit for SE Australia, despite the old ‘tunnelling through to China’ adage).
So much for our orientation on Earth, but where and how does it fit in relative to the bigger picture? Firstly, our planet spins on its axis once a day, so exactly where an individual’s head points also varies according to the time of day. Then there’s the matter of precession of the equinoxes, whereby the spin of the Earth is analogous to that of a spinning top, completing one ‘wobble’ every 26,000 years or so, further complicating the notion of direction.
We’re also subject to the Earth’s yearly trip around the Sun; while this doesn’t noticeably affect our orientation in an absolute sense, it certainly does in relation to the Sun and planets, and our location within the solar system varies by around 300 million km. Additionally, the rotational axis of our planet is tilted by 23.4° in relation to the plane of our orbit (causing the seasons), whereby one hemisphere is tilted first towards the Sun, and then away six months later.
While of supreme significance to us, the Sun itself is just a bit player in the galaxy, orbiting once every 250 million years or so, while at the same time bobbing up and down below the galactic plane (approximately four times per ¼ billion year trip around the galaxy); again, orientation is a factor, as the plane of the solar system is tilted by over 60° in relation to the plane of the galaxy, as illustrated below, courtesy of Physics Stack Exchange:
In relation to the above diagram, note that the solar system, which is obviously depicted grossly oversized compared to the galaxy, propagates to the left, whereby it is orbiting clockwise when seen from above the plane of the galaxy, while the planets orbit counter clockwise when seen from above the plane of the solar system (above left of the Sun in the diagram). Note also that the orientation of other solar systems throughout the galaxy, and the direction in which planets orbit their respective stars, is random, but the vast bulk of stars and their planetary attendants orbit the galaxy counter clockwise (when seen from ‘above’).
Expanding our view, the notion of where we are in the bigger picture of things is illustrated in the depictions to come. The first of these, courtesy of Wikipedia, shows the local group of galaxies, dominated by the two big boys – M31, the Andromeda galaxy, and the Milky Way, currently separated by around 2½ million light years, and each surrounded by many smaller galaxies.
Galaxies within the local group, which spans around 10 million light years, are gravitationally bound; M31 and the Milky Way are moving towards each other at around 100-140 km/sec, and destined to collide and merge in some four billion years. M33, the Triangulum Galaxy, is the only midsized member of the local group – the part it will play in the future mega merger is unclear; it may become as one with the two larger galaxies as they form a giant elliptical galaxy, or it may end up orbiting the combined pair.
On an even bigger scale, the local group, or local cluster, is just a small part of the local supercluster. This collection of galaxy clusters, some 110 million light years across, is known as the Virgo supercluster and is dominated by the Virgo cluster, which is analogous to our local cluster, but larger, in terms of both numbers of galaxies and their individual sizes; here’s a graphic, again sourced from Wikipedia:
This is where things get interesting as regards the direction in which the clusters which compose the supercluster are moving. While the components of the Virgo supercluster were previously held to be gravitationally bound, spectroscopic analysis of the light from individual galaxies within the Virgo cluster (stay with me, I’m referring here to the cluster which is the largest member of the supercluster) indicates that they are red shifted, and hence moving away from us. It now seems likely that the accelerating expansion of the universe as a whole (under the influence of dark energy, whatever that may turn out to be) is overcoming the gravitational attraction of members of the supercluster, which are moving away from each other, but at a slower rate than objects outside their mutual gravitational influence.
Relatively recently, it has become apparent that the Virgo supercluster is itself just a component of a larger concentration of galaxy groups dubbed Laniakea, some 520 million light years across and containing perhaps 100,000 galaxies; the following graphic is courtesy of The North Coast Journal:
The white lines in the above graphic are intended to depict the flow of components of Laniakea towards its gravitational centre, which has been dubbed The Great Attractor. There has been speculation that Laniakea may itself be but a component of an even larger grouping, of which other superclusters shown nearby would obviously be a part. Of course, the domination of gravitational attraction by the expansion of the universe, referred to previously, is even more pronounced in the case of these larger structures.
Finally, we portray the observable universe – once again courtesy of that venerable and often unfairly maligned source known as Wikipedia, 90+ billion light years across and composed of the various superclusters; while the depiction (note that it is oriented differently to that above) shows Laniakea at the centre, there is no suggestion that it is the centre of the universe as a whole, it’s just the region from within which we look out in all directions.
So there you have it: next time someone asks you over the ‘phone where you are, you’d better ask them in return “in relation to what?”
Coming back down to Earth after our trip through the cosmos, that’s all we have for this month folks; join us again in November when The Australian Night Sky returns to examine and muse on the inhabitants of the heavens, both near and far.
As always, any questions, comments or suggestions are welcome and may be directed to: firstname.lastname@example.org
Until next month: