Rug up and defy the mid-winter chill with the aid of :

Transcription

1 Rug up and defy the mid-winter chill with the aid of : The times, circumstances and star charts to follow are configured for the Melbourne Observatory in line with other publications of the Astronomical Society of Victoria (ASV), for which these viewing notes are principally compiled: Longitude ' 23.8" E, Latitude 37 49' 54.1" S [" denotes arc second = 1/60 th of an arc minute (symbol ') or 1/3600 th of a degree (symbol )]. The notes contain many references to rise and set times, all configured for the co-ordinates given above; the following procedure will facilitate determining the correct times for other locations. The adjustment required for longitude is straight forward for each degree east (of Melbourne Observatory) subtract four minutes; conversely, add four minutes for each degree west. Adjustment for a difference in latitude is dependent on the declination of the object concerned how far north or south of the celestial equator it lies. The table below, from the yearbook of the ASV, gives the required adjustment (in minutes) for any object at a declination of between +30 and -30 (by convention, north is positive and south negative) when viewing from any location between latitudes -28 and -44 ; interpolate for intermediate latitudes/declinations. Note: add these values to times of rising and subtract from setting times.

2 Whereas the declinations of stars and other deep sky objects are (very nearly) constant, those of the Sun, Moon and planets change considerably over time; a suitable resource for determining declinations is therefore required to obtain the adjustment for difference in latitude as described above. While the reader may have ready access to such resources, I offer the following suggestions for those who do not: the ASV yearbook, free to members and available at reasonable cost to others (Sun and planets only), the yearly publication Astronomy Australia (Sun, planets and Moon), and various astronomical software packages (typically catering for all celestial objects). Other sources may be available on-line. Note that some resources give co-ordinates for epoch denoted as J 2000 whereas others may quote real time figures, denoted J Now. The discrepancy between the two systems will not be significant for the purpose at hand. Now a few notes defining terms which may be encountered in the text to follow: For those not familiar with the terminology used to describe phases of the Moon, note that it is referred to as waning (hard a ) when the phase is decreasing daily and waxing (soft a ) when increasing, and as a crescent when less than half lit and gibbous when more than half. Thus you may find it referred to in the text as, for example, a waxing crescent Moon. Appearances of the abbreviation au in the text denote astronomical unit, the average Earth-Sun distance, currently defined as 149,597, km; sm denotes solar mass, the mass of our Sun. The abbreviation ZHR, where it appears in relation to meteor showers, refers to the zenith hourly rate, the number of meteors per hour which can be expected to be seen under ideal conditions, with the radiant (the point from which the meteors appear to emanate) at the zenith and a clear dark sky. The ecliptic, shown as a green line on many of the star charts to follow, is the path followed by the Sun and (very nearly) the planets as they arc across our sky daily. As the outer planets, those that orbit farther from the Sun than does Earth, move along their orbital paths, they move from west to east in our sky relative to the stars; this is referred to as direct or prograde motion. Around the time of opposition however, when we overtake them on our inner, faster orbital path, they change direction, moving east to west relative to the stars. Referred to as retrograde motion, this phenomenon is caused by the same effect as that seen when a car which is overtaken seems momentarily to move backwards in relation to a line of trees in the background. On any given night, a planet is said to be transiting (or culminating, there is a subtle difference between the two) when it reaches its highest point above the horizon in its passage across our sky that evening do not confuse this use of the term transit with the same word often used in these viewing notes to describe the passage of the moons of Jupiter across the face of their parent. Regarding the inner planets, Mercury and Venus, which never stray very far from the Sun in our sky, greatest eastern elongation refers to their maximum angular distance east of the Sun when they are visible in our evening skies; similarly, greatest western elongation refers to their maximum angular separation west of the Sun when visible in our morning skies. They are said to be in inferior conjunction when passing between Earth and the Sun on their inner orbital tracks, and in superior conjunction when rounding the far side of the Sun from our perspective. As a handy (excuse the pun) guide to estimating the angular separation of two objects, one finger held at arm s length typically spans a little over 1, a closed fist 10 and an open hand, thumb tip to tip of little finger, 20.

3 The symbol " denotes arc second = 1/60 th of an arc minute (symbol ') or 1/3600 th of a degree (symbol ). The notes to follow adopt the practise of quoting the span of planetary disks to the nearest arc second if that is sufficient to distinguish the values throughout the month in question and identify trends; one, two or occasionally three decimal places may be given if it is necessary to do so in order to achieve these stated ends. The same approach is employed in relation to the phase of planetary disks, denoting their degree of illumination, and the inclination of Saturn s ring system. Visual magnitudes are typically stated to the first decimal place; again, more precision is provided if required. All references to twilight in these notes relate to astronomical twilight, which ends/begins when the Sun is 18 below the horizon. Stars are generally referred to by their common names, if they have one, with their Bayer designations appended; Flamsteed numbers are often used for lesser stars, with HIP, TYC & USNO designations used to identify progressively fainter stars. Note finally that perihelion or aphelion of Mercury, Venus and other planets refer to the physical separation in space of the planet in question and the Sun, and are unrelated to their angular separation in our sky. What s in the sky this month; July 2018: 4 th Mercury visits the Beehive. 6 th Last quarter Moon. 7 th Earth at aphelion (farthest from Sun, 152,095,565.6 km / au). 10 th Aldebaran (Alpha [α] Tauri) occulted by Moon (not from Australia). 11 th Jupiter stationary (ends retrograde motion). 12 th Mercury at greatest eastern elongation, 26.4 ; Pluto at opposition. 13 th New Moon; Partial solar eclipse (visible from Antarctica, Tasmania and the far south of mainland Australia); Moon at perigee (closest to Earth, 357,431 km). 20 th First quarter Moon; Mercury at aphelion (farthest from Sun, million km / au). 25 th Mercury stationary. 27 th Mars at opposition (best since 2003); Moon at apogee (farthest from Earth, 406,223 km). 27 th Piscis Austrinids meteor shower peaks. 28 th Total lunar eclipse (visible from most of Australia, Melbourne included); Full Moon (farthest for the 406,099 km). 30 th Southern Delta [δ] Aquariids and Alpha Capricornids meteor showers peak. 31 st Mars closest to Earth (57.60 million km / au). N.B.: When reading the following, refer back to the explanatory notes at the beginning of this article for information on terminology, angular separation approximations and adjustment of latitude & longitude. The occultation of Aldebaran by the Moon is visible from the Arctic, Greenland and Canada; locally the Moon s limb only approaches to within a touch under 1¼ (at 8:10 pm), and that occurs more than 60 below our SSW horizon. The partial solar eclipse of the 13 th barely qualifies as such from our perspective, with the Moon obscuring, locally, just 2.3% of the Sun s diameter at best (the magnitude of the eclipse, defined as the maximum fraction of the Sun s diameter which is occulted by the Moon, is ), at 1:21 am; the

4 eclipse, such as it is, begins at 1:05 am and ends at 1:38 am. Only localities south of a line running approximately from Port Lincoln, S.A. to Lakes Entrance, Victoria will experience the eclipse. Southern Tasmania is the most favoured Australian location, with an eclipse magnitude at Hobart of , while even at the locality of maximum eclipse coverage, in far north Antarctica (the Sun has retired below the winter horizon over all but the northern tip of Antarctica), the eclipse magnitude is just i.e. around ⅓ of the Sun s diameter will be occulted. While solar eclipses have the potential to be much more dramatic visual affairs than do eclipses of the Moon, the total lunar eclipse on the 28 th is far more favourable than the partial solar eclipse detailed above. Locally, the penumbral phase of the eclipse, which is barely noticeable as the Moon enters the Earth s penumbra, or outer shadow, where only part of the Sun s disk is hidden by the Earth, begins at 3:13 am. The partial phase proper, or umbral phase, when the Moon starts to enter the Earth s umbra, or full shadow within which the Sun s disk is completely obscured, with a visible darkening beginning to creep across the Moon s surface begins at 4:24 am. Here s a generic diagram, courtesy of showing the difference between penumbral and umbral phases. Totality begins at 5:30 am, less than ½ hour before morning twilight commences at 5:53 am; mid-eclipse subsequently occurs at 6:22 am, in twilight, with the Moon at an altitude of 12 WSW. The sequence then plays out in reverse totality ends at 7:14 am, just over 10 minutes before sunrise, with the Moon just a few minutes short of setting, while the partial and penumbral phases end in daylight at 8:19 am and 9:30

5 am respectively. Viewers further west on the continent see more of the eclipse under a dark sky, with totality finishing ½ hour before the advent of twilight from Perth, W.A. The Moon will pass through the centre of Earth s shadow, whereby the event is termed a central lunar eclipse; this has not happened since June 2011, and will result in the longest lunar eclipse duration of the 21 st century more s the pity that part of it occurs in daylight. The animation below (click the link and choose allow if asked), courtesy of Wikipedia, shows the Moon s path through the Earth s shadow; the umbra is the inner grey region and the penumbra the annular region between the two circles. Three meteor showers peak this month, the Piscis Austrinids on the 27 th (the ASV s yearbook states the 28 th, the International Meteor Organization opts for the 27 th ) and both the Southern Delta Aquariids and the Alpha Capricornids on the 30 th ; unfortunately all three are heavily affected by the Moon, at and around full phase. The Southern Delta Aquariids are easily the strongest of the three showers, with a ZHR of 16 or more as compared to 5 for each of the other two. Meteor shower aficionados may benefit from combining views of the above-mentioned lunar eclipse of the 28 th with the opportunity to catch members of each of the three showers, as shower activity is spread out, with the Southern Delta Aquariids serendipitously displaying a particularly wide peak. Mars, only one day after its best opposition (and just a few days before its closest approach to Earth) in 15 years, adds to the spectacle. The chart below depicts the scene at 2:00 am on the 30 th. A preamble to the planetary notes is in order this month, as an opportunity exists to see all five naked eye planets above the evening horizon at the same time. Technically speaking, the spectacle begins on the 3 rd, when the westernmost member of the five, Mercury, sets at 7:08 pm, two minutes after Mars, at the eastern extreme, rises at 7:06 pm; it then extends throughout the month and beyond. In practice, the best viewing opportunities run from a few days before midmonth until the 28 th, the date Mercury last sets after evening twilight finishes. Here s where the planets sit on the 17 th, at 6:54 pm, the end of evening twilight.

6 More detail on the circumstances of the individual planets is to be found in the notes below. Mercury Having first set after the end of evening twilight late last month (on the 27 th ), Mercury soars high, by its modest standards, in the evening sky this month, not sinking back into the twilight zone until the 29 th, in its best evening apparition of On the 1 st it is 17 clear of the horizon at sunset, 5:11 pm, and still at an altitude of 2½ as the sky fully darkens at 6:45 pm, subsequently setting at 7:01 pm; spanning 7", its disk is 60% lit and shines vigorously at magnitude 0.0. The evening of the 4 th provides a great telescopic opportunity to watch Mercury invade the Beehive, open cluster M44, as shown. Mercury reaches its greatest angular distance from the Sun in our skies, 26.4, on the 12 th. The timing is quite pleasing as, with New Moon falling on the following day of Friday 13 th (which may well send a few astrologers into a tizzy ), we ll choose Saturday 14 th as our viewing night, only two days after the innermost planet appears farthest from Sol. On this night, Mercury is 22 high in the NW at sunset, 5:19 pm, still 6 as twilight is fully extinguished at 6:52 pm, and sets at 7:29 pm; its 8" disk presents a fine telescopic sight at a phase of 39%, still

7 shining brightly at mag 0.6. Here s the view at 6:52 pm. As the month of July concludes, Mercury is gathering pace in its descent back towards the evening horizon, being only 14 high at sunset, 5:32 pm, and setting at 6:49 pm, almost ¼ hour before the sky fully darkens; disk span is out to 11", the phase down to 9%, and brightness has fallen away markedly, to mag 2.6. It s worth targeting Mercury through your scope at regular intervals over the second half of the month as the shrinking crescent phase, augmented by the increasing span, is aesthetically pleasing, albeit challenged by an ongoing reduction in brightness and an ever brighter Moon, which nightly increases in phase (on the final two evenings of the month, it doesn t rise until after Mercury sets). The Messenger of the Gods passes from Cancer into Leo on the 15 th, before looping back into Cancer on the 5 th of next month. Venus The Evening Star continues to power away from the horizon and grow larger and brighter throughout July, dominating the early evening western sky in a manner no other planet can match. On July 1 st, it is 29 above the horizon at sunset, 5:11 pm, still 17 high as twilight ends at 6:45 pm, and doesn t set until 8:25 pm. Blazing fiercely at mag -4.05, it s arguably a better naked eye spectacle than a telescopic one its blinding but bland 15.7" disk is noticeably, but not greatly, out of round at a phase of 70%. Come our viewing night of the 14 th, the planet s altitude at sunset (5:19 pm) and twilight end (6:52 pm) has improved to 34 and 21 respectively, and setting time to 8:48 pm; the disk span is out to 17.4", at a reduced phase of 65%, and it shines yet brighter, at mag This is a good time to be chasing Venus with the naked eye earlier into the evening with each passing day, you may even be able to spot it before

8 sunset DO NOT attempt to do so with any form of magnification, just with the naked eye, lest the Sun enters the field of view and instantly damages your eyesight. The evening of the 17 th may provide an opportunity to use Venus as a guide to spotting, telescopically, the principle members of the Leo 1 group of galaxies, M95, M96 & M105, although the 24% illuminated waxing crescent Moon, just 15 away, may be problematic (as may brilliant Venus itself). The group lies just 2⅓ -3½ north (below right) of the planet. As the chart below shows (albeit not very well), M105 is closely attended by two other members of the group, NGC 3379 at bottom and NGC 3373 (also catalogued as NGC 3389) at top 3379 is of comparable brightness to the three Messier designates, while 3373 is a couple of magnitudes fainter, and hence considerably more difficult. If you re on a roll, you may care to try for the Leo triplet, just over 10 to Venus upper right as shown; the member labelled Leo Triplet is NGC At the end of the month, the brilliant orb continues to improve, both telescopically and to the naked eye; 39 high at sunset, 5:32 pm, and 25 as twilight wraps up at 7:02 pm; it doesn t yield to the western horizon until 9:14 pm. The planet s disk has swollen to 20.2", is more telescopically appealing at a phase of 57%, and shines at mag Venus spends the month traversing Leo, and by the 31 st is only a day short of crossing into Virgo. Earth As indicated in the monthly summary, the third rock from the Sun is at aphelion, its farthest point from Sol, on the 7 th, at a distance of 152,095,565.6 km / au. Mars Mars assumes centre stage this month, starring in its best opposition, on the 27 th, since 2003, which itself was the best in some 60,000 years. It rises at 7:15 pm on the 1 st, later transiting at 2:33 am; spanning a generous 21", its 97% illuminated disk shines brightly at mag -2.17, almost as bright as Jupiter. When we view on the evening of the 14 th, rise time has come forward to 6:17 pm, less than an hour after sunset (5:19 pm) and 35 minutes before the cessation of evening twilight; Mars transits at 1:36 am the following morning. The martian disk, now all but fully lit at a phase of 99%, has grown to 23" and puts even mighty Jupiter to shame, shining at mag as against its giant sibling s Although its sheer brightness makes a finder chart somewhat redundant, the chart below shows the sky at 9:00 pm to put the issue beyond doubt.

9 This month, with Mars disk looming large in the eyepiece, represents your best chance to spot the (currently shrinking) south polar cap and other surface markings. Mars reaches its much vaunted opposition on the 27 th, spanning a huge 24¼" and shining at a scintillating mag Note that, as shown in the monthly statistics towards the start of these viewing notes, Mars and Earth are closest on the 31 st, four days after opposition, a consequence of the facts that their orbits are elliptical, rather than circular and also not in quite the same plane; as a result, the span of Mars disk at the end of the month is a little larger than at opposition (see below). On the 31 st, having risen late afternoon (4:41 pm), Mars has already climbed 8 above the ESE horizon at sunset, 5:32 pm, and is 25 high in the east as twilight fades at 7:02 pm; time of transit has improved to just after midnight, 12:11 am. Still shining with a vengeance at mag -2.76, the span of its disk is a little larger, as indicated above, at 24⅓". Mars cruises through Capricornus throughout the month of July. Jupiter A couple of months after its May 9 th opposition, Jupiter remains an excellent viewing prospect, high in the sky early in the evening. As July begins, it is already 58 clear of the horizon as twilight ends at 6:45 pm and transits at the convenient time of 8:28 pm, not setting until 3:17 am; the jovian disk spans 41", is 99% illuminated (all month) and shines strongly, as always, at mag Views taken on the evening of the 14 th see the King of the Planets fully 65 high in the NNE as twilight fades away at 6:52 pm, and transiting (at an altitude of 67 ) less than ¾ hour later, 7:36 pm; disk span and brightness are slightly diminished, to 40" and mag Two earlier charts, one in the Mars notes above and the other immediately preceding the planetary section, show Jupiter s position in the night sky; the magnification below, configured for time of transit (7:36 pm) shows where the four Galilean moons sit.

10 As the chart shows, Io is transiting (east to west, or right to left on the chart) across the face of Jupiter s disk; the other three moons are moving in the opposite direction, receding from Jupiter. Looking very closely at the chart, you ll notice a black dot immediately above the o in Io s label that s the moon s shadow, also in transit. Here s a further magnification. Io first moves onto the face of the disk at 5:51 pm, exiting at 8:03 pm, while the corresponding times for its shadow are 7:02 pm and 9:12 pm. On the last day of July, Jupiter transits at 6:32 pm, one hour after sunset and ½ hour before the sky fully darkens, before setting at 1:22 am; the span of the disk is down to a still impressive 38", and brightness to mag Jupiter is to be found within the constellation of Libra until well into November.

11 Saturn Having been at opposition late last month, stunning views of the Ringed Planet are on offer this month and for the bulk of the remainder of Rising at 4:49 pm on the first day of the month, a little over 20 minutes before sunset, Saturn is at an altitude of 21 at the cessation of evening twilight, 6:45 pm, and transits just after midnight, 12:06 am. Its disk spans 18.4", the rings 41.7" at an inclination of 26.1 ; disk and rings together shine at mag 0.0. When viewing on the night of the 14 th, you ll find the planet has already climbed 15 above the horizon at sunset, 5:19 pm, and has distanced itself from the horizon to the tune of 33, in the east, as twilight is consumed by darkness, 6:52 pm. Saturn then transits at 11:11 pm and remains above the horizon until around ½ hour after morning twilight begins, setting at 6:28 am. The span of its disk and rings is 18.3" and 41.5" respectively, the rings inclined at an angle of 26.2 ; visual magnitude is 0.1. As a number of the above charts show, Saturn is easily recognizable as the bright star near the lid of the Teapot in Sagittarius; here s a magnification, configured for 8:30 pm with Saturn at a viewer friendly altitude of 52, showing its seven brightest moons. The visual magnitudes of the moons is as follows: Tethys 10.4, Mimas 13.1, Enceladus 11.9, Dione 10.6, Rhea 9.9, Titan 8.5 and Iapetus 11.5; typically, Titan will show up in a finder scope or binoculars, Tethys, Dione and Rhea in a four to six incher, Enceladus and Iapetus in a six to eight incher, with faint Mimas, close to the rings, probably needing at least a 12 incher. Only stars brighter than mag 13.0 are plotted; do not mistake the star of mag (TYC ) for a moon. Beware also of the unlabelled star just below Iapetus on the chart the moon is of mag 11.5 and the star (USNO J ) mag Of lesser concern is (TYC ), short of Iapetus; note that, while too far removed from centre stage to be a problem, is a double star, the combined luminance of which yields mag The three remaining unlabelled stars shine at magnitude 12.8; it may be useful, in identifying Iapetus, to be armed with the knowledge that there is a relatively bright star, of mag 9.2, out past Iapetus, about as far from the moon as is

12 At month s end, Saturn is 31 high at sunset, 5:32 pm, 49 at the cessation of twilight, 7:02 pm, transits at 9:59 pm and sets at 5:17 am. The disk spans 18.0", the rings 41.0" inclined at 26.4 ; disk and rings together shine at mag 0.2. Saturn won t first emerge from Sagittarius until March Uranus Three months past conjunction, Uranus is now a presence in the early morning sky. Rising at 2:19 am on the 1 st, it s still a tentative proposition, not transiting until after sunrise; shining at mag 5.84, its disk spans 3.48". By the time our viewing session of the 14 th comes around, or more correctly, the morning of the 15 th, rise time has improved to 1:26 am, and time of transit to 6:51 am, a little over half way between the beginning of morning twilight and sunrise; brightness and disk span stand at mag 5.81 and 3.52". The following chart shows where the ice giant sits in the sky at 5:00 am, 34 clear of the NNE horizon; the Great Square of Pegasus is a handy reference point for relating the chart to the night sky. There are no stars brighter than Uranus for more than 4 in any direction; the two of 4 th magnitude which are labelled in yellow are the closest which outshine it. That fact, along with its blue/green hue and steady shine relative to the twinkling stars, may allow you to hunt it down; in the months to come, as its altitude improves, more detailed charts will be provided. By the end of July, Uranus rise time has come forward to 12:24 am, and it transits at 5:49 am, a couple of minutes before twilight first starts to emerge in the eastern sky; span and brightness have improved incrementally to 3.57" and mag Uranus will remain within Aries until early December.

13 Neptune The outermost planet proper (since Pluto s demotion) is considerably better placed in the morning sky than its inner neighbour. At the start of July, Neptune breaches the eastern horizon at 10:31 pm, later transiting at 4:53 am, its 2.31" disk shining at mag Rising on our viewing night at 9:40 pm, it subsequently transits commensurately earlier, at 4:01 am; span and visual magnitude read 2.33" and The previous chart relating to Uranus shows where Neptune sits at 5:00 am; that below, configured for 2:00 am, enlarges the area of concern, showing in more detail where the planet is located relative to the Great Square of Pegasus, The Circlet in Pisces and the Y of Aquarius (you may also find it useful to look back at the wide field chart, configured for 2:00 am on the 30 th, in the meteor notes). The chart shows stars down to magnitude 6.0, a little fainter than most folk will detect with the naked eye, and is intended to allow you to identify the two stars labelled either side of Neptune by reference to the three constellation figures depicted. As you can see, the area between both the Y and the Circlet is devoid of stars of comparable brightness to Lambda [λ] Aqr, mag 3.71 and Phi [φ] Aqr, mag 4.21, whereby identification should not be problematic. Having these two, especially the latter, under your belt, proceed to the final magnification below to track down your planetary target.

14 Magnitude 9.5, typically around the limit for finder scopes, is the cut-off for this chart, which labels stars with their visual magnitudes. Through such an instrument, Neptune should be an easy capture by reference to Phi Aqr, the planet sitting just a touch over 1 (typically a little less than the span of a finger held at arm s length) from the star in the direction of Lambda Aqr, the only two intervening stars being those of 6 th & 9 th magnitude as shown. Note the two white crosses, denoting Neptune s position on the 1 st and 31 st of the month, respectively lower right and upper left. Confirm identification by switching to the main eyepiece at high power I recommend 200x or more to resolve the planet s tiny disk, shining with a steady blue-grey hue. On the final day of the month, Neptune rises at 8:31 pm then transits at 2:53 am; span and brightness have nominally increased to 2.34" and mag Neptune inhabits Aquarius until 2022/23. Pluto Pluto reaches opposition this month, on the 12 th ; the best time for hunting down the exiled member of the planetary clan is thus with us over the next few months, with detailed finder charts presented as flagged in previous editions of these viewing notes. As the month begins, Pluto rises in the ESE at 5:55 pm and has attained an altitude of 9 when twilight wraps up at 6:45 pm; transit follows at 1:09 am with Pluto s miniscule disk spanning 0.097" (all month, far too small to be resolved in amateur instruments) and shining at mag After reaching opposition on the 12 th, shining at mag 14.18, Pluto rises at 5:02 pm on the 14 th, is 2½ high at sunset, 5:19 pm, and 20 as all traces of twilight fade at 6:52 pm; the planet transits at 12:17 am (on the 15 th ), displaying a visual magnitude of

15 The wide field chart for Mars shows where Pluto sits at 9:00 pm in relation to other denizens of the night sky, in particular Mars, Saturn and the Teapot in Sagittarius; we need to view later when our elusive target is high in the sky, so here s an initial magnification configured for 12:17 am, when it transits. Begin by identifying the labelled (with visual magnitudes) arc of stars; starting at 3.50 and proceeding clockwise, they are: Xi 1 [ξ 1 ] Sagittarii, Omicron [ο] Sag, Pi [π] Sag, 43 Sag, Rho [ρ] Sag and Upsilon [υ] Sag (note, to avoid possible confusion, that the dimmer star, of mag 5.8, immediately above Rho Sag is designated Rho 2, and that Rho itself is consequently sometimes listed as Rho 1 ). By reference to the arc, principally the three brightest members first listed above, look for two stars shining at magnitude 5.56; one of them, 50 Sag, is labelled on the chart, and the other, HIP95077, is immediately above the l of Pluto s label. These stars will be near the limit of naked eye visibility for most folk, but identification will be aided by the fact that they are the brightest stars for at least a couple of degree in any direction. Use your finder scope if you can t see these two unaided; they (largely) delimit the first of the two charts on the following page. As the below chart shows, it is the labelled mag 5.56 star in the previous chart above that is the focus here, the other was originally highlighted as an aid to identification (two close together and of the same brightness). The chart shows stars down to around mag 13.7, still a little brighter than Pluto. Use it to identify, through a low-medium power eyepiece, the 8 th, 9 th and 10 th magnitude stars labelled in yellow, particularly that of magnitude (TYC , hereafter referred to, for convenience sake, as just TYC), then proceed to the final magnification, the second chart on the following page, to nail Pluto.

16 This chart shows all stars down to magnitude 15.5, considerably fainter than your planetary target. The angular distance from TYC to Pluto is 4' or one fifteenth of a degree, so use your highest practical

17 magnification, which will probably provide a field of view of somewhere in the region of 2-5 times this separation. Place TYC just to one side of the centre of the field of view, and look first for the mag 12.9 star (USNO J ), then the brighter USNO J , mag All three stars will appear close together, with intervening stars 15 th magnitude or fainter and thus unseen in other than quite large scopes. Adjacent to 11.9, just 1⅓' away, sits, as shown, a mag star (USNO J ); Pluto, virtually superimposed on this star, is represented by the white cross coincident with the star, the two only 4" just a little over the span of Uranus disk apart. Note the two further white crosses, signifying Pluto s position at the same time (12:17 am) on the two following mornings of the 16 th and 17 th a re-visit is the best way to confirm capture of the tiny frozen rock on the outskirts of the solar system proper. On July 31 st, Pluto is 18 clear of the horizon at sunset, 5:32 pm, 35 high when twilight fades at 7:02 pm, and transits at 11:08 pm; visual magnitude is minimally adjusted, to Pluto will be within the constellation of Sagittarius until 2023/24. Continuing our run of planetary nebulae features, this month we visit the constellation Ophiuchus to hunt down NGC 6572, first viewed by the German astronomer Friedrich Struve. One on-line reference calls it The Blue Racquetball, another the Emerald Nebula; distance estimates vary, 3500 light years is a ballpark figure. Regular readers of this column will be familiar with my oft stated contention that almost any inhabitant of the night sky is easy to find if the correct approach is taken; although considerable difficulty was encountered locating this planetary, the mantra holds true, as the initial approach was flawed. Providing the telescope being used has enough light gathering power to show a given target, then a failure to find it can only be for one of two reasons, looking in the wrong spot or looking for the wrong thing. My own episode involved the latter fault employing a 17mm eyepiece returning a modest magnification of 88x, I was expecting a small but noticeable green haze, but passed over the nebula many times until realizing more magnification must be required. Having a good idea where to look, I soon noticed a star with prominent green colouration that displayed, upon close examination, a slightly bloated appearance. Sure enough, targeting it with a magnification of 125x and then 250x, yielded the view expected earlier. It s not often that the highest magnification available gives the most satisfying view, but that was the case with regards to this planetary, which is variously quoted as spanning between 6" and 14" (it was a reference to the higher figure that caused the difficulty in the first place; 6" or less is a fair estimate of the view through the 12 inch reflector employed in researching this article). The nebula has no nearby strong pointers to its location, so an original approach was called for. After consulting Starry Night software, which is invaluable in the preparation of these viewing notes, the following approach was decided upon, with initial reference to the following chart, configured for 10:30 pm (on the 14 th ).

18 First identify the Teapot in Sagittarius and, currently conveniently situated nearby, Saturn. Next locate the long arc of four 2 nd magnitude stars, each circled, gently curving to the left and down from Saturn, which may be regarded as a remote member of the arc. The four stars in question are, from right to left, Sabik (Eta [η] Ophiuchi), mag 2.4, Han (Zeta [ζ] Oph), mag 2.5, Yed Prior (Delta Oph), mag 2.7 and Unukalhai (Alpha Serpentis), mag 2.6. Note that Yed Prior has a magnitude 3.2 companion, and Unukalhai sports 3 rd magnitude neighbours before and after it on the line of the arc. Next, look in the inward direction of the arc (NE) about halfway to prominent Altair (Alpha Aquilae), mag 0.75, and Vega (Alpha Lyrae), mag 0.0, for the trapezium of circled stars as shown; anticlockwise from top right they are: Cebalrai (Beta [β] Oph), mag 2.75, Kappa [κ] Oph, mag 3.2, Rasalgethi (Alpha Herculis), mag 2.75 and Rasalhague (Alpha Oph), mag 2.1. Refer then to the chart on the following page, which depicts stars down to magnitude 6.0 and is intended for naked eye use (the fainter stars will probably go unseen). Focus on the bottom two members of the trapezium and identify the close naked eye pair of 3.68 and 4.62 (respectively 72 Oph and 71 Oph) that they point towards. Look nearby, as shown, for the small asterism, traced in orange, of predominantly 4 th magnitude stars; regard that asterism as the apex of a shallow isosceles triangle with base 3.68 to 3.21, thus identifying the latter star (which is Eta Serpentis). Next, construct another shallow isosceles (almost) triangle with the same base, but pointing in the opposite direction, with its apex at the faint naked eye 4.84 (74 Oph). With this star logged, you are well placed to capture the planetary which, as the chart shows, lies about ⅓ of the way along a line from 4.62 to 4.84 (you may wish to start from the more prominent 3.68 and look slightly left of the line). Target the indicated portion of sky best done through a Telrad (a non-magnifying finder) or similar then, bearing in mind the difficulty described earlier of picking the nebula out from the star field at low magnification, use medium to high power (120x - 150x is a good place to start) to sweep the field looking for the very small green haze which is the nebula.

19 All star charts courtesy of StarryNight Pro TM Version /Simulation Curriculum Corp. Experienced observers may wish to look directly for 4.84 after the initial identification of the 3.68 / 4.62 pair; the more drawn out process is designed to negate the difficulty arising from the faint appearance, to the naked eye, of 4.8. That completes The Australian Night Sky for this month; we ll return next month to break the back of winter. As always, questions, comments and suggestions are welcome, and may be directed to: waynerobertsau@yahoo.com.au Until next month: