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In mid-winter, rug up and take advantage of extended viewing hours with:

The Australian Night Sky - July 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; July 2017:

 1st First Quarter Moon.
 4th Earth at aphelion (farthest from Sun, 152.0926 million km/1.016676 au).
 6th Moon at apogee (farthest from Earth, 405,934 km).
 9th Full Moon.
10th Pluto at opposition.
14th Neptune occulted by Moon (not from Australia).
17th Last quarter Moon.
20th Aldebaran (Alpha [α] Tauri, magnitude 0.8) occulted by Moon (not from Australia).
22nd Moon at perigee (closest to Earth, 361,236 km).
23rd New Moon.
25th Mercury occulted by Moon (not from Australia);
     Regulus (Alpha Leonis, magnitude 1.3) occulted by Moon (not from Australia).
27th Mars in conjunction with Sun.
28th Piscis Austrinids meteor shower peaks.
30th Alpha Capricornids meteor shower peaks;
     Southern Delta [δ] Aquariids meteor shower peaks;
     Mercury at greatest elongation east (of the Sun), 27.2°.
31st First quarter 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.

The Moon occults two planets and two stars this month – Neptune on the 14th, Aldebaran on the 20th and first Mercury and then Regulus on the 25th; unfortunately, none of the events are observable from Australia.

Neptune’s occultation is visible from most of Antarctica and New Zealand. The close miss in our skies sees Neptune less than 7' (1', or one arc minute, is 1/60th of a degree, and may itself be subdivided into 60", sixty arc seconds) to the NW, or lower left of the Moon’s limb, almost directly opposite the lower end of the terminator. Our natural satellite may actually help to pinpoint Neptune through binoculars or a finder ‘scope, there being no stars rivalling the planet in brightness between it and the Moon’s limb. The glare from the 80% illuminated waning gibbous Moon may be problematic in this regard; you should be able to counter this by placing the Moon just outside the field of view. Closest approach, at 4:17 am, occurs with the pair high in the northern sky, 59° clear of the horizon.

The event involving Aldebaran, which can be seen from India, Hawaii and Central & NE Asia, is less accommodating from our point of view – at closest approach, the star, 35° high in the north, is just over 1° above the limb of the 17% illuminated waning crescent Moon at 9:49 am.

As Mercury and Regulus are separated by just a little over 1° on the evening of the 25th, their dalliances with our Moon occur in quick succession, just over an hour apart; neither is an occultation from our perspective, and closest approach is below our western horizon in both cases. Mercury, which is occulted from N. Europe, Britain, most of Greenland and the northern half of Asia, is closest to the 6% waxing crescent Moon (see the explanatory notes at the beginning of these notes if you are unfamiliar with the terminology applied to phases of the Moon), just under 1° from the limb, at 8:56 pm, 15° below the horizon. Regulus (occultation visible from Indonesia, S. India, the Middle East and the northern half of Africa) passes considerably closer, less than 8', at 10:02 pm, 28° below the horizon.

As the monthly event summary above shows, three meteor showers peak towards the end of the month. First come the Piscis Austrinids on the 28th, which according to the IMO is not well documented. It has a modest ZHR of just five; some can be expected to be bright enough to leave visible trains. On the 28th, the radiant transits at 2:43 am, close to the zenith at an altitude of 82°; as the early morning hours are generally favoured for meteor activity (do you know why?), and the Moon sets at 9:44 pm on the 27th, conditions are favourable.

Two showers are set to peak on the morning of the 30th, each notable, and for differing reasons. The Alpha Capricornids are a minor shower in terms of a meteor count, with a ZHR of only 5. This is balanced, however, by the often spectacular nature of the individual meteors, which move relatively slowly across the sky but nevertheless are often bright, even attaining ‘fireball’ status, the descriptor denoting meteors as bright or brighter than brilliant Venus – i.e. magnitude -4 or better.

Rising at 5:46 pm on the evening of the 29th, the radiant transits at 12:19 am (on the 30th), at a healthy altitude of 62° in the north, and remains above the horizon until 6:53 am, over an hour after morning twilight commences at 5:51 am. Additionally, the Moon continues to co-operate, setting at 11:44 pm (on the 29th) as a 39% waxing crescent, leaving the morning hours unencumbered by its glare.

Sharing the morning glory are the Southern Delta Aquariids, which rates highly as a consistent and reliable southern shower. While meteors produced by this shower are relatively faint, they compensate with strong numbers, with ZHR quotes of 16-25 to be found in the literature. Rising at 7:34 pm on the night of the 29th, the radiant subsequently transits at 2:27 am, 68° clear of the horizon, and is therefore well placed in the Moon-free pre-dawn hours. This shower’s peak is broad, spread over a couple of days; peak hourly counts of 40 were reported on the night of the 28th/29th in both 1977 (from Australia) and 2003 (Crete).

The following chart shows the radiant of all three showers at 12:19 am on the 30th (when the Alpha Capricornids radiant transits). The radiant of the Piscis Austrinids, when it transits on the morning of the 28th, will be situated higher than depicted here, 82° vs 61°, and in the NNW rather than NNE; note that the star Fomalhaut (Alpha Piscium, magnitude 1.2) is concealed by the ‘P’ of the label Piscis Austrinids.

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THE PLANETS

Mercury

The innermost of all the planets is destined to ride high, by its modest standards, in the evening western sky this month and next. As July begins, it is still well within evening twilight, being at an altitude of just 7° when the Sun sets at 5:11 pm, subsequently setting itself at 5:58 pm; its 5.3" disk is 91% illuminated and shines at magnitude -1.1. You may be able to spot it with the naked eye, close to the horizon, a little north of where the Sun set; your chance of doing so will be greatly enhanced if you pinpoint it first with binoculars or a finder ‘scope – if you attempt to do so, make sure the Sun is below the horizon to avoid the prospect of instant and irreparable eye damage.

On the evening of the 10th, Mercury is only one day short of breaking free of twilight; it doesn’t set until 6:46 pm, 1½ hours after sunset (5:16 pm), at which time it’s 14° clear of the NW horizon. The span of the disk is out to 5.8", 76% illuminated; although its brightness has declined, to magnitude -0.40, the greater separation from the Sun will make it a comfortable naked eye target. The reason its circumstances for this particular evening are provided is that it skirts the northern (right) edge of the open star cluster M44, known as The Beehive. Although planet and cluster will be close to the horizon by the time the sky has darkened sufficiently to detect the latter, monitoring Mercury through your ‘scope after the Sun has set, with a little patience, a keen eye and a flat western horizon, should pay dividends.

We’ll nominate Saturday 22nd as our viewing night this month, New Moon falling on the following day. On the evening of the 22nd, Mercury is well clear of the twilight zone, not setting until 7:32 pm, more than ½ hour after the sky fully darkens at 6:57 pm, when it’s still 6° above the horizon; the 58% lit disk spans 6.8" and is prominent against the dark sky at magnitude 0.15. The below chart shows the scene at 6:30 pm, with the planet 11° clear of the WNW horizon. It also plots Regulus, 4½° away and shining at magnitude 1.3, which as indicated earlier is only just over 1° from Mercury three days later on the 25th – this serves to illustrate the planet’s speedy movement against the background stars, a trait which earned it the title Messenger of the Gods in earlier times. The other two stars shown, Algieba (Gamma [γ] Leonis) and Zosma (Delta Leonis), shine at magnitudes 2.0 and 2.5 respectively.


Mercury reaches its greatest angular separation from the Sun on the 30th, when it sets at 7:47 pm, more than 2¼ hours after sunset, 5:31 pm. Its altitude at 7:02 pm, when evening twilight fades, is 8°; the planet’s disk then spans 7.7", is 47% illuminated, and shines at magnitude 0.44. The following evening, being month’s end, sees the above figures little changed; they are, respectively, as follows: 7:48 pm, over 2¼ hours post sunset, 5:32 pm, 7:02 pm, 8°, 7.9", 45% and mag 0.48.

Mercury transitions from Gemini into Cancer on the 5th, and then to Leo, where it sees out the month, on the 17th.

Venus

Although still dominating the predawn sky, Venus rapidly loses altitude throughout the month of July, rising later while the Sun rises earlier. On the first of the month, it remains a beautiful, almost awe inspiring sight, rising at 4:09 am and sitting all of 19° high in the NE as twilight begins nearly two hours later, at 6:02 am. Blazing forth at magnitude -4.13, it displays, through a ‘scope, a span of 18.2" at a slightly gibbous phase of 63%.

Venus has a couple of interesting encounters this month. On the morning of the 13th, it acts as a brilliant counter balance to Aldebaran, anchoring the opposing leg of the A shape of the Hyades, completely altering the spectacle (for a few days either side, as well), while the 27th sees it just over ½° SSW (above right) of the famous supernova remnant M1, the Crab Nebula.

Our viewing night of the 22nd – or more correctly in this case, our viewing morning of the 23rd – falls between these two encounters, and sees Venus conspicuously closer to the morning horizon than at the beginning of the month. It rises just over half an hour later, at 4:40 am; that, in conjunction with the earlier rising time of the Sun and the shallower angle of the ecliptic (causing the planet to rise less steeply away from the horizon over a given time period), conspire to reduce its altitude at the commencement of morning twilight (5:56 am) to 12°. The disk has fattened to a phase of 71%, while reducing in size to span 15.4"; it still commands attention, shining as only it can (bar the Sun, Moon and some transient phenomena) at magnitude -4.03.

Come the end of the month, just over a week later, Venus’ retreat from the morning sky continues; rising at 4:51 am, it is 10° high when twilight commences at 5:51 am; phase, span and brightness stand at 74%, 14.6" and magnitude -4.01.

Venus spends almost the entire month in the constellation of Taurus, crossing into Orion on the 30th.

Mars

Mars is in conjunction with the Sun this month, on the 27th; as such it is not a viable target throughout July. The timing is unfortunate, as it passes through the open star cluster M44, The Beehive, over the period July 30th to August 2nd. The Red Planet transitions from Gemini into Cancer on the 17th.

Jupiter

Mighty Jupiter sets before midnight from the 16th onwards, but still rides high in the early evening, shining brighter than any star in the night sky. On the 1st, the King of the Planets transits at 6:35 pm, just over ten minutes before evening twilight fully subsides (6:46 pm), and remains above the western horizon until 12:50 am (on the morning of the 2nd); it displays a disk spanning 37.4" shining at magnitude -2.04.

Come our designated viewing night of the 22nd, the planet transits at 5:19 pm, just over five minutes before sunset; span and visual magnitude are down to a still generous 35.2" and -1.92. Jupiter is easy to locate with the naked eye due to its brightness, but the following chart, configured for 6:30 pm on the 22nd, is provided to assist in doing so. Saturn and Pluto are plotted for later reference.


The magnified view which follows, configured for the same time, shows Jupiter’s four Galilean moons; the time has been chosen to facilitate observation of the close approach of Io, mag 5.75 and Europa, mag 6.0 (Callisto and Ganymede shine at mag’s 6.4 and 5.3 respectively). Apart from one 8th magnitude star at or past the extreme bottom right of the chart, others in the area are of 12th magnitude or fainter, so none are depicted.


Note that Jupiter’s equator, and the line formed by the moons, will be closer to the horizontal than shown here; I was unable to fully correct for the inclination introduced when magnifying the view. Io, the moon with the innermost orbit, is at its maximum separation from Jupiter (from our perspective), and just beginning to move inward, while Europa is moving away from its parent. Callisto, with the widest orbit of the four, is moving in, while on the other side of Jupiter, Ganymede is at maximum separation and due to begin closing on Jupiter just before 7:50 pm. Io and Europa are 15" apart at 6:30 pm; closest approach will be under 9" just before 7:30 pm – watching through your ‘scope at high power over the one hour interval, you’ll be able to see the gap between the two dwindle. Unfortunately the Great Red Spot doesn’t begin to rotate onto the eastern edge of the disk until around 9 pm; you’ll probably have trouble spotting it before 9:45 pm or so.

On the 31st, Jupiter sets at 11:07 pm, spanning 34.4" and shining at magnitude -1.87. The giant amongst the solar system’s planetary inhabitants will traverse the constellation of Virgo until November, when it crosses the border into Libra almost in lockstep with Venus – the two brightest planets will be just over ½° apart very low in the east before sunup on Nov 13th and 14th (they rise a little over ½ hour before the Sun), a good reason to get up early

Saturn

Having been at opposition last month, beautiful Saturn is well placed for viewing throughout July; the fabulous ring system continues to open from our perspective, the inclination on its way to reaching a nearly 15 year maximum, just shy of 27°, in a few months.

As July gets underway, the Ringed Wonder is already at an altitude of 31° in the eastern sky when twilight subsides at 6:46 pm, and transits at 11:12 pm. Saturn’s disk spans 18.4", the rings 41.5" at an inclination of 26.68°; the system as a whole shines brightly at magnitude 0.1.

On our viewing night of the 22nd, the sky fully darkens at 6:57 pm, with Saturn 51° clear of the ENE horizon, subsequently transiting at the more convenient time of 9:44 pm. The span of the disk has decreased just a little, to 18.0", as has that of the rings, to 40.9", the inclination having increased to 26.76°; visual magnitude stands at a still prominent 0.2.

Consult the earlier charts in the notes on meteors and also on Jupiter to see where Saturn sits relative to Antares (Alpha Scorpii, mag 1.0), the luminary of Scorpius, and the teapot asterism in Sagittarius; relating the chart to what you see in the sky will quickly lead to identification of Saturn, brighter than any star in the area and shining with a subtle, but easily noticeable, yellow hue. Having bagged the planet, turn your ‘scope on it and hunt down its seven brightest moons with the aid of the two following charts, both of which are configured for 8:30 pm, with Saturn at an altitude of 67° in the NNE. The first chart shows five of the seven which sit, at that time, closest to Saturn; the other zooms out to pick up Titan and Iapetus. As with the magnified Jupiter chart, Saturn’s equator and ring plane will be closer to the horizontal in the sky than shown here.

As all the stars in the field of view of this chart are 16th magnitude or fainter, they are not depicted. The labelled moons shine at the following magnitudes: Tethys 10.4, Enceladus 11.9, Mimas 13.1, Dione 10.6 and Rhea 9.9. Four other fainter moons can also be seen just outside the ring system; for those with very large ‘scopes, they are Janus, mag 14.8, inside Mimas; Pandora, mag 16.8, very faintly visible inside Janus; Epimetheus, mag 16.0, at lower left, and Prometheus, mag 16.1, at lower right. The second chart:

The star field has been retained in this chart as an aid to identifying the two outlying moons. They will serve as a help rather than a hindrance, as the brightest stars in the area covered by the chart are those circled to the left of Saturn, of magnitudes 12.2 (USNO J1724207-214858) and 12.6 (USNO J1724286-215205), with all stars closer to the planet than the latter being more than a full magnitude dimmer than it.

First identify Titan, which at a relatively brilliant magnitude 8.6 will be unmistakeable – make sure you look in the correct direction; there is a star, HIP85247 (not shown) of magnitude 8.8, very similar to Titan, in the opposite direction from Saturn, almost twice as far from the planet as is the moon. Look then for Iapetus, magnitude 10.3, about 2¾ times as far from Saturn as is Titan, in nearly exactly the same direction as it. All stars closer to Iapetus than that of magnitude 13.6 (USNO J1724080-215410), circled above the moon, are even dimmer than that star, so identification should not be difficult.

Note that all magnitude ratings given above have been sourced from Starry Night software with the exception of that for Iapetus. The mag 10.3 rating is my (confident) estimate, necessitated by the fact that the software – along with all other resources at my disposal – does not take into account the significant variation in brightness caused by one hemisphere of the moon having a much higher albedo than the other (Starry Night quotes magnitude 11.3 – a discrepancy of 1.0 represents a 2½ fold difference in brightness on the logarithmic scale used for the purpose of assigning magnitude ratings to stars). At the end of July, Saturn sits 59° high in the NE at the cessation of twilight, 7:02 pm; it transits a little over two hours later, at 9:07 pm. The disk and rings span 17.8" and 40.4" respectively, the latter inclined at 26.79°; visual magnitude stands at 0.25.

Saturn will remain within the constellation of Ophiuchus until the second half of November, so those who place any store in the quaint practice of astrology are in strife until then

Uranus

Uranus continues to claw its way into our morning skies, although views in the pre-dawn hours are still required to catch it at a decent elevation above the horizon. At the start of the month, the innermost of the two ‘ice giants’ at the edge of the solar system proper (if you exclude poor old Pluto, demoted to dwarf planet status by the boffins) breaches the eastern horizon at 1:59 am. It becomes a respectable target, defined here as one at least 30° above the horizon, a little over an hour before the commencement of morning twilight, 6:02 am, by which time it sits 38° high in the NNE. The planet’s disk spans 3.5" and shines at magnitude 5.84.

When our viewing night rolls around on the 22nd, (the morning of the 23rd, in this case), rise time has improved to 12:34 am, and altitude as the first traces of light invade the eastern sky (5:56 am, just eight minutes before the planet transits), to 42° in the north. The disk, still spanning 3.5", shines imperceptibly brighter, at magnitude 5.80. The following chart, the faintest stars shown being around magnitude 5.5 (typically as faint as you’ll detect with the naked eye), shows the scene at 5:00 am, with the planet 40° clear of the northern horizon:

Begin your search with the naked eye, by relating the chart to what you see in the sky. While a number of reference points are labelled to help in orientation – Venus, open clusters The Pleiades and Hyades (with Aldebaran at the latter’s lower right) and Menkar (Alpha Ceti, magnitude 2.5) – you should begin your search by first identifying the Great Square of Pegasus, in particular the star at its top right corner, Algenib (Gamma Pegasi), magnitude 2.8. Next identify Eta [η] Piscium, fainter at magnitude 3.6, but still comfortably naked eye, and conspicuous by virtue of the fact that, as the chart shows, the patch of sky around it is devoid of stars bright enough to be seen without some form of magnification. From Eta, look then for Omicron [ο] Psc, the (unlabelled) star just to Uranus’ upper left on the chart, fainter again, but still naked eye (under a dark sky) at magnitude 4.25.

Having Omicron, which is just over 1° from Uranus, in your sights, you are now ready, with reference to the below chart (configured for the same time), to target the area of concern through your finder ‘scope; this should allow you to bag your quarry:

This chart, which labels stars with their magnitudes, shows only those of magnitude 9.5 or brighter, to approximate the view through a finder ‘scope. It basically magnifies the area of sky between Eta Psc, labelled at lower left as magnitude 3.59, and Omicron Psc, magnitude 4.25, also labelling Pi [π] Psc, magnitude 5.53. As indicated by the chart, there is only one star in the area in which you now know Uranus lies, which is as bright as the planet – that of magnitude 5.9, catalogued as HIP8588. This fact, along with Uranus’ subtle blue-green hue and relatively steady glow as compared to the flickering stars (look it up on the ‘web or drop me a line if you don’t know why this is), should allow you to make a positive identification.

The planet’s position at the start of July is marked with a white cross; at the end of the month it has barely moved from its plotted position on the chart – even by its relatively pedestrian standards, it’s moving only very slowly against the starry backdrop towards the end of July, courtesy of the fact that it is stationary on August 3rd, reversing direction as retrograde motion begins (see the explanatory notes at the beginning of these viewing notes).

Confirm capture by switching to the main eyepiece at high magnification (at least 150x is suggested) to resolve the planet’s tiny disk, the colouration of which will be considerably richer and more aesthetically appealing.

As the month of July concludes, Uranus is rising at 12:03 am and still sits 42° high in the North as twilight commences (at 5:51 am, almost twenty minutes after time of transit); the span of the disk is incrementally improved, to 3.6", as is the brightness, to magnitude 5.78.

Uranus will remain within Pisces until 2018/19.

Neptune

Neptune, officially the outermost planet (although Pluto always gets a mention here), is gaining altitude late in the evening as it progresses towards opposition in early September. On July 1st, it is rising at 10:19 pm and subsequently transiting at 4:43 am; the disk, which spans 2.3" all month, shines at magnitude 7.86.

On the 22nd, our nominated viewing night, the planet has become considerably more accessible, rising and then transiting at 8:55 pm and 3:19 am respectively; the planet shines incrementally brighter, at magnitude 7.83. Consult the chart in the meteor notes to determine where Neptune sits in relation to the asterisms The Circlet in Pisces and the ‘Y’ of Aquarius, each composed principally of 3rd and 4th magnitude stars and so visible to the naked eye. Refer then to the following chart (intended for use without visual aid), configured for midnight of the 22nd/23rd, and which magnifies the area from the ‘Y’ to Neptune and beyond, with the planet at an altitude of 35° in the ENE sky.

The chart, which labels stars with their magnitudes, excludes all those dimmer than magnitude 5.5 (the unlabelled star at far right shines at magnitude 4.96), and so approximates a naked eye view under a dark sky; it is intended to allow you to identify, with the naked eye, Lambda [λ] Aquarii, magnitude 3.71, and Phi [φ] Aqr, magnitude 4.21. As these stars are closer to the ‘Y’ than any other you will see, with the exception of ‘5.03’ (Kappa [κ] Aqr), and brighter or as bright as any other in the area immediately beyond them, identification should not be difficult. Having pegged these two stars, refer to the final chart below, which magnifies the region between them and should allow you to bag your planetary target. Note that it also includes markers (white crosses) to indicate where the planet sits at the start and end of July.

This chart is intended for use in conjunction with views through your finder ‘scope; as such, only stars of magnitude 9.5 or brighter are depicted (this is a subjective call; the limiting magnitude through your finder will depend on the instrument itself, sky conditions, and your eyes, but it’s in the ballpark). No stars below the ecliptic (the green line) are labelled, being away from the area of concern, nor are those above the ecliptic which are dimmer than Neptune, with one exception – that of magnitude 7.84 (TYC5813-25-1), which to all intents and purposes is the same brightness as Neptune and is labelled as a reference in that regard.

As indicated by the chart, Neptune is the dimmest member of a group of relatively bright stars about ⅔ of the way from Phi Aqr to Lambda Aqr (which delimit the view). This relationship, Neptune’s subtle blue-grey hue and, as with Uranus, its relatively steady glow, should allow you to locate the planet. When you think you have done so, seek confirmation by switching to the eyepiece at the highest practical magnification (I suggest at least 250x) to resolve its miniscule disk with close and careful examination.

On the 31st of the month, Neptune crests the eastern horizon at 8:19 pm, then transits at 2:44 am. Moving very slowly among the stars courtesy of its great distance from us, Neptune will inhabit the constellation of Aquarius until 2022/23.

Pluto

Pluto, the outermost planet of all (a little dig at the boffins) reaches opposition this month, on the 10th, so prime viewing time is with us over the months to come.

At the beginning of the month, the frozen orb in the outer reaches of the solar system still lies beneath the ESE horizon at sundown, 5:11 pm. It rises at 5:47 pm, and has attained an altitude of 10° when the sky fully darkens at 6:46 pm, subsequently transiting at 1:00 am. Pluto’s disk spans 0.098" (all month, far too small to be resolved by amateur instruments) and glows unseen by all but those with a will to pursue it, at magnitude 14.16 (memo to self – having made only one serious attempt, which failed, a re-visit is overdue!).

On the evening of July 22nd, almost a fortnight after opposition, the race is on, with Pluto already 11° above the horizon as the Sun goes down (5:25 pm). It is 29° high at the end of evening twilight, 6:57 pm, and transits at 11:35 pm; visual magnitude is unchanged to all intents and purposes, at 14.17. Wide field charts in the meteor and Jupiter sections show where Pluto sits in relation to the three stars below (from our perspective) the handle of the Teapot asterism in Sagittarius. Here’s a magnified view, configured for 11:00 pm on the 22nd (as will be those to follow in relation to Pluto), when the planet has attained an altitude of 72°, very high in the sky where the best views, conducive to spotting and identifying such a dimly glowing target among the rich star fields of Sagittarius, are to be had.

The chart labels, with its visual magnitude, Kaus Borealis (Lambda Sagittarii, mag 2.81, the star at the tip of the Teaspoon’s lid), as well as the three stars mentioned above – Albaldah (Pi Sag), mag 2.87, Omicron Sag, mag 3.75 and Xi22] Sag, mag 3.50. The next chart is delimited by Albaldah and the area of sky beyond Pluto; as well as plotting Pluto on the 22nd, it includes white crosses to indicate its position on each of the other Saturdays in July, from the 1st at right to the 29th at left.

Stars brighter than magnitude 9.0 are labelled here (the usual 9.5 would have cluttered the view a little); as such, it may be used in conjunction with your finder ‘scope, but might be better suited for the main eyepiece at low power. Relating the chart to what you see in the sky, the best approach would seem to be to first identify, by reference to Albaldah, the arc of 8th magnitude stars beyond Pluto (mag’s 8.46 through 8.21). The angular distance from Albaldah to these stars varies from around 1½° to 2°, so ideally, if employing the main eyepiece, you want to use an eyepiece which, when fitted to your telescope, would give a field of view of 2° or a little more; this requires a simple calculation.

The actual field of view is given by the eyepiece’s apparent field of view (AOV) in degrees multiplied by its focal length in millimetres (which should both be marked on the eyepiece itself or its packaging), and divided by the telescope’s focal length, again to be found on the ‘scope itself or its accompanying documentation (as the magnification of a combination is simply the focal length of the ‘scope divided by the focal length of the eyepiece, this can be restated as actual field of view = AOV divided by magnification).

As a general rule, the AOV of modern eyepieces varies from around 50° for an inexpensive item to 100° or more for those requiring deeper pockets. Therefore, to take a couple of examples, if your ‘scope has a focal length of 1200 mm, then a 40 mm eyepiece with a 50° AOV would give an actual field of view of 50 x 40 / 1200 = 1⅔°, while the same focal length with a 100° AOV would yield 3⅓°. Remembering that identifying the above-mentioned arc of stars using Albaldah as a starting point is best accomplished through a field of view of a little over 2°, it’s incumbent on you to use the above information to choose the appropriate eyepiece – this is a bread and butter exercise for experienced viewers, and I trust the explanation given above will suffice to guide those with less time at the eyepiece.

Having identified the stars comprising the arc, use the member of magnitude 8.21 (TYC6308-535-1) to identify the nearby star of essentially the same brightness, that of magnitude 8.18 (HIP94765), which will serve as your reference point in the penultimate chart below (note that ‘8.18’ is almost superimposed, in the above chart, on a star of magnitude 9.78, and that two such stars will together appear to shine at mag 8.0).

This chart shows stars down to magnitude 16.0, much fainter than Pluto (the software used in compilation of these charts is capable of showing considerably fainter stars still, but they would clutter the field, and go unseen in other than quite large ‘scopes); note that its position on the 15th is designated by a white cross. A straight line from ‘8.18’ through Pluto to the top left of the chart spans around ⅓°, so that is the field of view required in this case; adapting the examples given earlier, an 8 mm eyepiece would yield ⅓° or ⅔° depending on the AOV (and the telescope’s focal length). The chart, only labels those stars brighter than magnitude 11.0, and the best technique for using it to close on your target would seem relatively straight forward (in theory ), and is as follows:

Note that the trio of ‘8.18’, ‘9.78’ and ‘9.43’ form one group of relatively bright stars, and ‘10.31’ & ‘10.81’ another, albeit dimmer than the first. Having identified these two groups, ‘10.53’, a slightly lesser distance further on than the spacing between them, and of similar brightness to members of the latter, should be conspicuous, courtesy of the fact that the brightest star between that group and ‘10.53’ is much fainter, at magnitude 12.5. Once you’ve spotted ‘10.53’ (TYC6308-1161-1), refer to the final chart on the following page, which should allow you to identify Pluto.

This chart is delimited by ‘10.53’ at top right and the area beyond Pluto at bottom left. As it spans less than 3', or 1/20th of a degree, use the highest power magnification at your disposal – i.e. the eyepiece with the shortest focal length. Again reworking the earlier examples, even a 4 mm eyepiece, which is smaller than any in my kit, would deliver views 10' or 20' across (in a ‘scope with 1200mm focal length) depending on the AOV, so the area of sky covered by the chart will occupy only a relatively small area through your eyepiece in such a case.

Between Pluto and ‘10.53’, almost all the stars are dimmer than your target, which hopefully will simplify deciding which faint point of light is the culprit; if not, and the others are visible, star hop from one to the other to get to your destination. Take care not to confuse the planet with the nearby star of very nearly the same brightness, that of magnitude 14.10 (USNO J1915443-213257). At this magnification, Pluto moves noticeably relative to the stars over the course of a few hours, so its position at 9 pm and 3 am is also shown.

Note that the fuzzy appearance of Pluto at this resolution is due to the depiction of its moons by the software. Note also that this chart is angled slightly differently to the earlier three (compare the angle, between the last two charts, of a line from ‘10.53’ to Pluto); this was, as per the situation with charts on Jupiter and Saturn, an unavoidable artefact of magnification. A couple of more points of note regarding this chart – firstly, although set to show only stars brighter than mag 16.0, the software insisted on depicting one, near the top of the chart, of mag 16.05, hidden behind the last ‘5’ of the star of magnitude 15.55; secondly, there is a star of mag 15.50 very close to Pluto, behind the “Pl” of its label (both these stars will probably go unseen anyway, unless your ‘scope is quite large).

As Pluto’s disk is too small to confirm identification by resolving it, you may wish to take an image or make a sketch of the surrounding star field and return the following night to see which point of light has moved; to this end, its position at 11:00 pm on the 23rd is shown at the left of the chart.

On July 31st, Pluto is 37° above the horizon as twilight fades, 7:02 pm, and transits at 10:59 pm; its brightness has further reduced fractionally, to mag 14.18; it will continue to drift among the stars of Sagittarius until 2023/24.

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Our Monthly Feature

This month, we feature the globular cluster M5 in Serpens Caput; Messier 5 was discovered by Gottfried Kirch in 1702 while searching for comets. Charles Messier saw it in 1764, but thought it was a nebula without stars; William Herschel first resolved stars within it, counting approximately 200, in 1791.

M5 is a relatively large globular cluster, around 165 light years in diameter, but because of its large distance from us, some 25000 light years, it is quite small through the eyepiece of an amateur ‘scope. Containing upwards of 100,000 and perhaps as many as 500,000 stars, it is, like all globulars, very old – probably older than most – with one reference quoting an age of 13 billion years.

To locate M5, first refer to the following chart, which is configured for 7:30 pm on the 22nd, and concentrates on the region of sky between Arcturus (Alpha Bootis, magnitude -0.07), at bottom left, and Antares (magnitude 1.03) & Saturn towards top right.

To identify where the globular sits in the sky, the best approach is to first identify the 2nd and 3rd magnitude stars which form a rough line, perhaps better described as a gently curving arc, between Saturn and a point a little below Arcturus (were Saturn not situated where it currently is, the text would read “between Arcturus and the Teapot asterism of Sagittarius”). While the chart labels these 2nd & 3rd mag stars with their magnitudes to differentiate between them and denote their brightness, you will find it helpful to look back at the wide field chart in the notes on Jupiter, which lacks labels and shows the line of stars more clearly. The stars can be segmented into two groups, one closer to Arcturus and the other to Saturn. Turn your attention to the former, which is in the vicinity of the globular, and then refer to the following magnification.

This chart shows the afore-mentioned line of stars at bottom right; it also shows a line of generally fainter stars (one is 3rd mag) angling down to meet the first line, forming a lopsided ‘V’. While much fainter, all these stars are visible to the naked eye under a dark sky (note that ‘5.12’ is obscured, in the earlier chart, by M5’s label). Once you’ve spotted these stars, your task is all but complete, for – as the chart shows – M5 lies very close, just over ⅓°, from the central member, magnitude 5.03 HIP74975 (aka 5 Serpentis). The three 5th magnitude stars are quite faint to the unaided eye; best to begin from the brighter end, tracing out ‘3.71’ (HIP72220 or 109 Virginis), ‘4.37’ (HIP73620 / 110 Vir) and ‘5.03’.

Having identified ‘5.03’, target it with your finder ‘scope and you’ll immediately see the small fuzzy patch which is M5. With the globular centred in the field of view, switch to the main eyepiece and explore the view at a range of magnifications; as indicated earlier, it will appear quite small due to its distance from us, so the best view will be afforded by the highest magnification equipment and conditions will allow. The globular has a noticeably condensed core and is quite aesthetically pleasing; here’s an image; sourced from the venerable Wikipedia, which is close to what I see through my 12 inch Dob.

All star charts courtesy of StarryNight®ProTM Version 7.5.8.1252/Simulation Curriculum Corp.

That’s all we have for this month folks.

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