[ANS 2016] - ANS 2017 [Jan][Feb][Mar][Apr][May][Jun][Jul][Aug]

 

In mid-Autumn, with longer nights not yet too cold, go exploring with:

The Australian Night Sky - April 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]
Top of page

What's in the sky this month; April 2017:

 1st Aldebaran occulted by Moon (not from Australia);
     Mercury at greatest elongation east, 19°.
 2nd Daylight saving ends at 3 am, reverts to 2 am.
 4th First Quarter Moon.
 6th Saturn stationary (begins retrograde motion).
 7th Regulus occulted by Moon (not from Australia).
 8th Jupiter at opposition.
10th Mercury stationary.
11th Full Moon.
13th Venus stationary.
14th Uranus in conjunction with Sun.
15th Moon at apogee (farthest from Earth, 405,475 km).
19th Last Quarter Moon.
20th Mercury in inferior conjunction.
21st Pluto stationary (begins retrograde motion).
22nd Lyrid meteor shower peaks.
23rd Neptune occulted by Moon.
24th Pi [π] Puppid meteor shower peaks.
26th New Moon.
28th Moon at perigee (closest to Earth, 359,327 km).
29th Aldebaran occulted by Moon (not from Australia).
30th Venus greatest illuminated extent.

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 expires at 3 am on Sunday 2nd, when the time reverts to 2 am; there is, however, no reference in these notes to times between 2am and 3am on the morning of the 2nd. All times referring to AEDT in the text to follow will be labelled as such; all unlabelled times will refer to AEST.

Aldebaran (Alpha [α] Tauri, magnitude 0.8) is occulted by the Moon twice this month; unfortunately neither event is visible from Australia. The occultation of the 1st is accessible to viewers in NE Africa, Arabia, India, Mongolia, China and Japan, while on the 29th, those in North America, Cuba, Europe and northern Africa are favoured.

Locally, closest approach on the 1st occurs at 9:12 pm AEDT, with Aldebaran, 1¼ hour short of setting, at an altitude of 13° in the NW, and situated a little under 1° SE (above left) of the surface of the waxing crescent Moon, which is 24% illuminated. The event of the 29th is yet more poorly positioned for us, closest approach of just over ½° occurring at 4:16 am, 66° below our SSE horizon.

On the 7th, Regulus (Alpha Leonis, magnitude 1.3) is occulted from our location, but unfortunately below our horizon. The event begins at 1:14 pm and ends at 2:03 pm, with the planet still 24° below the eastern horizon; Regulus rises at 4:01 pm, a little over a degree clear of the illuminated limb of the waxing crescent Moon, which rises three minutes later.

We are more fortunate as regards the occultation of Neptune on the morning of the 23rd. The illuminated limb of the 18% waning crescent Moon slides in front of Neptune at 4:57 am, with the planet, having risen at 3:51 am, 12½° above the eastern horizon. Neptune reappears from behind the dark limb at 5:53 am, at an altitude of 23½°. As twilight commences at 5:26 am, the planet’s reappearance plays out under a brightening sky.

If you monitor this event, be careful not to confuse Neptune, which shines at magnitude 7.9, with the quite noticeably, but not greatly, dimmer star TYC5241-749-1, magnitude 8.9. The star trails Neptune into the morning sky only 1½' to the east, or lower right (' denotes arcminute; one arcminute is 1/60th of one degree). Neptune’s pale blue/grey hue will help to distinguish it from the star, as will the fact that the planet will be clearly visible in a finder ‘scope, while the star will be near the limits of resolution.

Two meteor showers peak this month, the Lyrids on the 22nd and the probably minor Pi-Puppids on the 24th. The Lyrids are due to peak, according to the International Meteor Organization (IMO), at 10 pm (on the 22nd). The shower’s ZHR tends to vary between 14 and 23 – 18 is usually quoted – but as with all showers, unpredictability is the name of the game, with a short burst of 80 recorded in 1982. The shower’s parent body is comet Thatcher (C/1861 G1), which has a 415 year orbit around the Sun, the last pass in 1861; no exceptional activity is expected this year, but you never know.

The Lyrids are predominantly a northern shower, which reduces prospects for we in the southern hemisphere. Additionally, the radiant – the point from which the meteors appear to emanate – doesn’t rise until 12:24 am, almost 2½ hours after the predicted peak, further dampening expectations. The Moon cooperates to a certain extent however, not rising until 2:46 am (on the 23rd), and then only as an 18% waning crescent, so some worthwhile activity may be forthcoming.

With this in mind, the following chart is configured for 1:30 am, with the radiant at an altitude of 8° in the NE; the radiant transits – reaches its highest point – at 4:21 am, when it sits 18° high in the north.

The Pi-Puppids, due to peak at 3 am on the 24th, are not expected to put on a good show this year. The comet whose orbital debris is responsible for this shower, 26P/Grigg-Skjellerup, has a period of 5½ years. While ZHR counts of about 40 were seen in 1977 and 1982, years when the comet rounded the Sun, other such visits, and years in between, have not proved fruitful. As the last two cometary visits, in 2008 and March 2013, were non-productive, and the comet will next reach perihelion (its closest point to the Sun) in October next year, activity will probably be low or even non-existent on this occasion.

On the positive side of the ledger, the Moon is completely absent from the sky until 3:51 am, more than ¾ hour after the radiant, which is high in the sky at sunset, sets at 3:04 am. The chart below is configured for midnight of the 23rd/24th, with the radiant at an altitude of 22° in the SW – note that at the time the peak (if one occurs) is expected, the radiant will be only minutes away from setting.

Top of page

THE PLANETS

Mercury

Mercury spends the first couple of weeks of April hopelessly engulfed in twilight in its worst evening apparition of the year. It is stationary relative to the stars on the 10th as it heads back towards the evening horizon, its inner orbit subsequently carrying it between Earth and the Sun on the 20th (when it’s said to be in inferior conjunction) before emerging into our morning skies for its best morning apparition of 2017.

On the 1st it sets at 7:49 pm AEDT, having been at an altitude of only 6° at sunset 35 minutes earlier; despite shining brightly at magnitude 0.0, its 7.6" disk, 42% illuminated, is consumed by the glow of twilight.

As New Moon falls late on Wednesday 26th, we’ll nominate the following Saturday 29th as our viewing night this month. Because views of Mercury at this time are in the predawn hours, on the morning of the 30th, no further end-of-month data will be required.

On the 30th, the planet has transitioned to the pre-dawn eastern sky as detailed above. Although only ten days past inferior conjunction, it is considerably better positioned than it was at the start of the month, courtesy of the fact that the ecliptic, the path followed by the Sun and, very nearly, the planets, rises steeply away from the horizon on April mornings. Rising at 5:46 am, Mercury attains an altitude of 14° by the time the Sun rises at 7:00 am. This early in the apparition, however, it still sits within twilight (which commences at 5:31 am), whereby its 9% lit disk, spanning 11" and shining relatively feebly at magnitude 2.7, fails to pierce the twilight.

On this same morning, Mercury is positioned only ⅔° to the upper right of Uranus, but the adverse circumstances render views of the conjunction unviable – on the previous morning of the 29th, the gap is just 1/5°.

Mercury crosses from Pisces into Aries in the early hours of the 1st and then, after reversing direction on the 10th as mentioned earlier, crosses back into Pisces on the 21st, remaining within that constellation for the rest of the month.

Venus

Having been in inferior conjunction late last month, Venus now begins another apparition as the Morning Star; although it re-enters morning twilight after the first week of September, it will continue to rise before the Sun for the rest of the year.

As April begins, Venus crests the eastern horizon at 6:55 am AEDT. At sunrise 39 minutes later, it sits just 7° clear of the horizon, but it’s worth targeting in the interim because the razor thin crescent spans a huge 58" [" denotes arc-second = 1/60th of an arc-minute (symbol ') or 1/3600th of a degree (symbol °)] and shines at magnitude -4.2. If you do so, take extreme care to avoid allowing the Sun to enter the field of view provided by your optics, lest instant and irreparable eye damage results.

Venus breaks clear of twilight on the 8th, and is said to be stationary on the 13th, as it ceases to move westward relative to the stars; it still continues to increase its distance from the eastern horizon, as it rises earlier daily while the Sun rises later.

Come the morning of the 30th, which equates to our viewing night of the 29th, Venus rules the predawn sky in all its glory. The brilliant orb rises at 3:50 am (remember that daylight saving is no longer operable), well over three hours before the 7:00 am sunrise, at which time it’s at an altitude of 35° in the NE. Its disk is still a magnificent sight, 26% illuminated, spanning 39" and shining at a punishing magnitude -4.5, its brightest for this morning apparition.

Venus resides within Pisces throughout April.

Mars

Mars’ eastward movement relative to the stars almost matches that of the Sun; consequently it lingers in our western sky far longer than the other outer planets, not destined to reach conjunction until late July. Nevertheless it is waning as a spectacle, dropping into evening twilight on the 5th and appearing tiny through the eyepiece due to its great, and increasing, distance from us.

At the beginning of April, the Red Planet sets at 8:42 pm AEDT, almost 1½ hours after sunset (7:14 pm AEDT), when it sits 15° high in the NW; its disk spans a mere 4.2", is 96% illuminated (Mars is the only outer planet to show an appreciable phase – it dropped below 85% last September, but you are extremely unlikely to be able to discern the phase this month) and shines at magnitude 1.5.

On our viewing night of the 29th, having almost kept pace with the Sun, setting time is 6:55 pm, still an hour and twenty minutes after sunset (5:35 pm), when its 98% illuminated disk spanned 3.9", shined at magnitude 1.6 and sat at an altitude of 12°. At month’s end on the following night, Mars and the Sun set at 6:53 pm and 5:34 pm respectively; the other data remains as above.

The following chart is configured for the 29th at 6:20 pm, ¾ hour after sunset, as a compromise between the planet maintaining some altitude and allowing the sky to darken a little; its tiny disk will be completely featureless.

Mars begins the month in Aries and passes into Taurus on the 12th, where it sees out the month apart from briefly revisiting a corner of Aries for less than 3½ hours on the afternoon of the 14th.

Jupiter

Giant Jupiter rules the evening sky over the coming months, as it reaches opposition this month on the 8th (note that Jupiter’s documented rise time at this opposition agrees closely with time of sunset; regular readers will be aware that I have flagged that this is, puzzlingly, not always the case – resolution of this conundrum is still a work in progress).

Earlier, as the month commences, the King of the Planets rises at 7:32 pm AEDT, having sat 4° below the horizon at sunset, 7:14 pm AEDT; its disk spans 44.1" and shines at magnitude -2.46. Jupiter transits at 1:52 am AEDT (on the 2nd).

When the 29th, adopted as our viewing night, arrives, rise time is 4:33 pm, now more than an hour before sunset (5:35 pm), at which time the planet sits almost 12° above the eastern horizon; transit time is in to 10:49 pm. Reflecting the fact that the gap between Jupiter and our home planet has begun to increase, the span of the disk is down marginally, to 43.7", as is the brightness, which stands at magnitude -2.42.

The following chart is configured for 10:00 pm, with Jupiter optimally positioned high in the sky, just over ¾ hour short of transiting. This time has been chosen to allow you to witness Europa being occulted by Jupiter, as it slips behind its parent between 10:18 pm and 10:23 pm (there is no need for a wide field finder chart for Jupiter itself – it’s the overwhelmingly brightest ‘star’ in the heavens, high in the sky).

Two other of Jupiter’s Galilean moons are also shown, Ganymede and Io, as is Amalthea (unlabelled), a dim moon of Jupiter at magnitude 14.4 – it’s the barely visible dot midway between Europa and Ganymede. You won’t see Amalthea unless you have a large ‘scope (even then, Jupiter’s glare will likely overpower it); it is included merely to demonstrate that no stars in the field of view of the chart can be mistaken for moons, as the brightest star, USNO J1259036-043607, visible as an equally small dot near the bottom of the chart below the left hand side of Jupiter’s disk, shines dimly at the same brightness as Amalthea.

While Europa will emerge from behind Jupiter, at the other (eastern) side of the disk, over a period of a few minutes between 10:42 pm and 10:47 pm, it will do so unseen as it re-emerges into the shadow cast by Jupiter. That is to say it re-emerges from occultation into eclipse; for those willing to extend their viewing session, Europa can be seen to ‘magically’ appear between 1:45 am and 1:50 am at a distance of about ⅓ the span of Jupiter’s disk from its upper right (on the chart) limb.

At this time, Io will be closing in on Jupiter, about ⅓ as far from the disk as is Europa. Here’s the view at 1:50 am, just after Europa has fully emerged from Jupiter’s shadow; the view zooms out a little to include the fourth Galilean Moon, Callisto:

If you wish to further extend your viewing session, you can watch as Io begins to transit across the face of Jupiter at 2:08 am, followed by its shadow at 2:38 am; Io will depart Jupiter’s disk at 4:21 am, its shadow at 4:51 am (by which time Jupiter will be only ¼ hour from setting).

As is to be expected, the stats for the end of the month are little changed – Jupiter rises at 4:29 pm on the afternoon of the 30th and the Sun sets at 5:34 pm, with the King just under 12½° high; time of transit is 10:45 pm, and the disk spans 43.6", shining at magnitude -2.41.

Jupiter is in Virgo, and will remain there for some months to come.

Saturn

Saturn officially became eligible for the title of evening object last month, rising before midnight. Destined to reach opposition in mid-June, it is becoming accessible by our viewing night late this month.

On April 1st, the Ringed Wonder requires very late night viewing sessions – it doesn’t rise until 11:14 pm AEDT, and transits at 5:29 am, just over 20 minutes after twilight starts to paint the sky the next morning. Its disk spans 17.0", the ring system 38.6", inclined at 26.4°; disk and rings together shine at magnitude 0.41. The inclination of the ring system reverts, around mid-month, from a short term closing trend (due to our differing viewing perspective as we progress along our orbital track) to its current long term opening trend (symptomatic of Saturn’s orbital progression). Consequently, the inclination is 26.44° at the beginning and end of the month, bottoming out at 26.42° mid-month; it will reach a maximum of almost 27° in October.

Saturn is stationary against the starry backdrop on the 6th as it begins retrograde motion and starts to backtrack (relative to the stars) to the west.

Come the 29th, our nominated viewing night this month, the planet’s circumstances have improved considerably (aided by the cessation of daylight saving). It then rises at 8:22 pm before transiting at 3:37 am; the span of the disk has increased a little, to 17.7", and that of the rings to 40.3", and the planet has brightened to magnitude 0.26.

The chart to follow, configured for 11:30 pm on the 29th, shows where Saturn is positioned in relation to the prominent constellation figures of Scorpius (with Antares [Alpha Scorpii], magnitude 1.0, labelled as a reference) and Sagittarius; Saturn has, by this time of the evening, attained a respectable viewing altitude of 35° in the east.

The next chart shows six of Saturn’s seven brightest moons at the same hour; they shine at the following visual magnitudes: Titan 8.6, Rhea 10.0, Mimas 13.2, Enceladus 12.0, Tethys 10.5 and Dione 10.7 (just visible below the ring system is unlabelled Janus, magnitude 14.9, too faint and close to the ring system to be visible in other than large ‘scopes).

All stars in the area covered by the chart are dimmer than magnitude 15, too dim to be seen here or to be confused with the moons. Note that whereas the software generally does a good job of matching the brightness of a given body with the size at which it depicts their symbol, if fails to do so in relation to Saturn’s moons, with Mimas being depicted too small, while the other five are all plotted at the same size, despite the wide variation in brightness.

Titan will be visible in any ‘scope, probably even a finder ‘scope or binoculars, while Rhea, Tethys and Dione will show in a six incher; an eight incher may be necessary for Enceladus, and Mimas may call for a twelve incher. This is only a rough guide, visibility will depend on a number of factors, including light pollution, clarity of optics and visual acuity.

The separate treatment of Iapetus in this column is twofold in that it not only requires a separate chart due to its wide orbit, but also that a different approach is required in determining its visual magnitude. Whereas brightness ratings for the six moons covered earlier are taken directly from Starry Night software, that for Iapetus derives from my own calculation. This is necessary because the brightness of Iapetus, as seen from Earth, depends significantly more on the moon’s 79 day orbit of Saturn than on the Earth-Saturn separation, due to the fact that one hemisphere of Iapetus is much more reflective than the other, the software unfortunately ignoring this significant 79 day cycle. Iapetus has its bright hemisphere pointed almost directly at Earth on our viewing night, the calculated brightness being magnitude 10.4 – the folly of relying on the software is glaringly obvious in this case, as is gives a value a full magnitude higher, 11.4. This final chart zooms out a little to incorporate Iapetus:

Identifying this outlying moon is not as straight forward as for its inner brethren; as the chart shows, there are a number of relatively bright stars in its general location. Nevertheless, these stars can serve as a help, rather than a hindrance, as not only will they serve as reference points, but all are dimmer than the moon with the exception of one.

Left to right on the chart, the labelled stars shine at the following magnitudes: 12.5, 11.1, 12.7, 12.8 and 9.6. The star of magnitude 11.1 should serve as your primary guide, being closest to Iapetus both spatially (on the chart) and in terms of luminosity. To avoid confusion, it should be pointed out that this star, TYC6261-1553-1, actually shines at magnitude 11.4, a little dimmer than indicated here, but is superimposed on another unseen star, designated USNO J1747082-220222, which shines at magnitude 12.3 – together they shine at magnitude 11.1 as indicated.

Iapetus is, at this time, just under 7' – a little over 0.1° – from Saturn (as a rough guide, one finger held at arm’s length spans a bit more than 1°, a closed fist 10°, an open hand, tip of little finger to thumb tip, 20°); looking in the appropriate direction and comparing the star field with the chart should enable you to identify which ‘star’ is actually the moon.

As the month draws to a close on the evening of the 30th, Saturn rises at 8:18 pm, then transits at 3:33 am; its disk spans 17.8", the ring system more than twice that (across the major axis), at 40.4", visual magnitude being 0.25.

Saturn’s usual leisurely stroll against the starry backdrop is accentuated by its stationary status early this month; consequently it remains within the constellation of Sagittarius throughout April, albeit close to the border with Ophiuchus.

Uranus

Uranus is in conjunction with the Sun on the 14th and hence unobservable. For the record it’s at an altitude of 5½° at sunset on the 1st (7:14 pm AEDT), setting itself at 7:45 pm AEDT. After passing through conjunction and transferring to our morning sky, it rises on the 29th and 30th respectively at 5:53 am/5:49 am, attaining an altitude of 12°/13° by sunrise (6:59 am/7:00 am); the planet’s disk spans 3.4" and shines at magnitude 5.9 all month.

Uranus will travel through Pisces until 2018/19, albeit coming within ¼° of the border with Aries early this August.

Neptune

Having itself been in conjunction with the Sun early last month, Neptune is now gaining altitude in our morning sky. At the start of April, it rises at 5:14 am AEDT and is at an altitude of 27° when the Sun crests the horizon at 7:34 am AEDT; its disk spans 2.2" (all month) and shines at magnitude 7.95.

It is certainly worthy of observing time on our viewing night of the 29th – or more correctly, the morning of the 30th – when it rises at 2:24 am, and is 36° above the NE horizon when twilight first challenges the darkness at 5:31 am, sunrise being at 7:00 am; brightness has improved imperceptibly to magnitude 7.93. Here’s a wide field chart configured for 5:15 am:

Use brilliant Venus, at half Neptune’s altitude, to identify the two naked eye stars shown just above left and below right of Neptune (planet and star separated by approximately 2° and 3¾° respectively), then refer to the following magnification, which labels stars with their magnitudes:

This chart is delimited by the two naked eye stars referred to above; they are Phi [φ] Aquarii, mag 4.21, at bottom right of the chart and Lambda [λ] Aquarii, mag 3.71, at top left. All the unlabelled stars among those which are labelled are dimmer than Neptune; that fact, in conjunction with Neptune’s subtle blue-grey hue and its relatively steady shine compared to the twinkling stars, should aid in identification through your finder ‘scope. Note also that Neptune’s brightness of mag 7.95 is very close to that of the mag 7.84 star (TYC5813-25-1) at Lambda Aqr’s lower right. The planet’s position at the start of the month is indicated with an orange cross.

Once you think you’ve identified your target, switch to the main eyepiece at the highest practical magnification (I suggest 250x or more) to confirm capture by resolving its tiny disk – you’ll need to look carefully to do so.

Neptune will remain within Aquarius until 2022/23.

Pluto

Pluto rises at 12:53 am AEDT when the month of April is less than one hour old; its miniscule disk, far too small to be resolved in amateur instruments, spans 0.095" and glows faintly at magnitude 14.29. With daylight saving ending the following morning, Pluto rises twice on the 2nd – at 12:50 am AEDT and 11:46 pm.

By the evening of the 29th, rise time has improved to 10:00 pm, and the planet (dwarf planet, officially) transits at 5:12 am (on the 30th), a little over ¼ hour before morning twilight begins; its particulars are incrementally adjusted, spanning 0.096" and glimmering gently at magnitude 14.25.

This column only presents detailed finder charts for the tiny frozen orb – revealed in glorious detail recently by images sent back by the New Horizons spacecraft – in the months around opposition. The following chart, configured for 2:00 am on the morning of the 30th, shows its general location near the three naked eye stars Albaldah (Pi Sagittarii), mag 2.9, Omicron [ο] Sag, mag 3.75 and Xi22] Sag, mag 3.5; Saturn and the constellation shapes in Scorpius and Sagittarius are shown to aid in orientation.

Moving along its orbital path at a great distance from us and at a cosmic snail’s pace (4.7 km/sec, just 16% of Earth’s velocity), Pluto remains within Sagittarius until the opening days of 2024, with the exception of a foray the previous year into Capricornus between the start of March and early July.

Top of page

Our Monthly Feature

This month’s feature is a DIY guide to finding open star clusters in Crux, the constellation of the Southern Cross. Virtually every observer has seen the Jewel Box, aka NGC 4755, 1° from Mimosa (Beta [β] Crucis, magnitude 1.25), but have you taken the time to troll through Crux hunting down the other open clusters it contains? Although the Jewel Box is definitely the stand out, there are seven other open clusters with NGC designations, and yet more from other catalogues.

The following chart, configured for 10:00 pm on the 29th, shows all open clusters within Crux as white circles, labelling those with NGC designations; the others tend to be very small and/or difficult to discern.

No further directions will be given here; Crux is a small constellation – the smallest of all – so just trot out your ‘scope and go hunting. Some of the clusters are best identified in a finder scope (or steadily held/mounted binoculars), others through the main eyepiece, at a variety of magnifications. Good luck.

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

That’s it for the April edition of The Australian Night Sky

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