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THE ORCADIAN SKY NOTES
The Orcadian Sky Notes for Late April 2013.
Opposition in longitude between Saturn and Sun 2013 Apr 28 08:
We featured the opposition of Saturn in some detail last year since it had a strong personal significance for me. See item below:
The Orcadian Sky Notes for APRIL 2012. Make the Most of Saturn—Opposition 2012 April 15 .
I pointed out that for northern observers Saturn is becoming progressively less favourable from an observational point view.
It is interesting to compare the positional data at opposition for 2013 and 2012.
Date: 2012 April 15 Date: 2013 April 28
Right ascension: 13h 41m Right ascension: 14h 27m
Declination: -7° 31' 2.5" Declination: -11° 41' 9.7"
Constellation: Virgo Constellation: Libra
It will be seen that the planet has moved into the constellation Libra with a reduction in declination of a little over 4°. This means that as viewed from the latitude of Orkney Saturn’s maximum elevation above the horizon meridian passage (due south) on the 28th April will be a low 19° 12'. This limits the period during which the planet may be usefully observed in the telescope using moderate to high powers.
Fig. 1 gives a diagrammatic view of Saturn with its principle satellites at 23h 59m UT on the April 28. The brightest star in the field (to the right) is TYC 5571-780-1 mag. 10.18.
Unlike Jupiter, Saturn has only one natural satellite (Titan) that is readily seen in smaller telescopes. The apparent visual magnitudes for the brighter satellites of Saturn are given in order of brightness in the table below.
Iapetus 10.2 to11.9
(Note: names are abbreviated.)
Rhea and Tethys should be visible in a telescope with an aperture of 80mm, though both always appear reactively close to Saturn itself, Tethys seldom straying more than 25" from the outer ring whilst Rhea may reach twice that distance
The important point to note is that we shall be looking down on Saturn’s north pole making the rings appear “open” and placing the satellites well out of the planet’s equatorial plane (this is particularly noticeable in the case of Titan and Iapetus
Saturn and its major satellites for 2013 April 28 at 23h 59m UT. (Left click to enlarge.)
Phase angle: 0.3°
Equatorial diameter: 18.77"
Polar diameter: 16.75"
Distance (AU) 26 8.8162
Light time: 1h 13m 19.3s
Ring System Information:
Major axis: 42.6"
Minor axis: 13.2"
To the naked eye Saturn appears a brilliant 0.1 magnitude, similar to Arcturus (some 30° above) in both brightness and colour. See general sky view below.
Sky looking south 2013 April 28 at 23h 30m UT. (Left click to enlarge.)
Please go to the NOTICE BOARD for other updates.
During the period The Orcadian Sky Notes is being featured here there will be the opportunity to update information more promptly. More substantial items will be featured from time to time as of old, the one for MAY 2012 appears below. We suggest also, that the GALLERY page be visited as it features sundry images from time to time.
Earlier editions may be found towards the end of the present page or on the site's ARCHIVES page.
The Orcadian Sky Notes for Early February 2013.
Planets Mars, Mercury and Neptune together in the evening sky early February.
There will be an interesting alignment of the planets Mercury, Mars and Neptune low down in the SW evening sky February 06 - 08.
The diagrams below show the sky at 17h 35m UT looking SW on the three evenings in question. At this hour the Sun will have set by some 50 minutes.
Mercury should be visible to the unaided eye at magnitude –1.0 (a little fainter than Sirius the brightest star) and this should guide the observer to Mars at magnitude 1.2, or a little fainter than the star Aldebaran currently well seen below Jupiter in the eastern sky at this time. You will probably require binoculars to locate Mars and Neptune at magnitude 8 will require a small telescope.
On the 8th Mars and Mercury will be separated by around half the Moon’s apparent diameter.
Be under no illusion, this is a challenging observational opportunity requiring a good horizon and clear weather but worth a shot none the less. (Left click images to enlarge.)
The sky looking SW on 2013 Feb 06 at 17h 35UT.
The sky looking SW on 2013 Feb 07 at 17h 35UT.
Large Scale showing disposition of planets and stars to magnitude 9 (visible in a 8x30 binocular) on February 07 at 17h 35m UT.
The sky looking SW on 2013 Feb 08 at 17h 35UT.
(Left click images to enlarge.)
The Orcadian Sky Notes for Early December 2012 - the “ageing moon” as it passes beneath the planets Saturn, Venus and Mercury on the mornings of December 10, 11 and 12.
Looking SE to SSE 08h 00m UT on 2012, December 10, 11 and 12. (Lecft click to enlarge.)
The morning skies of December 10, 11 and 12 will offer an interesting spectacle as the aging Moon passes a little beneath the three planets Saturn, Venus and Mercury, in that order. As may be seen from the diagram, the juxtaposition on the 12th will have the Moon well to the east (left) of Mercury and close to the SE horizon. The Moon’s phase on the 12th December at 08 00 UT will be 1.6% of full. It will conjunct with the Sun (new moon) at 08h 41m UT the following morning. Therefore, at 08h 00m on the 12th the “old moon” will be just 24h 41m from new moon.
The thinnest crescent ever seen by the naked eye was for a “new moon” at 15h 32m from “new” observed by Stephen James O’Meara in May 1990 May. The record for the youngest Moon ever seen with optical aid, 11h 40m past new, was by Mohsen G. Mirsaeed of Tehran in 2002 September 7.
The Orcadian Sky Notes for Early September 2012
The End of Northern Summer—Seeing the Milky Way
Astronomical twilight (Sun between 12˚ and 18˚ below the horizon) is present all night from the latitude of Orkney between April 24 and August 18. Although in a sense this designation is arbitrary, it does indicate that within this period there is a sufficiency of scattered sunlight in the Earth’s upper atmosphere to record on long exposure photographs of the night sky on a clear night. This is not to say that useful work cannot be done by way of photography in twilight conditions, but for the recording of very faint, extended objects (nebulae and comets for example), a truly dark sky is desirable if not essential. (See GALLERY page.)
These arguments apply in general, though less crucially, to observations made with the unaided eye.
From the UK, and from Northern latitudes in particular, the best time to look for the Milky Way is in late August/early September as soon as the sky is sufficiently dark. Diagram 1 shows the reason for this. The line rising up almost vertically from the southern horizon represents the plane of the Milky Way: this is called the Galactic Equator.
Dia. 1 A representation of the sky looking south at 22h 45m UT on August 24. stars to magnitude 6. (left click to enlarge.)
The Solar system is positioned in the galaxy someway toward the perimeter of that great structure (estimated to be at least 100,000 light years in diameter). What we see when we look to the galactic equator is a vast assemblage of material including stars, dust and gasses, with the galactic centre itself appearing in the direction of the constellation Sagittarius. Above Sagittarius will be seen the so-called summer triangle comprising (in order of brightness) the stars Vega, Altair and Deneb.
Vega is the second brightest star in the northern hemisphere and the fifth brightest star in the entire sky. Altair is the 13th brightest star in the entire sky and Deneb one of the faintest of the first magnitude stars: all three are at the “blue” end of the spectral classification.
Both Vega and Deneb are circumpolar (never set) from most locations in the British Isles. As seen from Exeter, Vega “scrapes” the northern horizon, at Plymouth the star sinks below the northern horizon for a little over one hour. Here in Orkney Vega has an altitude above the northern horizon at lower transit amounting to a little over 8°. We mention this so as to emphasize the significant latitude difference between north and south, a fact invariably overlooked by advocates of permanent summer time.
Returning to our galaxy, The Milky Way, the succession of diagrams 2a to 4a indicates the appearance of the sky as we progress towards ever-fainter stars. This clearly demonstrates that the greatest concentration of light (mostly from stars) is to be found neighbouring the line of the galactic equator itself. Diagram 2 includes stars to magnitude 6 and well within reach of the average, adult human unaided eye in its prime; diagram 5 includes stars to magnitude 9, the approximate limit for a 30mm aperture glass (8x30 binocular for example).
As the days progress so the nights darken, but with this the position of the Milky Way inexorably sweeps towards the west and away from the meridian. For a few weeks the early gathering darkness permits observations to be made earlier thereby making it still possible to see the Milky Way close to or a little west of the merdian.
Dia. 2 A representation of the sky looking south at 22h 45m UT on August 24. stars to magnitude 7.
Dia. 3 A representation of the sky looking south at 22h 45m UT on August 24. stars to magnitude 8.
Dia. 4 A representation of the sky looking south at 22h 45m UT on August 24. stars to magnitude 9.
Those wishing to photograph the Milky Way may find they can achieve reasonable results with a static camera (mounted on an ordinary tripod that is) using wide apertures, a high ISO setting and exposures in the region of 15 to 20 seconds. Longer exposures will show star trailing when the image is magnified. (There are small equatorial and altazimuth mounts with electric drives that can compensate for the Earth’s Diurnal motion thus making it possible to use longer exposures with small cameras.)
Fig.1 shows the Milky Way imaged with a Nikon D100 SLR and 24mm f/2.8 Nikkor lens. The camera was mounted on an EQ6 equatorial mount via a dovetail bar and exposed for 90 seconds at the sidereal rate. The undulating dark band is caused by massive accumulations of dust and gasses within the galaxy, sensibly blocking off all starlight from behind.
Fig. 1 The Milky Way imaged 2010 September 10 at 21h 02m UT. A 100 sec. exposure Nikkor 24mm f/2.8. Nikon D100 ISO 800, The Summer Triangle is offset to the left of the central line in this picture with Deneb top left. The stars of constellation Scutum appear lower right.
When I first came to Orkney in 1970 it was possible to see the Milky Way from horizon to horizon. Conditions today make this almost impossible. Clear skies are at a premium anywhere in the UK, but maritime areas, such as the Northern Isles, are at an even greater disadvantage with sea mist and low cloud often a feature of our weather. Add to this the intrusion from excessive artificial lighting, aircraft condensation trails and the like, one’s opportunity to appreciate a truly clear,, starlit sky diminishes by the year.
Sky Notes August 2012: Mercury, Venus & Jupiter together in the morning sky – mid to late August
Another fine opportunity will present itself to enable the illusive Mercury to be found, this time in the morning sky, using the brilliant planets Venus and Jupiter.
One should commence their search from August 16 (Dia. 1b). Twilight is always a key factor in these observations but with a clear sky and good eastern horizon (afforded to some of us on the islands) the feat is achievable—I have managed it in the past in fine weather.
Dia. 1b: The early morning sky looking east at 03h 45m UT. August 16th. (left click to enlarge.)
On August 16 at 03h 40m, Mercury will be at magnitude 0.1, identical to that of the bright star Capella, to be seen at nearly 60˚ elevation directly above Venus. Below Mercury will be the waning Moon at a phase of 2.7%.
Within less than a week Mercury will have doubled its apparent brightness and should be a relatively easy object. Mercury will continue to brighten and will be at magnitude –1.4 on August 31 (as bright as Sirius, the brightest star in the entire sky).
Dia. 2b The early morning sky looking east at 04h 00m UT. August 24th. (left click to enlarge.)
Should weather permit, it may be possible to follow Mercury using Venus and Jupiter as “pointers” right up to the end of August and even into the first day or so of September.
The Orcadian Sky Notes for July 2012
Venus and Jupiter—the sparing continues & Equinox and Solstice.
Readers will recall the stunning appearance of the two brightest planets (as seen from Earth), Venus and Jupiter, in the spring evening sky. Since then the two planets have left the scene, both passing their conjunctions with the Sun only to appear together in the early morning sky.
The summer season is still in its early stages to afford the appearance of the two planets against a relatively dark sky, at least until well into July.
Diagram 1 shows the early morning sky looking north of east at 02h UT on July 10th. It will be seen that Venus appears to lie a little to the north of the bright, red star Aldebaran (the eye of the Bull).
Dia. 1 Looking NE on July 10 at 02h 00m UT. The two planets should be identifiable to the unaided eye in a clear sky despite strong twilight. Attempt to locate Jupiter a little earlier, say around 01h 30m UT. (Left click to enlarge.)
As the days pass Venus will pull way from Aldebaran but its position in relation to Jupiter will not seem to alter much for the first week; thereafter Jupiter will climb into the morning sky eventually leaving Venus well behind.
There will be a close conjunction between Jupiter and the waning Moon on the morning of the 15th July at 02h 15m UT (Dia. 2). Jupiter will appear only 160 arc-seconds from the Moon’s limb, close enough to present an interesting challenge for naked eye observers. (The average adult human eye is reckoned to separate two point sources no closer together than 180 arc-seconds.)
Dia. 2. A diagrammatic representation of Jupiter appearing close to the Moon on the morning of 15 July at 02h 15m UT. (Left click to go to the real thing!)
From Orkney we have the following rising times (UT) for the Sun, Jupiter and Venus:
July 01 03h 03m 01h 05m 01h 47m
Both planets are slowly increasing in brightness. Venus is approaching maximum brilliance at magnitude –4.5 and Jupiter will be at magnitude –2.1 by the end of July, approximately half its brightness (mag. –2.8) at a favourable opposition (2012 December 03).
Equinox & Solstice
Summer Solstice 2012 June 20 at 23h 09m UT.
It appears to be assumed by many (some UK weather forecasters and the broadcasting media included) that equinoxes occur on the 21st day in the months of March and September and solstice on the 21st day in the months June and December of each year.
This is far from he case in fact.
The reason why these events appear on different days from year to year has to do with the Earth’s orbital movement about the Sun which does not conform to the calendar we have constructed for civil convenience.
The calculations are quite complex but the result is that we have the process of leap years in order to bring things into some sort of order. Without the astronomer’s contribution people would find it impossible to keep in step with the real environment. (Many folk today would appear to inhabit a sort of cyberspace.)
The reasons for this false assumption are more difficult to fathom but may have something to do with rumour—it may be easier to replicate than to go to the source for reliable information!
See also BASICS page.
The Orcadian Sky Notes for June 2012.
Never observe the Sun with or without optical aid unless appropriate filters are used. To look at the Sun without protection can lead to permanent blinding. Optical devices concentrate the Sun’s light and heat into a small area thereby raising temperatures to inflammable values within seconds.
Venus Transit Report from Rousay, Orkney.
Apart from a brief view at shortly after sunrise, thereafter the Sun was obscured by cloud.
No images were possible. Visually, using a 100mm achromatic refractor at power 45x and Orion Glass Filter, the appearance of Venus against the Sun’s surface impressed in terms of apparent size and “blackness” as indeed it did at the previous transit.
120606 05 00 UT
Venus Transit of the Sun 2012 June 05/06.
A Transit of Venus occurs when the Earth, Venus and Sun are in line and in, or very nearly in, the same plane. (See Dia. 1.) Were the centres of all three bodies to be exactly in line, then Venus would be seen from the Earth to pass in front of the Sun on a diameter to the Sun’s centre. This would give a maximum duration for the event approximating to 7h 45m.
Dia. 1 The orbits of the inner planets (so-called terrestrial planets) of the Solar System showing the positions of the planets for 2012 June 06 00h 01m UT. Note the Earth, Venus and Sun are in line. (Left click to enlarge.)
Occurrence of Transits of Mercury and Venus.
Both of the so-called “inferior” planets Mercury and Venus may appear to transit the Sun’s disc. The requirement is that the planet must be at inferior conjunction and close to a node. (Intersection of the planet’s orbit with the plane of the Earth’ orbit.) Since Mercury circuits the Sun more rapidly than Venus (mean synodic periods 115.88 days and 583.92days respectively), transits of Mercury are much more numerous than those for Venus.
The most recent Mercury transit took place on 2006 November 8 but was not observable from the UK. The transit previous to that in 2003 May 07 was observable from Orkney during the early morning. The weather here was favourable in the main and the entire event was imaged from Rousay (see fig. 3). The next Mercury transit will take place 2016 May 09 and will be ideally suited for observers in Orkney. This transit will be over 7 hours in duration, with the Sun well above the horizon.
As has just been mentioned, Venus transits are much rarer than those for Mercury. The last transit took place 2004 June 07 and, despite indifferent weather, most of the event was seen from Orkney with the Sun well above the horizon. (See Fig. 1) The transit pervious to that was in 1882 December 06, an interval of 121.5 years. The details for this year’s transit are given bellow. No one reading this is likely to see the next Venus transit since it will occur 105.5 years hence on December 10, 2117 and will occur with the Sun below the horizon from Orkney.
Fig. 1. 2004 June 08. Venus imaged at 07h 56m UT. 100mm achromatic refractor fitted with Baader filter and using a Minolta Dimage F300 digital camera. Note: there are two very small sunspots above Venus.
The transit to take place on June 05/06 this year will last 6h 40m, commencing at 22h 09m 41s on June 05 and ending at 04h 49m 30s on the following day, 6 June 2012. This means that from the UK the Sun will be below the horizon for most of the event. However, the Northern Isles will be at some advantage over southern locations since on June 06 sunrise at Kirkwall takes place at 03h 02m 58s UT. The appearance of the Sun at this time is shown in Dia. 2. (This does not allow for the possible presence of sunspots, which are unpredictable of course.) At Kirkwall at the end of the transit, the Sun will have an altitude of 9° 56' in azimuth 61° 46' (well north of east).
Dia. 2 The position of Venus against the Sun’s disc as it should appear at 2012 June 06 at 03h 03m UT. (Note: The Sun will be in the constellation Taurus.)
Dia. 3 The path of Venus against the solar background during the entire transit.
The safest and easiest way to observe the transit will be to use an optical device to project the Sun’s image onto a white screen. (See fig. 2.) In this way a number of people may be able to view the event at the same time. Be careful not to intercept the light beam from the eyepiece of the telescope or binocular, and at all costs do not allow young people too close to the arrangement. The immerging beam from the eyepiece concentrates the Sun’s light and heat into a small area close to the eyepiece, raising temperatures to combustible levels in seconds!
Fig. 2. A typical projection set up with an equatorially mounted refractor. (Image by kind permission of Lyn Smith, British Astronomical Association Solar Section Director.)
Fig. 3. The transit of Mercury in 2003 May 07, 06h 18m UT. Sun’s image projected onto white screen as described in Fig. 2 and photographed with a Minolta Dimage F300 digital camera. Telescope 3.3 inch Wray equatorial, Springfield, Rousay. Mercury is the small "dot" top right close to the solar limb. Note: A moderately sized sunspot appears below not far from the solar central meridian.
Those with suitable solar filters may find it more convenient to use this method of observation particularly if there is wind about. Either method allows a good opportunity to image the progress of the transit with a digital camera.
The thing that impressed me most when I first saw Venus projected on the Sun’s face in June 2004 was the apparent largeness and “blackness” of the planet’s disc. At around 1 arc-minute this is approximately 1/30th of the apparent diameter of the Sun. (At transit this year Venus will have an apparent diameter of 57.78 arc-seconds.) Unlike sunspots, which would appear bright against the sky without the dazzling background of the Sun’s photosphere, Venus at transit offers an unlit surface earthwards. This comes as close to a true “black body” that one is ever likely to see in space.
Venus approaches inferior conjunction.
Venus imaged 2012 May 25 at 16h 04m UT, 100mm apochromatic refractor 180x. (Poor seeing.) Venus data: magnitude: -4.2 phase: 4.6% diameter: 54.11" elongation from Sun 17.4°.
The Orcadian Sky Notes for May 2012.
May 7 2012: Matters Astronomical, please go to: Visitor's Forum page.
Full Moon at Perigee
Full Moon* at Perigee, 2012 May 05/06.
The Moon imaged 20h 44m UT from Springfield seen close to the uninhabited building at Classiquoy. The red ring around the Moon is an optical “artefact” resulting from the light contrast between the Moon and the relatively dark sky, and exacerbated by computer processing.
In contrast to most of the British Isles, Orkney was blessed with a fine evening and night in which to observe this very rare event. It is to be hoped many of our readers were able to brave the cold to see at least part of the spectacle of a bright, full moon “close” to Earth.
On Rousay we were denied a view of moonrise due to a low bank of cloud but within 40 minutes the Moon had climbed above the cloud and for the rest of the night and early morning the sky was relatively clear.
* Full Moon: a collection of poems 2004, ISBN 1-902582-51-9
Venus heads towards a transit of the Sun’s disc and Full Moon at Perigee.
Venus is now well past maximum eastern elongation and is moving steadily towards its rendezvous with the Sun on June 5/6, when it will transit the Sun’s disc. (More on this next month). Venus is now at maximum brilliance with a magnitude of –4.5 and is readily seen in full daylight with the unaided eye provided one knows exactly where to look. The planet reaches a maximum northerly declination of +27° 50' on May 05 and so is well above the ecliptic at this point. (Rise time: 4h 19m, Set time: 0h 58m UT. Compare with Sun, rise time: 4h 00m 44s set time: 20h 12m UT.)
Venus imaged 2012 April 27, 15h 02m UT. 120mm apochromatic refractor 350x.Minolta Dimage F200, 1000sec. ISO400. JCV
Full Moon at Perigee.
There are a number of features to the Earth/Moon system that make it unique within the Solar System’s retinue of planets. First, and most significantly, is the relative size (mass and volume) of the two bodies. The Earth has an equatorial diameter of 12,756 km compared to the Moon’s diameter of 3,476 km. In other words, our natural satellite has a diameter approximately 25% that of the Earth’s.
With no other planet is the this ratio so large. Mar’s two satellites are but a fraction of the size of Mars itself. In the case of the giant planets their satellites, though comparable to and exceeding in size our Moon in some instances, are none the less relatively small compared to the planets themselves. (Incidentally, it is interesting to note that neither Mercury nor Venus possess natural satellites.)
The second important fact concerning the Earth/Moon system is the Moon’s path or orbit about the Earth. The mean or average distance from the Earth makes it possible for the Moon to just cover the Sun during a solar eclipse. This fine juxtaposition of circumstance gives us the opportunity to observe the tenuous, outer atmosphere or corona during a total eclipse of the Sun. (Some have invested a spiritual or creative significance to this fact. That is to say, the deity arranged it thus to enable us to appreciate the wonders of a solar eclipse, or something on those lines. I wish to make no further comment either way on that one!)
The dynamics of the Earth/Moon system are complex and have been the subject of mathematical analysis in what has become known as “lunar theory” over many centuries, long before the invention of the optical telescope. Only in comparatively recent times, however, has it become possible to predict with fine accuracy the position of the Moon in relation to other bodies in the solar system and in particular to that of the Earth itself.
Seen from Earth, the Moon presents phases that relate to the relative position of the Earth/Sun/Moon at any given instant. A full moon occurs when the Moon appears in the opposite part of the sky to the Sun. For example, on May 6, 2012, full moon will occur when the Sun is in the constellation Aries and the Moon in Libra. As the Moon circuits the Earth so its distance from us changes. At its closest it is said to be in perigee and at it farthest in apogee. When a solar eclipse occurs close to apogee the Moon will appear just too small to entirely cover the Sun’s disc, giving rise to an “annular” eclipse of the Sun. Maximum duration for a solar eclipse will occur when the Moon is close to a perigee.
By a similar argument, when a full moon coincides with or is close to perigee, then it appears to the eye significantly larger than when full moon occurs close to apogee. (See the accompanying diagram.)
This year on May 6 the two events, full moon and perigee, occur within a little over one minute of time of one another. Our calculations show that for a period of just over 200 years there are only four occurrences comparable to that on May 06, 2012. Not only that, but the perigee on May 6 this year is one of the closest within the annual cycle of perigees.
When this phenomenon first became apparent to me I contacted Jean Meeus, a renowned authority on astronomical algorithms, to ask for his verification of my deductions. You will see from the table below the results of our combined efforts.
Figures in brackets are from Jean Meeus himself. The concordance will be seen to be very close.
In my analysis the original table of concordances for the 221 year period amounted to over 300 hundred entries with differences in the two events amounting to no more than 10 hours. The five dates given in the above table stood out from the rest indicating that the May 06 event is exceptional by any standard.
The fact that on May 06 as seen from parts of the UK the Moon will be in a twilight sky, and within an hour of setting, is to an extent unfortunate but insignificant compared to the juxtaposition of the two events themselves. (From Orkney moonset occurs at 03h 47.3m UT.)
Seen from Orkney the Moon will be due south approximately six minutes before midnight on the 5th May. At an altitude of a little over twelve degrees, and only a few hours from perigee, this full Moon should look all of that!
The Moon rises at Kirkwall on May 5 at 19h 54m UT, 21 minutes before sunset.
I should like to thank Jean Meeus for his prompt and helpful input to this investigation.
2012 April 26
The Orcadian Sky Notes for APRIL 2012.
Make the Most of Saturn—Opposition 2012 April 15.
In a BBC Radio Orkney appearance by Michael Sinclair (co-founder of the recently formed Orkney Astronomical Society) when asked the question “What got you going astronomically?” Michael replied in a little detail “Saturn”.
It was the same with me. As a young schoolboy recently immerging from the traumas of WW2, and finding myself in a new home surrounded by the quiet of an Essex countryside, the winter night skies beckoned immediately. Then, in 1948, there was little light pollution and the skies were free from aircraft and artificial Earth satellites.
My father would take me on long evening walks from where I had my grounding in astronomy—identifying the stars and constellations by name, the appearance and movements of the planets, and so on.
Then, in the summer of 1950, my father presented me with a smallish coastguard telescope purchased for £7 10s (old money). This fine instrument by Watson had an aperture of 60mm and a magnification of 30x. I quickly put together a mounting form a large Meccano collection and set about a study in detail of the night sky. (Hitherto I had made do with a small pair of opera glasses, no bad thing since one quickly learned to navigate the sky over a larger area.)
Outside in the quiet of a spring evening in 1951 I turned the telescope onto a quite ordinary-looking star only to find this small yellowish disc with a spike sticking through it: it was Saturn! (fig.1) Opposition that year occurred on March 20 and, as may be seen from the diagram, the ring system appeared virtually “edge on”. As a result Saturn appeared to the eye only a little brighter than the star Spica in the constellation Virgo.
Since then Saturn has completed a circuit of the stellar background twice: I am now of an age when I shall never see that unique planet high in the sky from the latitude of Orkney again.
I make no apology for this preamble. Astronomy as a study is an art as well as a science (after all this compartmentalization of knowledge is itself somewhat arbitrary and of our own design—“in nature’s physical world neither number nor formula count for anything”.
This year (2012) Saturn comes to opposition on April 15 at 18h 26m UT. To the unaided eye it will appear as a bright, star-like object rivalling the brilliant star Arcturus (above), both pale orange in colour. (See Dias. 1 & 2.)
Dia. 1 Looking south from the location of Orkney 2012 April 15 at 23h 59m UT.
Dia.2 The same aspect as Dia. 1 but with fewer star and constellation names to assist clarity.
At meridian passage, seen from Orkney, the planet will have an altitude of 23° 22' above our southern horizon. All things being equal the higher in the sky an object appears the clearer it should be when viewed through the telescope. But much depends up atmospheric conditions (upon which we humans now impose a heavy burden) and I have frequently observed finer image quality at low altitudes than when closer to the zenith.
Fig 1 The appearance of Saturn in a small (80mm aperture) refractor power 120x. From 1951 May 04.
Fig. 2 The appearance of Saturn at opposition Apr15 2012 as it might be seen in a telescope of similar size magnification to the above.
By comparison with nearby Mars (magnitude by this time having dropped from –1.2 at opposition to –0.4, a virtual halving in apparent brightness), Saturn presents a pale image in the telescope. This can be an advantage when viewed against a dark sky since contrast is a subtle medium and too much of it can blur clarity. (I have seldom had good views of Mars even with large instruments.)
Unlike Jupiter, Saturn has only one natural satellite (Titan) that is readily seen in smaller telescopes. It is an interesting coincidence that Titan is to be seen to the east of the planet on April 15 in a similar position to when I first observed the pair those many years back in 1951. (Refer to diagrams above.) The apparent visual magnitudes of the brighter satellites of Saturn are given in order of brightness in the table below.
Rhea and Tethys should be visible in a telescope with an aperture of 80mm, though both always appear reactively close to Saturn itself, Tethys seldom straying more than 25" from the outer ring whilst Rhea may reach twice that distance.
Data: Saturn 2012 April 15
(Opposition in longitude between Sun and Saturn 2012 Apr 15 18h 26m.)
Light time: 1h 12m 31.1s
Ring System Information:
Major axis: 43.0"
The Orcadian Sky Notes for February/March 2012
A Parade of Bright Planets in the late Winter Sky
There will be a rare opportunity (again weather permitting) of seeing our four brightest planets: Mercury, Venus, Jupiter and Mars, all in the sky at one and the same time.
For about fourteen days from February 26th to March 10th the early evening sky will host the first three mentioned in the SW sky, together with Mars in the east; moreover, all will be brighter than first magnitude, with Venus heading the list close to maximum brilliance at mag. –4.3. Use the two brightest planets Venus and Jupiter as pointers for Mercury low down closer to the western horizon.
The first diagram depicts the sky looking close to due west on the evening of February 26 at 18h 45m UT. The presence of twilight shouldn’t interfere with the observations provided the sky is clear.
The second diagram for the same date and time gives the sky looking east.
The configuration will change rapidly in the case of the SW sky, with Mercury disappearing from the scene after the 12th March. By the 15th Venus will appear above Jupiter by a separation of a little over 3°. At the same time, Mars will rise into greater prominence in the east (in terms of elevation) but will start to fade shortly after opposition (see below).
Mars at Opposition in Leo 2012 March 03.
Mars is by now a conspicuous object in the early evening sky rising a little north of east at around 18h 30m UT on February 20th.
Readers should refer to the map in the Archives section (The Orcadian Sky Notes November 2011) showing the apparent movement of Mars over the past months and into the immediate future.
Closet approach to Earth in the case of Mars does not necessarily coincide with opposition date. This is because the planet’s orbit about the Sun is not quite circular [eccentricity (e) 0.093 compared to that for the Earth’s orbit 0.017].
This year opposition occurs at 20h UT on the 3rd March whereas closest approach to Earth takes place on the 6th March.
Perhaps we should explain the terminology here for those less familiar with the subject.
Opposition refers a position in a superior planet’s orbit when lying opposite the Sun in relation to the Earth. (A superior planet following a path or orbit outside that of the Earth and therefore farther from the Sun.)
If all the orbits of the superior planets and the Earth’s were to be perfect circles with the Sun at the centre, then quite clearly closest approach to the Earth of a superior planet WOULD occur at opposition. But no planet has a perfectly circular orbit and so the Sun’s position is not truly symmetrical but rather appears off centre. For most practical purposes we may regarda planets orbit as elliptical in form with the Sun lying in one foci.
Again, from a practical point of view, the difference between opposition date and closest approach to Earth is insignificant. The table for Mars shows that for a period of eight days the planet has sensibly the same apparent diameter of 13.9 arc-seconds.
Observers should make a direct comparison where possible with Jupiter (still very much visible in the western sky).
Thus for Jupiter on February 20 we have:
and for Mars:
Note: the polar and equatorial diameters for Mars are virtually identical.
The point has been made in previous articles that the present opposition compares less favourably with closer approaches in other years. At the favourable opposition for southern observers in July 2018 the planet will have a diameter of 24. 26 arc-seconds and an apparent visual magnitude of –2.8 (similar to the maximum brightness for Jupiter). However, the elevation abnove the southern horizon here in Orkney at this opposition will be a mere 5.5 arc-degrees with a strong twilight lit sky.
Data for Mars
Date Rise Diam. Phase Mag .
The Orcadian Sky Notes for February 2012
Sky Notes February 2012—More on Solar System Planets.
Always go to the SKY VIEW page in order locate the positions of the planets. This feature is updated on a weekly basis with full explanatory notes.
Since our evening skies are presently dominated by the Solar System’s brightest planets (as viewed from planet Earth) Venus and Jupiter, I have chosen to extend our coverage of solar system objects, and the major planets in particular.
Therefore, we are again to delay the analysis of Comet Garradd, hopefully to no longer than late March or early April, by which time twilight will start to make observations problematical from the latitude of Orkney.
What we will do at this point is to give two, full hemisphere maps of the sky to show the apparent positions for the planets Neptune, Uranus, Venus, Jupiter, Mars and Saturn, along with comet Garradd. You will notice that by the end of February the comet will appear almost overhead in the early hours.
Currently we are at a juncture in time when it is possible to observe all the known planets (with the exception of Pluto, which has been relegated to the title “dwarf planet”) within a 24 hour period. True you will need a telescope to observe Mercury, and powerful binoculars for Neptune, but the rest are all observable in a relatively dark sky between Jan 24 and Jan 27, by which time Neptune will be close to the horizon of an evening.
The remaining five planets are all accessible to the unaided eye up to at least mid-February, when Uranus becomes enmeshed in evening twilight. Thereafter Venus, Jupiter, Mars and Saturn will all be observable in a dark sky up to February 24. (Note that the presence of the Moon in the sky, although a hindrance where the fainter stars and planets are concerned, only when the Moon is close to full will it detract from an appreciation of the brighter planets.)
In the case of the brighter planets it is worth considering the conditions that offer the best telescopic views.
First, for Saturn, a dark sky is essential due to the planet’s relatively low surface brightens (albedo). Atmospheric conditions are seldom predictable but good seeing is essential in order to make out fine surface detail on the planets and the Moon. Transparency is of secondary importance up to a point and indeed, good seeing frequently occurs at time of relatively low transparency.
Jupiter, on the other hand, may be observed to advantage in a twilight lit sky and, unlike Saturn, may be picked up with relative ease in the telescope in full daylight when separated from the Sun in the sky by 20° or more. (Of course this pr-supposes one is equipped with a telescope capable of being directed to the appropriate position of the object in the sky.)
Whereas both Saturn and Jupiter vary in brightness approximately within a single magnitude range, Mars can vary in brightness from a brilliant –2.8 (as bright as Jupiter at is brightest) at opposition to a little below 1.4, (or fainter than Regulus, the faintest of the so-called twenty first magnitude stars), when most distant from Earth. For this reason the conditions for best observing the planet are more tricky to define.
Obviously for a planet that reaches a maximum apparent diameter of only 25.1 arc-seconds (about half that for Jupiter at a good opposition), Mars needs to be close to opposition for a reasonable telescopic view. And because Mars has a relatively eccentric orbit opposition distances may also differ significantly.
At the forthcoming opposition on March 3, 2012, Mars will have a magnitude of -1.2 (a little fainter than Sirius, the brightest star). With an apparent diameter of only 14 arc-seconds it will require a good refracting telescope of at least 100mm aperture to show any detail on the disc. This combination of small apparent size and moderate brilliance means that Mars may best be observed against a twilight sky. (Mars will feature in greater detail on next month’s Sky Notes.)
There is no ambiguity in the case of Venus: observe it through the telescope against bright twilight or, preferably, in full daylight. In this way you will also gain from the advantage that maximum altitude for Venus can only be achieved at or close to meridian passage and this always occurs in daylight. For this reason morning apparitions are more suited to the casual observer than evening apparitions (as at present) because having located the planet (no problem there!) in a relatively dark sky, it is then possible to follow it with the telescope or binocular with rising altitude into full daylight. And, of course, close to maximum brilliance Venus is visible to the unaided eye in daylight.
At the time of issue of this feature (January 24) Venus has an apparent diameter of 14.46 arc-seconds (compare Mars at opposition as mentioned above) and a phase of 76.5%. Next to Mercury Venus appears to move most rapidly against the stellar background at the same time with changes in apparent diameter (brightness changes more leisurely). Thus, by the end of February 2012 Venus will have an apparent diameter of 18.33 arc-seconds with a phase of 64%.
Mercury is in a category of its own in respect of the arguments discussed above. Therefore a feature will be devoted to Mercury later in the year.
Perhaps I should comment on the frequent use of the word “apparent” in these articles. By this we leave no ambiguity between actual physical size and size as measured by the observer at a given distance from the object. I could enlarge on this aspect of the “apparent” versus the “real” from a metaphysical standpoint but will spare readers the tedium! (I thrash out this topic in a book I have been working on for many years entitled Big Bang—Fact or Fiction?, See http://www.spanglefish.com/springastlix/index.asp?pageid=234479
A 10 pence piece (diameter 28mm approximately) will subtend an angle of one arc-minute (60 arc-seconds) at a distance of 95 metres.. .
*2012 is a leap year.
A full “hemisphere” projection of the sky for January 24 2012 at 18h 00m UT.
A full “hemisphere” projection of the sky for February 29*, 2012 at 04h 00m UT.
The Orcadian Sky Notes for January 2012
January Sky 2012—Early Evening (planets: Venus, Neptune, Uranus and Jupiter, together with Comet Garradd).
The evening skies of January 2012 illustrate a number of salient astronomical details.
The first of the two diagrams below (Dia.1) indicates the positions of the four planets (working from west to east) Venus, Neptune, Uranus and Jupiter, together with the waxing Moon as they will appear in a clear evening sky looking SSW on January the 1st 2012 at 17h 00m UT.
Also shown are the constellation boundaries, including all stars names for those brighter than magnitude 2.5, along with the Ecliptic.
Note the first magnitude star Fomalhaut close to the southern horizon. This is the farthest south first magnitude star visible from Orkney and offers something of a challenge to the naked eye observer—a good, clear horizon is essential. (I have seen the star from the road above the Midhowe Broch on Rousay just touching the hills of Mainland and now festooned with wind turbines!)
The second diagram (Dia.2) for the same date and time shows the sky NWW and includes Comet Garradd.
Dia. 1 (see text above)
Dia. 2 (see text above)
The points to note are, first, how the regular solar system members all lie close to the ecliptic, whereas the comet is far from that line as it appears in the sky. This is because, despite being in orbit about the Sun, Comet Garradd has a highly inclined orbit (i 106.18° to the plane of the Earth’s orbit about the Sun). (Compare: Venus i 3.4°; Neptune i 1.77°; Uranus i 0.8°; Jupiter i 1.3°.)
The relative positions of the planets will be seen to change during the course of January and beyond. As mentioned on the “Sky View” page, the Moon appears to move against the star background approximately 26 times its own apparent diameter in the course of 24 hours, bringing it above Jupiter by the early hours of January 3rd.
Apart from the Moon, within the present grouping, Venus will appear to move the fastest so that by the early afternoon of January 13th Venus will pass 1.1° below Neptune. The conjunction should be observable in small telescopes, say, by 18h 00m UT, when the two planets will be around 8° above the horizon. Venus will outshine Neptune some 63,000x! This means that if the two planets were to appear much closer the greater brilliance of Venus would make it difficult to see Neptune.
Jupiter continues in steady forward motion crossing from Pisces into Aries on 9th January. Although slow compared to Venus, Jupiter’s movement from night to night against the star background can be monitored quite easily with binoculars. On the 1st Jupiter passes a little above the 8.1 magnitude star TYC 626-597-1 (a good 2 magnitudes fainter than the four Galilean satellites) so that this will make a good reference point for the assessment of Jupiter’s forward movement. Another comparatively close “encounter” with a star will take place on the 30th when Jupiter passes above TYC 626-597-1, magnitude 6.5 (a shade fainter than the Galilean satellites).
The thin, crescent Moon reappears on the scene during the early evening of January 24th. For those with a clear SSW horizon this will offer yet another challenge since the Moon’s phase will be a mere 2.4% (“age” 30.5 hours).
By the 27th January the Moon will be above Uranus (see Dia. 3a). The diagram gives a field of 8° diameter typical for a 8x40 binocular. Use the Moon to locate Uranus (magnitude 5.9) in the context of the nearby stars, this will prove useful in locating the planet on subsequent nights. In a good, dark sky Uranus can be seen close to the threshold of naked-eye detection (see below).
Dia. 3a. See text above.)
Uranus is in slow forward motion, as may be seen from dia. 3b. Although Uranus is close to the threshold of naked eye visibility, the presence of the Moon will make it necessary to use some optical aid from the 27th on for at least a further twelve days.
Dia. 3b (see text above). Note: the Moon will of course appear in the field only on the evening of the 27th.
On February 8th Venus will be in conjunction with Uranus, and this will be featured in the February Sky Notes. Also, a detailed appraisal of Comet Garradd will now appear in the 2012 February Sky Notes
Sky Notes Features Archive: http://www.orcadian.co.uk/skynotes/archive.htm
Sky Notes Aurora Borealis Archive: http://www.orcadian.co.uk/skynotes/aurora.htm
(All observational data is for the location of Kirkwall, Orkney.)