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SKY NOTES

 

SKY NOTES TENTH ANNIVERSARY - SPECIAL FEATURES

The first Orcadian Sky Notes appeared on The Orcadian’s own site on 8th October 2002. Many, but not all, of the 100 or so articles are still available through the archives at: http://www.orcadian.co.uk/skynotes/archive.htm
 

As most readers are aware, we now feature the notes on the ACTIVE SKY page (and others) of the NORTHERN SKIES site.

For the purposes of this celebratory year we have a number articles that will appear during the course of the next few weeks to be found on this special SKY NOTES page.

2012 October 16 The Orcadain Sky Notes 10th Anniversary Feature (2) COMETS. Please refer to COMET page.

"Planets and More  . . . ”

October Skies 2012: JUPITER &VENUS.

venus continues to dominate the early morning sky for at least two months yet, disappearing from the scene as the year-end approaches. (See: SKY VIEW page.) However, it is imperceptibly fading as the phase increases. Rising at 01h 57m UT on October 1st, 02h 44m UT on the 15th and 03h 41m UT on the 31st of this month.

By contrast JUPITER (unmistakable in Taurus not far from the bright, red star Aldebaran – see Sky View page) is slowly increasing in brightness as it approaches opposition in December this year. Rising at 19h 33m UT on October 1st, at 18h 38m UT on the 15th and 17h 42m UT on the 31st.

URANUS

Uranus “passing by” the star 44 Piscium on 2012 September 22/23

2012 September 23, 23h 54m UT 120mm aperture f/7.5 apochromatic doublet: Nikon D300 SLR at the prime focus. Exposure 20 sec. ISO 2000. FIELD DIAMETER: 15 arc-minutes approx. (Left click to enlarge.)
 

Two of Uranus’s satellites are plainly visible. Oberon (13.9) to the right of GSC 3-110 (12.33) and Titania (13.7) below Uranus. On the original image there is an indication above Uranus of Umbriel (14.8).

The point to notice is that the satellites appear on a close to vertical line passing through the centre of Uranus itself. This is because the inclination of the planet’s axis to that of its orbital plane is 97°.77, close to a right-angle.

Other stars of interest in the field are, from left to right: TYC 3-498-1 (8.48); GSC 3-133 (12.84); GSC 3-290 (12.83); 44 Pisc. (5.78), separation from Uranus 82 arc-seconds. (Apparent visual magntudes in brackets.)

Both Uranus (left) and 44 Piscium (right) are of course excessively over-exposed. The planet and the star should appear of equal brightness.

Uranus Conjunction with 44 Piscium 2102 September 22/23

What made this conjunction of special interest?

First, the closeness of the approach itself. At the time of observation as recorded on the image shown here, the apparent separation was 82". This is below the resolution capable of the average unaided human eye. In other words, the star and the planet would have appeared as a single, very faint star almost at the threshold of visibility without optical aid in terms of brightness.

Second, the almost identical visual magnitude of the pair. Because of this fact the combined magnitude would have been approximately 5.5. (Note the combined magnitude is not the simple arithmetic or algebraic sum of the two magnitudes. This aspect of things will be treated in the “BASICS” page at a later date.)

Closest approach took place in daylight on the next day, 23rd September (position “B” dia. 1).

Dia.1. The path of Uranus with respect to the star 44 Piscium. At time of observation on 22 September 23h 56m UT in position “A”; closest approach at “B”. (Left click to enlarge.)

A third consideration is more difficult to quantify and has to do with the probability of such a close approach to a star of similar or even brighter magnitude. The odds for an occurrence may appear relatively favourable in view of the number of stars in the entire sky at visual magnitude 6 or brighter totalling over 4,000 in number.

However, the planets in general (and Uranus in particular) stick pretty closely to the ecliptic throughout their cycle of the heavens. Moreover, the stellar distribution is far from even-handed so that the number of candidates is a great deal below this figure.

The sidereal period (roughly the time taken to circuit the heavens with reference to the stellar background) for Uranus is around 84 years, which is about 4.3˚ per annum. In common with all planets moving outside the orbit of the Earth, the apparent path of the Uranus against the stellar background doesn’t proceed in a smooth West to East direction but is prompted by a series of annual, retrograde loops. (See dias. 2 and 3.)

Dia.2 Part of the “retrograde” loop, 2012 Sep. 20 to 2013 June 17. The motion is retrograde to 2012 Dec 19. where a “stationary point” is reached. The planet is then shown to move forward motion up to 2013 June 17. (Left click to enlarge.)

Dia.3 A smaller scale diagram showing the motion of Uranus from 2012 Sep. 20 to 2013 June 17. (Left click to enlarge.)

Thus, Uranus’s course is a slow one, which reduces the chances of it appearing to pass close to a star. However, there is one compensation in the fact that in retracing its steps, so to speak, a star missed in forward motion may become a candidate in retrograde motion and or visa versa. (Again, see dia. 2.)

Looking forward over a period of some 70 years there would appear to be no more than around a dozen stars that are likely to offer anything comparable to the recent conjunction with 44 Piscium. But of course there are other considerations which reduce the chances of observing these events including the one-to-one chance (more or less) that they will occur in daylight hours, not to mention poor weather.

I therefore regard my observation of 22 September a pretty rare deal!

JCV

12/10/01

Update October 05.

 

2012 October 05 19h 21m UT. 132mm f/7 apochromatic triplet. 30 sec. ISO 2000 D300 SLR. Uranus centre/right. (Left click to enlarge.)

44 Pisc./Uranus separation = 29' 40"

 

Diagram showing distribution of bright satellites at time of the above observation.

Uranus imaged 2012 October 10, 21h 25m UT. 200mm f/8 Ritchey–Chrétien Cassegrain. A 20 sec. exposure. Nikon D300 ISO 1600.

Poor transparency, but seeing good for the altitude of Uranus at a little under 19˚. Three satellites visible below the planet (compare diagram to right), the two above may be artifacts caused by terrestrial atmospheric disturbance. 

Comparisons with images with the smaller aperture apochromatic doublet (120mm f/7.5) on this night would indicate some advantage in quality being given to the latter.

 

 




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