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Venus in 2011-'12: third interim report.

Dichotomy

By the time this report appears in the Journal, Venus will just have passed through dichotomy. Predicted half-phase will have occurred on 2012 Mar 29.34 according to calculations by Jean Meeus. (1) In reality the more shaded terminator causes the disk to look dichotomised some 4-7 days ahead of schedule. Good estimates of phase will be of value to continue the historical record of exactly how many days early observed dichotomy actually is, for the effect is variable from elongation to elongation. Estimates in white light or with Wratten 25, 15, 44, 47 and 58 filters are worthwhile. The phase is best measured from 50mm diameter drawings. Images can also be useful here, but only if they are not enhanced, for increasing the contrast will reduce the apparent phase by darkening the terminator.

Typical views of the planet near dichotomy are shown in the fine series by the late Paul Doherty, taken from the Section records for the 1977 E. elongation: see Figure 1. This was a very well-observed elongation, (2) when I too made many observations.

Around dichotomy is the best time to look for any irregularities along the terminator and at the cusps. It is interesting that modern observers show few irregularities in the terminator of the crescent. Dark markings, poor seeing and optical illusion contrive to produce such appearances, and doubtless we are all more aware of this today, and reluctant to record impressions on the boundary between suspicion and reality. Older observers such as Beer & Madler (3) (Figure 2) more freely showed irregularity in their drawings, but were also well aware of the presence of illusion in planetary observation.

CCD and webcam images almost never show obvious terminator (or limb) irregularities, but should still be scrutinised for possible occurrences. However, both the old and new observations often reveal that the S. cusp near dichotomy can appear more rounded ('blunted') than the north. We have discussed this phenomenon in detail in our exhaustive reports covering the years 1991-1998 and 1999-2006, which are available on the Section's website.

[FIGURE 1 OMITTED]

Phenomena of the narrow crescent phase

The current (2012 March) issue of Sky & Telescope contains a thoughtful piece by Tom Dobbins about the Ashen Light, (4) with historical notes and sound observational advice. The author cites my BAA Ashen Light statistics paper as 'an exhaustive review of the British Astronomical Association's observational archives', but in fact it was an exhaustive account of the work of just two of the BAA's long-standing observers, namely Henry McEwen and Patrick Moore. (The paper itself (5) is not actually cited by Dobbins.) As Dobbins writes, this is a mystery that has never been satisfactorily resolved. The effect was particularly well seen during several elongations of the 1950s.

[FIGURE 2 OMITTED]

One person who intensively studied the phenomenon in the 1950s was the late Valdemar Axel Firsoff (1910-'81), (6) who often sent his work to the BAA. In Figure 3 we give some of his drawings. (7) Before inferior conjunction in 1958 there was a fortnight (1957 Dec 29-1958 Jan 13) during which ten BAA observers including Firsoff saw the Light prominently and often without doubt. (8) At the 1959 morning elongation, Firsoff observed it on Oct 8 (Figure 3). Firsoff did not contribute to the Section Report at that elongation, (9) but at least one other observer recorded the Ashen Light, on Sep 28. Like others who saw the phenomenon well, Firsoff described lighter areas within the general faintly illuminated nightside. In his book Life Among the Stars, (10) Firsoff recalled a later view on 1961 Mar 27 under superb seeing conditions when the Ashen Light stood out clearly on the dark side: '... as I watched it out of the corner of my eye, I could discern little points of light, like misty stars, scattered in it here and there'. The BAA Report of the same elongation (11) contains a few positive records by others during 1961 Feb 20-Mar 26. Moore's notebook (5) contains a remarkably accordant record of it for Mar 26, one day before Firsoff. The Light was suspected, Moore noted: '... a slightly different hue from the sky; mottled--the full circle.'

Observers can make a great contribution to our work by undertaking a regular Ashen Light patrol. An aperture of 75mm or above will suffice. You need a good horizon so that Venus can be observed (at relatively low altitude) once the sky is reasonably dark. Observe the planet to see whether or not the dark side is faintly illuminated. Record details such as instrument, magnification, date, time and conditions. You may be able to make several patrols per night. Negative observations should be logged and sent in too. (Any daylight report of the dark side appearing darker than the sky is a well-known illusion.) It will be worthwhile trying out different filters.

If the Ashen Light is seen, try to hide the bright crescent by an occulting bar in the field (or failing that by the edge of the field of view), and note whether the light remains visible or not. Obviously confirmatory images would be very desirable. Please contact the Director at once by phone or e-mail if you do have any success.

In the June Journal we may have more to write about the crescent phase, the nightside thermal emission, the cusp extensions, Ashen Light and the forthcoming transit. For now, let us admire some of the detailed and careful observations by H. C. Russell made in 1874, which I have been able to reproduce from an original source (12) (Figure 4). The illuminated arc along the limb of Venus is the main point of interest, as it will be this June.

[FIGURE 3 OMITTED]

Current observations, 2011 Dec-2012 Feb

As I complete this report on 2012 February 15, we are finally receiving a decent number of observations of Venus (from 16 observers), even if the snow of the last week or two has temporarily reduced the flow of work. However, we still need more visual observations and observers. Figure 5 shows a comparison of ultraviolet and infrared images. Sean Walker's images of Feb 5 and 9 were made at closely similar atmospheric longitudes and the general similarity of the UV markings is obvious. The images by Daniele Gasparri on Jan 16 were also taken at a similar longitude. Finally, note the greatly increased size of the S. cusp cap on Feb 11 on the image by Jean-Jacques Poupeau. These sudden increases in size have also been witnessed by Venus Express.

[FIGURE 4 OMITTED]

The images also show elusive details in the infrared. Gasparri achieved a high level of resolution by stacking some 12,000 images. This is practicable with a slowly rotating planetary atmosphere, but would be impossible with say Jupiter.

The data also reveal nearly exact accord between independent and near-simultaneous drawings by Gianluigi Adamoli and the Director on Jan 3 and 12. A similar accordance was found between the Director's drawings of Jan 23 and 31 and ultraviolet images by both Poupeau and John Sussenbach on Jan 27, when broad dark shading adjoined the N. cusp (Figure 5).

Other images by Walker record obvious markings in the visible region which vary with wavelength. Dave Tyler on Feb 1 also imaged a horizontal shading near the S. cusp in red light. Observers must use the infrared blocking filter when imaging Venus in the visual waveband if their work is to record these features.

[FIGURE 5 OMITTED]

Observations for the next report (for the June Journal) should reach me at the latest by the middle of April. Good observing!

Richard McKim, Director

References & notes

(1) Times of theoretical dichotomy of Venus from 2000 to 2040 have been published in: J. Meeus, J. Brit. Astron. Assoc., 110, 83 (2000)

(2) J. Hedley Robinson, ibid., 88, 73-78 (1978)

(3) W. Beer & J. H. von Madler, Beitrdge zur Physischen Kenntniss der Himmelischen Korper in Sonnensysteme, Weimar, 1841

(4) T. Dobbins, Sky & Telesc, 123(3), 50-54 (2012)

(5) R. J. McKim & P. A. Moore, J. Brit. Astron. Assoc., 117, 265-272 (2006)

(6) I knew V. A. Firsoff slightly. Contrary to published obituaries, he was born in the Ukraine. His family moved to Poland after the Russian Revolution and he grew up in the Tatra mountains, later studying English, Philosophy and Astronomy at Krakow University. After briefly working as a professional astronomer in Sweden, he came to live in England at the outbreak of WW2, making a living by translating patents and writing popular books about politics, military skiing, travel, and--finally--astronomy moving first to Scotland, and later to Glastonbury. In his later works Firsoff became more controversial, but his writing is always interesting. I owe my early interest in Mercury, Venus and Mars to his little paperbacks.

(7) These particular versions were taken, for convenience (with slight enhancement in contrast for clarity) from V. A. Firsoff's book The Interior Planets, Oliver & Boyd, 1968.

(8) P. A. Moore, J. Brit. Astron. Assoc., 69, 22-26 (1959)

(9) P. A. Moore, ibid., 71, 146-148 (1961)

(10) V. A. Firsoff, Life Among the Stars, Allan Wingate, 1974, p.77

(11) P. A. Moore, J. Brit. Astron. Assoc., 72, 262-265 (1962)

(12) Observations of the Transit of Venus, 9 December 1874; made at stations in New South Wales under the direction of H. C. Russell, Government Astronomer, Charles Potter, Sydney 1892; plate 25.
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Title Annotation:Mercury & Venus Section
Author:McKim, Richard
Publication:Journal of the British Astronomical Association
Geographic Code:4EUUK
Date:Apr 1, 2012
Words:1560
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