A.L.P.O. Venus Section
Visual observations of the planet Venus should be carried out in a
cooperative systematic research program to maximize opportunities for
useful scientific contributions. In the A.L.P.O. Venus Section, our
objective is to observe Venus on every possible clear night throughout
an apparition of the planet, which runs from conjunction to subsequent
conjunction with the Sun. Venus is an inferior planet, meaning that it
has a smaller interior orbit to that of the Earth, and it exhibits
phases just like the Moon. Because Venus is comparatively near the
Sun, it is characteristically ve ry bright, and the high albedo
produces an excessive amount of glare. The rather faint and elusive
markings on the disk of Venus, normally of very low contrast, become
difficult to see as a result. Considerable controve rsy exists over the
true nature of these dusky amorphous or somewhat stre aky atmospheric
features. It is not at all unusual for two observers, working on the
same date with comparable instrumentation, to see striking dissimilar
atmospheric phenomena on the planet.
Even though Venus may reach a maximum angular distance from the Sun of
ar ound 47, the planet is still observed near the times of sunrise or
sunset 2E If Venus is seen against a dark sky, the effects arising from
the excessive brilliance of the disk are pronounced. At eastern
(evening) apparitions, Venus is frequently low in the western sky
where the effects of atmospheric differential refraction and prismatic
dispersion destroy good image quality. Seeing conditions are so poor
at such times that most observers have adopted a practice of viewing
Venus only when has an altitude of 20 deg. above the horizon. At times
of western (morning) apparitions, it is possible to wait until the planet
gains altitude and the background sky brightens considerably, and Venus
can readily be followed into daylight. It is perfectly desirable to
observe Venus during daylight hours when most of the prevailing glare
associated with the planet is gone or reduced, but observing Venus too
far into the daylight hours can become a problem as solar heating
produces turbulent air and resulting poor seeing. While it may seem
difficult to look for Venus in daylight, it should be recalled that
the planet is comparatively bright, and in practice, the observer can
usually find Venus if knowledge of exactly where to look is obtained
before the observing session. It is worth mentioning that observers
find that the presence of a slight haze or high cloud often stabilizes
and reduces glare conditions while improving definition.
Wdely-spaced observations are of very little value, and we must stress
the important point of striving for systematic, regular studies
carried out by a large team of experienced, dedicated individuals
using similar equipment and methods. We are seeking an intensified
effort to increase the incidence of simultaneous identification of
atmospheric phenomena on Venus to improve the objectivity of our data,
chiefly because of the elusive nature of features in the atmosphere of
the planet.
With the highest optical quality and mechanical stability assumed, the
minimum recommended aperture for useful observations of Venus and
participation in all aspects of our programs is about 15.2 cm. (6.0
in.) for reflectors and 7.5 cm. (3.0 in.) for refractors. When
observing with smaller a pertures, one must seek a suitable combination
of large angular diameter of Venus and large phase for successful
detection of the elusive disk features. This combination of factors
occurs about midway between the times of greatest western elongation
and superior conjunction, as well as betwe en superior conjunction and
greatest eastern elongation. During these intervals, one may note that
Venus has an angular diameter of approximately 16.0 and a gibbous
phase. Use of magnifications within the range of about 50D to 80D (where
D is the diameter of the telescope in inches) is often practical when
Venus is high in the sky.
It is important not to overlook various accessories that come with or
arepurchased to accompany one's telescope. Eyepieces need to be of
equal quality as the main optical system of the instrument, and filters
of known wavelength transmission must always be used in conjunction
with eyepieces employed for observations. Admittedly at the cost of a
little comfort and convenience, one should avoid using diagonals or any
devices that orient the image of Venus contrary to what is normally
seen and adopted as the view of a planet through an astronomical
inverting telescope. Low-transmission filters increase contrast and
definition while limiting the effect s of irradiation. For Venus,
Wratten blue (W38A) and violet (W47) filters are useful for
uncontrasted detail, mainly because of the yellowish atmospheric cloud
layer. Alternating color variations from red to blue are frequently
detected in the south polar region of Venus using Wratten red (W 23A or
W25), yellow (W12 or W15), and green (W57 or W58) filters. Variabl
e-density polarizing filters add to the visibility of faint markings by
reducing glare. Once at the telescope, there can be no substitute for
actual observing, and as we shall see, it is just as vital to report
negative results (no markings) as it is to record features that are
present. The fundamental goal is objectivity and realism.
Utilization of the appropriate drawing blanks and report forms is
require d by the A.L.P.O. Venus Section if participants in our program
expect to have their data included in our analysis and subsequent
apparition reports. If observers wish to have drawings considered for
publication, they must be submitted as originals rather than xerox
copies. Forms are available at the cost of reproduction and postage
from the A.L.P.O. Venus Section 2E
It is the ultimate goal of the A.L.P.O. Venus Section to attempt to
assem ble a completely homogeneous mass of accurate, reliable
observational data collected over many apparitions, permitting an
exhaustive statistical a nalysis. It is hoped that we might derive
enough from painstaking observa tions and analysis to help provide some
answers to questions that continue to perplex us about Venus.
Observations of the atmosphere of Venus are organized into the following
routine programs:
1. Visual observation and categorization of atmospheric details in
dark, twilight, and daylight skies.
2. Drawings of atmospheric phenomena.
3. Observation of cusps, cusp-caps, and cusp-bands, including defining
the morphology and degree of extension of cusps.
4. Observation of dark hemisphere phenomena, including monitoring
visibility of the Ashen Light.
5. Observation of terminator geometry (monitoring any irregularities).
6. Studies of Schroter's phase phenomenon.
7. Visual photometry and colorimetry of atmospheric features and
phenomena.
8. Routine photography (including UV photography), CCD imaging,
photoelectric photometry, and videography of Venus.
Julius L. Benton, Jr., Ph.D., F.R.A.S.
Executive Director
ASSOCIATION OF LUNAR AND PLANETARY OBSERVERS (ALPO)
Coordinator - ALPO Venus and Saturn Sections
c/o ASSOCIATES IN ASTRONOMY
P.O. Box 30545
Savannah, GA 31410 USA
Telephone: 912-897-0951
E-Mail Address: jlbaina@msn.com
Website: http://www.lpl.arizona.edu/alpo
eGroups: Saturn-ALPO@yahoogroups.com, Venus-ALPO@yahoogroups.com