MIRA 28
May 1990

Petavious


CRATER . . . . . . PETAVIOUS
LOCATION . . . . 25° S 60° E
DATE  . . . . . . . 1990 / 1 / 13
TIME  . . . . . . . 20.30 to 21.15 U.T.
MOON  . . . . . . 17 days old
OBSERVER . . . I Clarke, Warwickshire
CONDITIONS  . Good Ant. II to III, some shimmer, cold.
INSTRUMENT. . 114 mm Catadioptic reflector x167


The crater PETAVIOUS at 25°S 61°E is a large 100 miles plus crater, which Patrick Moore calls "magnificent, certainly one of the finest on the moon".  The floor Petavious is convex, so that at sunrise and sunset the whole of the bottom looks domed.  The central mountain range which rises to hight of over 5,500 feet and surrounding area looked like an island inside a black moat with the brilliantly lit outer mountain ring adding to the effect.  The area just outside the North West wall looked full of detail which could only be seen fleetingly in the unsteady air.  The central peak was very bright, as was the richly terraced walls.  To the north of the central peak was a small crater like depression which seemed to be casting a shadow onto the eastern walls which are over 11,000 feet high.  The North and South ends of the crater seem to be slightly 'pointed'.  The crater to the West is WROTTESLEY 34 miles in diameter, it didn't show much detail, the central area was in shadow.




A letter from Rob Moseley



I was most interested to see Ivor Clarke's excellent lunar drawing on the front cover of the January edition of MIRA.
Not being familiar with the formation, and noting that it was not identified, I did a little research, Ivor may be pleased to know that I would put my shirt on the bet that this particular portion of the Moon has never been drawn before.
Identification was a little tricky, and I had to consult Viscardy's Atlas, Hatfield's Atlas and my collection of LPL prints before I was satisfied.  The drawing shows an undesignated, ruined ring plain, lying due south of Piazzi.   The size of this feature is quite tremendous. Piazzi is nearly 60 miles in diameter — but dark's feature is nearer 100 miles across. It must vie as the largest previously undrawn feature on the Moon.
Why am I so certain it has never been drawn before.
Firstly, my job within the BAA Lunar Section has given me insight of the majority of drawings produced over the last hundred years.  Secondly — it is not a named formation. It is odd to note the effect that naming a feature has on successive generations of observers. The named features are drawn and studied in detail, while the spaces in between are largely ignored.
The bright line crossing the feature does indeed appear to be a ridge, but it is a very gentle feature, only showing well under morning illumination.  With lighting reversed it is very difficult to discern as it conforms to the general alignment of the ejecta from the Mare Orientate.
Even though it is in a sad state of repair, and only prominent when close to the terminator, it hardly seems right that a crater 100 miles across should go without a name.  Can I suggest that at least among members of our society, this object should rejoice in the name 'Clarke'!



Subjects of Astronomical Interest

Although the solar wind was first directly detected by the Soviet Lunar 3 spacecraft in 1959 and later by the American spacecraft Explorer 10 and Mariner 2, Johann Kepler, during the 17th, century first suggested the idea that the tails of comets are pushed away from the Sun due to the pressure of sunlight.

I would be most pleased if any member could kindly contribute something of the above for future issues of Mira.

Thanks Ed. V.C.



Junior Astronomical Society

Aurora Section

Part Two

Auroral Recording

Several aspects of auroral activity are worth recording, the most basic being the presence of aurora on a given night.  It is also desirable, however, to define what types of activity occurred, and at what times and positions in the sky.  When recording aurora, remember to use Universal Time  (UT = GMT = BST - 1hr), and quote the date as a double date eg. Oct 26/27 - the night of 26th to the morning of the 27th October.

Record what types of auroral features are present at times accurate to the nearest minute in UT, and how bright they are.  Auroral/brightness is measured on a four-point scale:-

I       Weak, comparable to the milky way.

II      Comparable to moonlit cirrus cloud

III    Comparable to moonlit cumulus cloud

IV     Stronger than III, possibly even strong enough to cast shadows.

Make measurements of position in altitude and azimuth.  Such measurements are not hard to make, and can be done with no more complicated equipment than a 6" ruler!  Held at arms length, this subtends 20° of arc for the average person.  Eye estimates of positions can, therefore be made, using the ruler as a standard.

Azimuth:  This is a measure, in degrees, of a feature's position round the horizon, with 0° due north, 90° due east, and so on.

Altitude:  Altitude is the height in degrees above the horizon at which a feature is seen.  Always use the TRUE horizon, not the apparent horizon; the latter is usually higher due to local topography (houses, hills, etc).  Altitude 0° is on the horizon, 90° in the zenith.  Polaris is also a useful marker for auroral measurements, being at 0° azimuth, and at an altitude equal to the latitude of the observing site.


Two types of altitude measurements are useful for aurorae:-

^:  This symbolises the top of a feature, measured at its highest point.  The azimuth of this point should also be recorded.

h:  A measurement of the highest point on the base of a feature.  Again, quote the azimuth of this point.  In many cases, only measurements of ^ will be possible, as the base of the feature lies below the horizon.

A diagram showing these types of measurement is included at the end of these notes.

Positions of stars, planets, meteors, etc, are sometimes given in altitude and azimuth.   These should not be confused with the standard RA and Declination positions quoted in books,  as they are simply a measure of where the object is in the observer's sky at a particular time.

Abbreviations: During an active display, the features will change quite rapidly, and the observer will have little time in which to record the details.  For this reason, standard abbreviations are used.  Some of these are mentioned above, next to the auroral forms which they describe.  Usually, they have to be used in combinations e.g. rayed band becomes RB.  "Homogeneous" is abbreviated to H, so we have HA for homogeneous arc, and so on.

Features may.be further described as quiet (q), or active (a).

Typical extracts from an observer's notes might read:

UT          h         ^         azi              Feature       Brightness

22.15     10°     15°       350°           qHA              II

22.40    20°      25°       330-020°     aRB              III

i.e. a quiet homogeneous arc at 22.15, becoming an active rayed band at 22.40, increasing in brightness.

Auroral Forms

Maximum height (^), Base height (h) and azimuth for features in a rayed arc.

The arc has (^) = 52°

                  (h) = 7°

The extreme eastern ray is at 35° azimuth. 

REPORTS:  The JAS Aurora Section does not issue report sheets, but observers should try to present their results in a tidy format, set out as above.  Remember to give the double date, your name and address, and the location from which you saw the aurora.

Send your observations in to the section within, say, a month of sighting the auroral display reported.  Observations will be summarised in the JAS Nes Circulars, and in an annual Section Bulletin.

Do keep a lookout for the aurora; all you need to observe it are a 6" ruler, the naked eye and a bit of patience.  Good Luck.





Observing Guide to Venus

Part Two


3. Night side events

These include (i) the strikingly beautiful effects of the illuminated atmosphere, produced by sunlight filtering through the upper part of the planet's atmosphere and (ii) the enigmatic visibility of the dark side, of which very little can be affirmed and for which no cause has yet been discovered.  Both appearances are accessible to modest telescopes but the latter is so feeble that great care must be exercised in its observation.  The illuminated atmosphere usually shows after greatest elongation east.  It consists of faint cusp extensions that gradually creep around the dark limb until at inferior conjunction they link to encircle the planet in a ring of pearl-like light.  The cycle is reversed after inferior conjunction.

The visibility of the dark side is divided into three phases:

(a) In daylight or bright twilight the unilluminated side is always darker than the sky.  The contrast is exceedingly slight but unmistakeable.  It is thought to be optical but opinion is evenly divided.

(b) The bright phase, otherwise known as the Ashen Light.  It manifests itself as a faint glow on the dark side.  It is variable, often  patchy, though sometimes affects the whole of the unlit side.  It has no preferred point of display and is very feeble.  Its detection requires favourable conditions and an acute eye.

(c) The neutral phase.  Occurs when the Sun is about 6 degrees below the horizon, roughly 30 minutes after sunset and the same interval before sunrise.  It apparently marks the transition from dark to bright at an eastern elongation and from bright to dark at a western.

An occulting bar fitted in the eyepiece is a necessity when searching for the Ashen Light for at that time Venus is being observed against a dark background.  However, it is a somewhat imperfect device as glare does tend to creep around each side of the bar. Accordingly the observer needs to develop an expertise in order to discriminate between  the elusive reality and what is patently illusion.

4. Contour anomaly

This is an irregularity in the apparent outline of the telescopic image.  The terminator can have an uneven curvature and the cusps can have a slightly different shape (especially around dichotomy when a blunting of the southern cusp cap is regularly seen).  Projections are occasionally seen along the limb but more often down the line of the terminator.  A careful watch should be maintained and the observer should not expect to see the geometric norm.

5.  Use of colour filters

Colour filters form a significant extension to the programme of the Venus observer.  At present, the range most widely used is the Kodak Wratten series of gelatin-based filters.  These come in various colours and densities and may be conveniently cut to fit eyepiece filter adaptors etc.,  Filters are useful in three areas;

(a)  To reduce the glare of reflected sunlight from the top of Venus cloud deck and hence render the subtle cloud shadings a little more apparent.  Neutral density filters are often used for this purpose.

(b)  To investigate the vertical structure of the atmosphere by measuring the observed phase in different filters.  This phase is usually seen smallest in filters admitting the shorter wave lengths of the visible spectrum, and largest in those admitting the longer wavelengths.

(c)  To standardise drawings made by different observers whose sensitivity to the visible spectrum may vary.  For this purpose the Kodak W15 yellow filter is most suitable.

PHOTOGRAPHY

Astronomical photographic techniques can be applied to Venus with successful results. The observer should experiment with a range of exposures to determine what is best for his or her own individual method and instrumentation. The techniques of prime-focus and eye-piece projection are the most popular.
Ultra-violet photography is difficult but may be attempted. The rewards far success are substantial, though, since Venusian cloud features, delineated by the UV absorbers in its atmosphere, become apparent. The main difficulty is caused by the absorption of UV by ordinary glass. Hence photography must be undertaken at the prime focus or with a quartz eyepiece (available from suppliers as single-element eyepieces). For optimum results, a UV filter should have its maximum transmission between 3200A and 3700A. The Kodak W18A filter, among others has this characteristic. Ordinary emulsions such as Kodak Panatomic X, Plus-X and Tri-X are sensitive in the UV.
After April 1989, Venus moves into the evening sky. If you would like to join in the work of the BAA Venus Group (you could either be an individual BAA member or your local society may be affiliated) then write to John McCue at the address below for observing forms and more details. Good observing!


This guide was prepared by;

Mr David Graham, JAS Planetary Director.

Mr Richard M Baum, BAA Terrestrial Planets Director.

Dr Julius L Benton, Jnr, ALPO Venus recorder.

Mr John Nichol, BAA deputy Venus co-ordinator.

Mr John McCue, BAA Venus co-ordinator.