MIRA 27
Jan 1990


Lunar Ridge near Schickard

FEATURE . . . . Ridge near Schickard

LOCATION . . . . 42° S 66° W
DATE  . . . . . . . 1989 / 11 / 11
TIME  . . . . . . . 17.45 U.T. to 18.10 U.T.
MOON  . . . . . .  13 days old
LUNATION  . . .  827
COLONG  . . . .   67
SEL. LIBRATION  -1.5 long. -6.0 lat.
OBSERVER . . .  I Clarke,  Warwickshire
CONDITIONS . . Good, clear sky, seeing Ant. II TO III
INSTRUMENT . . 114 mm Catadioptic reflector x167 & x250


The moon had not long risen when I started my observation of the Western limb, it was a mild evening and the sky was not fully dark.  On the South West section I saw what looked at first glance like a crater cut through by a giant knife.  So out with the pencil (and rubbers).  The bright line across the centre is most probably a small cliff or ridge or a fault, just at the right height to catch the morning sun.  It looked quite striking and the whole area was full of detail to small to be drawn, notice the "Birds Head" feature.  Later, after tea, I got out my copy of Patrick Moore's book 'The Moon' and tried to find what I had seen and drawn. After rechecking the observation and the local area along side my map I realised that it was not marked on page 72 (Near Side, Mercator projection), nor was it mentioned in Moore's 'Guide to the Moon' or Fred W. Price's 'The Moon Observer's Handbook'.  By now I was rapidly running out of books to consult.  Later that night, 20/21 UT, I could only just make out where the object was as the terminator had moved on so much and the sun had filled most of the shadows leaving the area featureless.   But I did find it and made a note of the surrounding area to identify it later.  The bright 'wall' runs towards the large crater Schickard, 134 miles in diameter.  The whole area is covered with the ejecta from the Mare Orientale impact, so maybe that is what the 'wall' is, a layer of dust and rock.  Later that night I found on page 83 of 'The Moon' an Orbiter 4 picture of the Mare Orientale which had, 22mm from the bottom and 22mm in from the right, an old ruined crater with a bright line across it!





JUNIOR ASTRONOMICAL SOCIETY

AURORA SECTION

Observing Aurora Part 1


Notes for observers



The aurora is one of the most beautiful natural phenomena, resulting from the excitation of atoms and molecules of the upper atmosphere at heights of 100km. At such heights, the atmosphere is a good approximation to a vacuum, and the conditions and processes are very similar to those in gas-discharge tubes.Aurorae are produced when fast protons and electrons, injected into the solar wind by solar flares or from X-ray "hotspots" on the sun's surface, arrive in the earth's environment. The Earth's magnetic field guides these particles onto the atmosphere at certain locations, by a number of complicated processes.
Generally, it can be said that the aurora is located in two oval regions, one in each hemisphere, around the magnetic poles. At times of intense solar activity, these ovals are pushed towards the equator, and the aurora is seen at lower geomagnetic latitudes. It is at such times that the aurora is seen from the more densely-populated regions of the UK.
Auroral activity follows the solar cycle fairly closely, and usually has two peaks. The first precedes sunspot maximum by about 12 months, and is due to solar flares. One or two years after sunspot maximum, a second auroral peak, resulting from coronal holes and other X-ray wavelength solar activity, is seen. Even at times of low solar activity, it is not unknown for impressive aurorae to be seen at temperate latitudes, and observers should keep an eye open for aurora at all times. It is safe to say, however, that the aurora is most often seen around sunspot maximum.

The aims of the section are two fold:-

a) To train observers in the standard method of auroral recording.

b) Co-operation with the BAA Auroral Subgroup; there already exists a small network of observers, who routinely watch for aurorae.  Results of sufficient quality from JAS members will be forwarded to the BAA group, which collects observations from all over the world.  These results are made available for professional use.  We would also hope to make observers at more southerly latitudes, e.g. the English Midlands, aware of the fact that aurora can be seen several times per year from their locations, and encourage such observers to regularly look out for activity.


Auroral forms and how to record them



The aurora is not easily predictable, unless radio or geomagnetic equipment is available. Therefore, the best idea is probably to keep a watch on the sky to the north every hour or so. Observers engaged in other fields, such as meteor or variable star work, may also see aurorae from time to time; reports of such casual sightings will also be appreciated.
Auroral activity in temperate latitudes follows a fairly well-defined pattern, and has easily-identified structures. These are described below, and in the diagrams at the end of these notes.

Glow (N)   From the southern parts of Britain, this is the most common form of auroral activity, if any is present at all.  Simply, it consists of ill-defined light on the north horizon, and represents the top parts of a display which will be more impressive further north.  Such glows can remain unchanged all night, or may disappear after a time.  Beware of misidentifying town lights in the line of sight for this type of aurora.  At lower latitudes, the glow does sometimes precede the onset of more spectacular activity.

Arc (A)   When the aurora comes south over the UK, the glow resolves itself into better defined forms of  activity.  The first of these is usually the arc, a bow-shaped curve with its highest point almost due north.  The arc may be featureless (homogeneous arc - HA), or may brighten at certain points along its length and develop rays, to produce a rayed arc (RA).

Band (B)  As activity picks up during an auroral substorm, the features rise higher in the sky and begin to move about, sometimes quite rapidly.  One form of this activity is the band, which develops when the arc bends round on itself to produce a ribbon-like structure.  Sometimes the band will have several kinks in it, giving a very impressive multiple form.  Bands; like arcs, can be rayed or homogeneous.

Rays (R)   Rays are vertical shafts of auroral light, extending upwards from other features as mentioned above.  Occasionally, rays are seen sticking up over the observer's horizon in the absence of other activity.  These are actually part of a display just below the horizon, and have a very eerie appearance, especially if moving, resembling searchlight beams.  Rays are often described in auroral notes by a subscript number to indicate their length, R1 being short to R3 long.  Auroral rays mark the positions of magnetic field lines, and stick up through the atmosphere for considerable distances.  Since conditions vary along the length of the ray, colour differences are often seen.  Typically, rays are reddish at the top, and greenish or white at the base.  Purple colours are sometimes produced in rays sticking up into the sunlit zone of the atmosphere as a result of fluorescence.

Patches (P)   Out with the main body of an auroral display, the observer will sometimes record patches of ill-defined auroral light.  As with other features, these may be rayed or homogeneous.  Patches often change in brightness and size, appearing to "switch" on and off over periods of some minutes.  One particularly interesting aurora, on August 29/30 1978, consisted entirely of this type of activity.  Patches are also referred to as surfaces by some observers.

Corona (C)   At the peak of a particularly strong auroral display, the activity may actually pass overhead from the observer's location, and even into the south of the sky.  When this happens due to perspective, a corona will form.  Typically, a corona takes the form of bundles of rays apparently radiating away from a single point; this point is actually the observer's, magnetic zenith, which lies 10 - 15° south of the  true zenith from the UK.  After the production of a corona, the display may collapse back to a less active form, or even subside entirely.  During particularly strong auroral storms, however, activity may peak  several times in the course of a single night, and there may be aurora on several successive nights.

Veil (V)   Normally, during an active aurora, the observer will see a faint background glow — the veil — extending above and beyond the main display.

The features listed above can also under-go changes in brightness which can be described under two categories:

a) Pulsing:  Slow, rhythmic changes in brightness and size, already mentioned in relation to patches.

b) Flaming:  More rapid changes, on the order of seconds.  Flaming consists of waves of brightening passing from the horizon upwards; one description compares flaming with the passage of wind over a field of corn.  As each wave passes, local features of the aurora are brightened.  Flaming usually signals a decline in activity, but can also precede formation of a corona.

Part 2 in MIRA 28





OBSERVING GUIDE TO VENUS

Part 1

INTRODUCTION




Venus is our nearest planetary neighbour, and the most conspicuous of the planets known since classical times. It is the second in order out from the Sun and, being inside the orbit of the Earth, passes through a cycle of phases like the Moon. But the phase is just about the only feature visible through the telescope. All Venus ever shows is a bright, slightly yellowish surface which is the upper deck of a thick layer of cloud beneath which the solid globe of the planet rotates unseen by the optical astronomer.
Confounded by a brilliance thats "dazzles the sight and exaggerates every imperfection of the telescope" (Sir John Herschel), it is not unknown for the would~be observer to give up in sheer frustration and turn to a more amenable object. And yet under favourable conditions when its brilliance is tempered by haze or a man-made filter, Venus is a different subject and the practised eye is quick to spot a pattern of shadings, less distinct than those of Mercury or Mars but as certain. With experience still more will be seen for then the observer will be observing rather than looking.
Scrutinising the image, noting every nuance of shade and irregularity, accurately recording that which is affirmed, indicating that which is elusive, it is a gradual process. Patience, care, determination and a methodical approach are the elements which in time will unite into a knowledge of the subject that transcends the books.

WHEN IS VENUS BEST SEEN?

By definition, an inferior planet can never be seen al1 night long (the maximum angular separation of Venus from the Sun can never exceed 47 degrees) but when well placed, e.g. when greatest elongation occurs near the vernal equinox, the planet may be visible for more than 4 hours after sunset for  observers resident at northern temperate latitudes.  At this time the ecliptic presents a steep gradient to the western horizon.  A similarly favourable presentation in the morning sky occurs when Venus comes to greatest elongation west near the autumnal equinox.  Conversely, when greatest elongation east  occurs near the autumnal equinox the ecliptic makes a shallow gradient to the horizon and Venus is poorly seen in the evening sky, as would be the case for the morning sky if greatest elongation west occurs near the vernal equinox.  However, the latter set of circumstances find Venus at its best for observers resident in the southern hemisphere.

AT WHAT TIME OF DAY SHOULD I OBSERVE?

The period between superior conjunction and greatest elongation is about 31 weeks, that between greatest elongation and interior conjunction about 10 weeks.  The apparent diameter of the disk ranges from approximately 10 arcseconds at superior conjunction to over 60 arcseconds at inferior conjunction but it should be remembered that increasing disk diameter means a decreasing phase.  Venus reflects 76% of the incident sunlight which explains the brilliant spectacle it makes in the twilight sky.  The planet reaches magnitude -4.5 at greatest brilliancy, 35 days after eastern elongation and 35 days before western elongation.  The brilliancy of the disk means that telescopic observation of the planet in a twilight or dark sky is hampered by glare which subdues subtle detail.  For this reason, Venus is best seen against a bright sky foreground; at sunset or sunrise is an ideal time.  During the winter months Venus may be usefully observed at any time during the daylight hours but this is not practical during the summer when the seeing is often poor while the Sun is above the horizon.

HOW DO I FIND VENUS AGAINST A BRIGHT SKY?

An equatorial mount fitted with Setting circles will be an advantage in this respect.  There are various methods by which Venus, when an evening object, may be found using setting circles but perhaps the easiest way is simply to sweep in the planet's declination, the planet coming into view in the telescope's finder.  When a morning object, Venus may be found with the naked eye and followed through the telescope as dawn gets under way.  However, whatever method is used extreme care should be taken when Venus is in proximity to the Sun.


OBSERVABLE FEATURES

1 - Phase Anomaly

The observed phase may differ slightly from the predicted value, this being known as the Schroter effect.  Dichotomy is seen to occur a few days early at eastern elongation and likewise late at western elongation.  There are three ways in which the observed phase may be determined and these are:

A.   Direct measurement with an eyepiece micrometer

B.   Comparison of the observed disk with a sequence of pre-drawn disks showing different phase values, (see diagram 1)

C.   Measurement of a disk drawing, the diameter of which would be 50 mm at full phase.  Care should be taken when attempting to determine the phase from a photograph as the outline of the planet varies with the exposure at the telescope and at the processing stage.

2 - Albedo detail

Although often appearing completely devoid of detail to the novice, with patient practice subtle brighter or shaded areas may be discerned on the disk.

The terminator often appears shaded and diffuse but bright areas have been observed.  The apparent poles are often bright, these being known as the cusp  caps and these in turn may be bordered by dark cusp collars.  All these features are nearly always of an ephemeral nature and an intensity scale can be used to describe them.  The scale, due to Patrick Moore, is as follows;

0 Extreme brilliance (exceptional white spots)

1. Bright areas (cusp caps etc.

2 General hue of the disc

3 Elusive shadings, on the limit of visibility

4 Shadings which can be seen without doubt

5 Unusually dark shadings

It is a good idea to record intensity estimates on a separate sketch.  Cloud shadings recorded carefully can often show the four-day retrograde rotation of the atmosphere.