The drawing from the 13th. April to the 27th. May are not to scale as Mr. Clarke observed Venus whilst it had an apparent diameter of 26.7 sec. of arc whilst mine, six weeks later, Venus had increased to 51.8 sec. of arc, and so would have to have drawn my drawing a little over half as big again to maintain the correct degree of angular scale.
The above effect is simply due to perspective as Venus is drawing closer to us, hence demonstrating the rapid change in the size of the disk during those six weeks.
The Apparition of Mars 1988
By Vaughan Cooper
Of all recent apparitions of Mars, this coming one will be the most favourable, as Mars will attend the closest position to Earth on 22nd. September, reaching opposition on 28th. September and will have a relatively high declination in the sky of around —2°.
Even during June the diameter of Mars will attend 10.8 seconds of arc, and by using a magnification on your telescope of x240, the image of Mars will nearly be as half as big again as the full Moon appears to the naked eye, so this is a very favourable apparition and must not be missed.
Every opportunity should be made to observe this planet, and the following details may help you what to look for.
Twilight Observations
Provided the planet has a moderate altitude, as it will be around early June, good views may be obtained to some advantage as the diminished glare helps to bring out faint markings which otherwise tends to be swamped out when seen against a dark sky, also a slight mist or haze can be effective in eliminating the glare, so don't let the local atmospheric conditions put you off from observing during this important apparition.
The Polar Cap
The south polar cap will be pointing towards the Earth and will be evaporating, and so visible shrinking during the period up to opposition and may or may not totally disappear.
Usually though, a small remnant remains so look out for cap rifts, bright portions and detached parts.
The regression of the south polar cap may be hindered by Martian cloud activity.
The largest of these observed rifts was first recorded in 1845 and named the Mountains of Mitchel after it's discoverer. We now know that these 'mountains' are actually low areas where ice is preferentially retained.
Cloud Activity
At irregular and unspecified intervals the surface markings are veiled by clouds in the Martian atmosphere these are of two kinds, yellowish and whitish.
The yellow clouds occasionally cover great areas of the planet and so you may find it difficult to identify certain landmark features. On other occasions cloud may be suspected when any well known dark markings is perhaps whitish in tone or colour or partly invisible, or appears unusually pale whilst neighbouring areas retain their usual intensity and distinctness.
It's possible during late September or early October a major storm will begin, but where will it originate from we can not predict, in previous pre—perihelic years major dust storms have occurred in Hellas Iapigia as in 1986 and before then in 1971, or Solis Casus as in 1973.
If you take the trouble to observe Mars consistently during the coming weeks your observations will make very useful contributions to following any cloud or storm activity and bring right up to date the latest developments on Mars.
The yellow clouds occur low in the Martian atmosphere and are believed to be clouds of dust, the white clouds are less easy to see, they occur high in the Martian atmosphere and believed to be due to solid carbon crystals.
The use of the Wratten range of filters are especially versatile, either a W25 or W29 red, will penetrate the atmosphere to emphasise the dark areas whilst a W44A or W47 blue will high light atmospheric phenomena.
Dust Storms
It's possible during late September or early October a major storm will begin, but where will it originate from we can not predict. In previous pre—perihelic years major dust storms have occurred in Hellas Iapigia as in 1986 and before then in 1971, or Solis Lacus as in 1973.
A list of the names of features which have been known to change in previous apparitions and will require detail observing, see map for positions on the Martian disk.
Hellespontus
Pandorae
Solis Lacus
Phasis
Achillis Fons
Idaeus Fons
Aetheria
Nodus Alcyonius
Gomer Sinus
Hesperia
Mercater projection showing the principal features along the temperate and equatorial region of Mars.
The shaded areas do not represent albedo values.
The size of the Martian disk with dates and magnitude.
June 8 10.8" -0.4
June 18 11.7"
June 28 12.7"
July 8 13.8" -1.0
July 18 15.0"
July 28 16.5"
Aug 7 18.0" -1.7
Aug 17 19.6"
Aug 27 21.3"
Sep 6 22.7" -2.4
Sep 16 23.5"
Sep 26 23.6"
Observers who are not good draughtsmen or are unable to make drawings, can do useful work by making notes descriptions of the appearance of the features which can be identified with certainty, the visibility of the fainter markings, especially those known to vary, the whitening of specified areas when near the limb and their changed appearance when near the centre of the disk, such notes are generally more useful than inferior sketches.
Drawing scales
2in. in diameter is recommended although a scale of 3mm to the second of arc has been advocated as this avoids over crowding detail at perihelic oppositions.
DO NOT LET THIS APPARITION OF MARS GO BY UNOBSERVED
Pisces
Movement of Mars to be observed with the unaided eye in the constellations of Aquarius and below Pisces.
Additional notes on features mentioned or annotated on the map
Hellas
A vast circular plain of approximately 2,000km. in diameter and often appears very bright and white as seen from Earth, and has been known to be so prominent as to be mistaken for a extra polar cap.
Before space vehicles studied Mars surface Hellas was regarded to be an elevated plateau, upon which white polar deposit persisted when the covering had disappeared from the adjacent areas.
In fact Hellas is a depression basin, the lowest known feature, as it lies 3km. below the mean surface and where the atmospheric pressure is high enough for liquid water to exist.
The western boarder of Hellas is better defined than the eastern boarder as this is probably due to prevailing wind directions on Mars.
Hellespontus
A heavily cratered area to the west of Hellas
Solis Lacus
One of the most variable areas on Mars and since 1877 systematic telescopic observations have noted pronounced changes in shape and intensities of this features. In recent years major changes have occoured in 1971 and 1973.
Nodus Gordil
A lofty volcanic cone of 20km. high and is certainly one of the highest points so far measured on Mars.
Antoniadi noted that it was one of the few features which could still be identified when the planet was still covered with dust.
Syrtis Major
This feature was recorded telescopically by Huygens in the 17th. century and is conspicuous in any moderate telescope. It has always been regarded as a deep depression but in fact it's a smooth elevated plateau sloping off to either side, but what is surprising from the Mariner photographs, there is nothing to distinguish this feature apart from it's colour.
To the north of Syrtis Major lies Casius nicknamed the Wedge also a albedo feature as is Nilosyrtis undoubtedly one of Lowell's so called canals.
Syrtis Major region has been known to show considerable variation from year to year, particularly the area to the east of Syrtis.
Aetheria
A large area where photographs show relatively few craters.
Acidalium
A large smooth depressed plain, a feature easily seen in moderate telescopes.
Herodotus
