Chapter 6 – The Planets (Planets through a Telescope)

Solar System Trek, view the solar system from the skies of any planet on any date
Planetary Ephemeris, trace the planets in the sky for any date
The Moons of Mars, animated presentation of Mars and its moons
Jupiter’s Satellites, identify and name the positions of Jupiter’s four moons on any date
The Rings of Saturn, simulation of Saturn and its rings
Saturn’s Rings, brief outline only
Saturn Draw, a Computer Aided Design
Planets through a Telescope, relative sizes of the planets
Globe-pixel, plots a globe divided at 10° intervals of latitude and longitude
Globe Projection, as before, using lines.

Planets through a Telescope

  • How large does Jupiter appear in binoculars magnifying x 10?
  • Are Mercury and Mars sometimes as small as Uranus in the telescope?
  • What range of sizes does Venus appear to be when closest and at its most distant?

This program answers these questions and gives an impression of the relative size of all the planets as seen through a telescope for any magnification up to x 300. Included in the printout to the screen is the maximum and minimum size of each planet in seconds of arc and a scaled disc according to the magnification INPUT. Figure 6.10 is a typical screen copy.

Figure 6.10
Relative size of all the planets in a telescope magnifying x 123.


The display uses a black screen (BORDER 0: PAPER 0: INK 9) and the outline for each planet is coloured via the conditional INK command in Line 110. This command is matched reasonably well to the actual colours of the respective planets — Mars is red, Jupiter and Saturn are yellow, and so on. In the case of Jupiter, two equatorial belts are added and Saturn has an edge-on ring system.

The program
The layout of the program is both economical and straightforward, with the DATA on the planetary diameters, both maximum and minimum, stored in Line 90. These are READ in Line 110 with variable a for the minimum diameter and variable b for the maximum diameter of each planet, using the FOR/NEXT n loop for the seven planets.

The CIRCLE commands in Lines 130 and 140 produce an outline of each planet immediately the DATA is READ but modified according to the magnification as INPUT via the variable c. Lines 160 to 250 PLOT and DRAW the belts and rings of Jupiter and Saturn respectively, and correctly scaled. The planet names are PRINTed to the screen via Line 40. It is often more economical to use this simple arrangement than DATA/READ. Note the three apostrophes (”’) between each planet name — these are to give two blank PRINT lines below each name.

Interpreting the results
The results need to be interpreted if the information is to be meaningful. The program indicates the minimum and maximum size the planets can appear as seen from Earth through a telescope.

The minimum diameter applies when a planet is at its most remote from Earth in what is called the far side of the Sun or ‘superior conjunction’ — ie the Sun is between us and the planet. On these occasions the planet is unobservable due to its apparent proximity to the Sun. (This restriction does not necessarily apply to radio astronomers who could communicate with and receive data from the Viking spacecraft on the Martian surface during these periods, the radio signals being beamed within a degree or so of the Sun’s position.)

Best viewing
When the planets from Mars through to Neptune appear at their largest, they are normally due south at midnight and are seen at their best. Mars’s orbit is eccentric to the extent that the full diameter is not experienced at each ‘opposition’ as it is called, ie opposite to the Sun. In the case of Mercury and Venus, these planets are permanently nearer to the Sun than the Earth and so at times either can appear to pass in front of the Sun’s general position at what is called inferior conjunction. Again, these two planets are unobservable due to their proximity to the Sun even when they are at their closest to Earth and at their largest apparent diameter. On extremely rare occasions, the silhouette of Mercury or Venus can be seen (with special equipment only) in transit across the Sun’s disc.

In general, then, for normal viewing the planets tend to fall into an intermediate size between maximum and minimum. The greatest range of apparent sizes is for Venus, the nearest planet to Earth. The least range for Neptune — the remotest planet.

10 REM Planets through ‘scope
20 LET d=120: LET e=180: BORDER 0: PAPER 0: INK 9: CLS
30 PRINT PAPER 5;”Planet when remote/nearest”
40 PRINT PAPER 1””Mercury””'”Venus””'”Mars””'”Jupiter””'”Saturn””'”Uranus””'”Neptune”
50 INPUT “Magnification x”;c
60 IF c>300 THEN GO TO 50
70 PRINT PAPER 3;AT 0,26;”x”;c
90 DATA 4.7, 13, 11, 63, 3.5, 26, 30, 48, 14, 20, 3, 4, 2.3, 2.3
100 FOR n = 1 TO 7
110 READ a,b: INK 5+ (1 AND n>3 AND N <6) – (3 AND N=3)
120 PRINT AT n*3,8; a; CHR$ 34; TAB 28;b; CHR$ 34
130 CIRCLE d, INT (170-n*24), INT (a*c/500)
140 CICRLE e, INT (170-n*24), INT (b*c/500)
150 NEXT n
155 INK 6
160 LET r=c/18: x=c/9
170 LET r1=C/11: LET x1=c/5.5
180 PLOT d,73-c/d
190 DRAW r,0: DRAW -x,0: DRAW 0,c/50: DRAW x,0
200 PLOT d,50
210 DRAW r,0: DRAW -x,0
220 PLOT e,50
230 DRAW r1,0: DRAW -x1,0
240 PLOT e,73-c/d
250 DRAW r1,0: DRAW -x1,0: DRAW 0,c/50: DRAW x1,0
260 PRINT #0; FLASH 1; “Press C to continue”: PAUSE 0: RUN

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