# Chapter 6 – The Planets (Saturn’s Rings)

## Saturn’s Rings

This program is similar to the previous one but stripped to the minimum to produce an outline of the planet and its ring system. It indicates how a program can be brief and still prove satisfactory. Figures 6.7 and 6.8 are COPYs of the screen.

Figure 6.7
Saturn with rings wide open. Figure 6.8
Saturn with rings edge on. The program
With this program you can simulate the appearance of Saturn and its ring system tilted at any chosen angle from 0 to 90 degrees. If 0 is ENTERed, the planet is drawn as viewed directly over its equator with the rings shown edge-on. If 90 is ENTERed, a polar view is presented with the ring system completely encircling the planet. Any intermediate angle will tilt the planet. As seen from Earth the appearance is limited from 0 to 26.73 degrees of tilt in both north and south directions plus or minus 0.49° depending on the relative positions of Earth and Saturn in their orbits at the time.

For simplicity, no attempt has been made to delete the ring system as it passes behind the planet and it is left to you to decide which portion of the rings is nearest to you. The Spectrum will influence your initial decision by drawing across the lower section of the globe first.

The Spectrum draws the globe and ring system with the PLOT command, using a modified Ellipse program (see Chapter 9). In the case of the globe the reduction in oblateness from 10% to zero (full circle) is applied progressively through the change of tilt from 0 to 90 degrees.

10 PRINT “Saturn’s Rings “;
30 INPUT “Tilt (0″; CHR\$ 130;”-90″; CHR\$ 130;”)”,z
40 PRINT “Tilt=”;z;CHR\$ 130
50 LET ob=1.1*z/100
60 LET z=1/SIN ((.1+z)/180*PI)
70 LET x=128: LET y=88
80 LET r=30: LET h=70
90 GO SUB 180
110 FOR n=1 TO 1.6 STEP .1
120 FOR f=0 TO PI*2 STEP .05
130 LET sx=INT (SIN f*h)
140 LET cy=INT (COS f*h/z)
150 PLOT x+sx/n,y+cy/n
160 NEXT f: NEXT n: STOP
180 FOR f=0 TO PI*2 STEP PI/100
190 PLOT INT (x+SIN f*r),INT (y+COS f*r*(.89+.11*ob))
200 NEXT f: RETURN