# Chapter 6 – The Planets (Globe Projection)

## Globe Projection

This program DRAWs a globe divided into lines of latitude and longitude at 10° intervals rather than as a single pixel to mark each division as via the Globe-pixel program.

This program is not as accurate as the previous routine because it uses the Spectrum’s DRAW command which, although executed rapidly, cannot produce the required elliptical lines but only simple arcs. The largest errors, such as they are, occur adjacent to the edge of the disc and in the polar regions. Nevertheless, the simulation is quite effective and the user has the option to show either a polar or an equatorial view. The latter may be tilted up to 12° in a north or south direction. Figures 6.13 and 6.14 are typical COPYs from the Spectrum screen. The REM statements show the general structure of the program.

Figure 6.13
The globe can be drawn for a polar or equatorial viewpoint. The latter can be tilted up to ±12°. Figure 6.14
Polar viewpoint. 10 REM Globe Projection
30 BORDER 5: PAPER 5: CLS : PRINT PAPER 4;”Globe projection [10″; CHR\$ 130;” int] ”
40 LET a=148: LET b=80: CIRCLE a,b,b
60 DIM x(9): DIM y(9)
70 FOR n=1 TO 9: READ y(n)
80 LET x(n)=63*(1.24*ATN (PI/180*n*10.5)): NEXT n
90 DATA 79,75,70,64,56,47,36,27,0: RESTORE
110 INPUT “Polar or Equitorial (p or e)? “; LINE a\$: GO TO (a\$=”e”)*120+(a\$=”p”)*270
130 PRINT “[max tilt=12 HR\$ 130;”‘”Equatorial “;
135 INPUT “Tilt (-s): “;k: IF ABS k>12 THEN GO TO 135
140 PRINT k;CHR\$ 130: LET k=k/14
150 FOR n=0 TO 36 STEP 2
160 LET f=n-18: LET g=2.71
170 LET h=g*ATN (PI/180*-f*7.3)
180 PLOT a,0: DRAW 0,b*2,h
190 NEXT n
210 PLOT a-b,b: DRAW b*2,0,k
220 FOR n=1 TO 9: LET c=y(n)
230 PLOT a+1-x(10-n),b+x(n)
240 DRAW c*2,0,k
250 PLOT a+1-x(10-n),-b+x(n)
260 DRAW c*2,0,k: NEXT n: STOP
280 PRINT “Polar”
290 FOR n=0 TO 72 STEP 2
300 LET d=n/36*PI: LET z=b*SIN d: LET yy=b*COS d
310 PLOT a,b: DRAW z,yy: NEXT n
330 FOR n=1 TO 9
340 CIRCLE a,b,x(n): NEXT n
350 STOP