The eclipse pictures printed in Figure 7, which show lunar and solar eclipses at various stages, were produced by subroutine DISPLAY listed later in this section. Unfortunately, it is not possible to write code which will work equally well on every computer since the mechanics of producing graphics displays varies so much from machine to machine. You will certainly have to modify DISPLAY to suit the particular characteristics of your own computer. To help you do so, I have collected all the graphics commands into a number of function calls; for example, FNCLGS clears the graphics screen to black. You can use DISPLAY just as it is provided you can define the functions so that they work on your machine. Alternatively, you may replace the functions with calls to procedures or subroutines to do the same thing. Thus, if your BASIC includes the graphics command ‘HOME’, for example, which clears the screen, you could substitute ‘GOSUB 9000’ for ‘X = FNCLGS’ and include the extra subroutine

9000 Home : RETURN

at the end. Obviously, more complex procedures will require several lines of code each.

The graphics screen on my computer measures 392 pixels in the horizontal, or X direction, and 256 in the vertical, or Y direction. You can think of the screen as a large sheet of graph paper, with the point X = 0, Y = 0 at the bottom left-hand corner of the screen.