ZX Computings - OpenLibra

Orbit
S M King from Bristol takes
you and your ZX81 far out
into space, where you can
obsereve the four
innermost planets of the
solar system — Mercury,
Venus, good old Earth and
Mars — happily circling the
sun.
S M King from Bristol takes
you and your ZX81 far out
into space, where you can
observe the four innermost
planets of the solar system
— Mercury, Venus, good old
Earth and Mars — happily
circling the sun.
Orbit is a program which
was originally intended to
produce an animated simulation
of the movement of
the four inner planets in
our solar system around the
sun. However, it soon
became apparent that producing
accurate elliptical
orbits was going to be
rather difficult, especially
with the limited 64x44
resolution available. The
result is that the program
uses circular orbits to
display the relative motion
of the planets at the expense
of accuracy. Whilst it
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may still find use in an instructive
role it would be
much better placed in, say,
a "Star Trek" type of program
as a "long-range scanner
report".
The Program
The program was written
on a Sinclair ZX81 with 8K
Floating-point BASIC and
16K of RAM, although it by
no means uses this much.
With the omission of REM
statements and with the
use of other space-saving
tips (which will be discussed
later) the program should
fit easily within 7K — with
four planets.
Documentation is given
to aid conversion to other
micros and the program
should RUN unaltered on a
ZX80 with the 8K BASIC.
PRINT the introduction. Line 35, for
example, PRINTs one blank line,
cause the introduction to be displayed
for 4000/50 (80) seconds. The POKE
instruction simply resets the system
variable. These lines could be replaced
by a FOR/NEXT loop.
CLS is the "Clear Screen" instruction,
assign variables to the number of
radians each planet moves per day. The
numbers are expressed in scientific
notation: ie, 57 • 1 E-03 - 0 • 0571.
pixel co-ordinates for the start position
of Mercury (Hermes),
pixel co-ordinates for the start position
of Venus.
pixel co-ordinates for the start position
of Earth.
pixel co-ordinates for the start position
of Mars.
assign variables to the "orbital radii" of
each planet. The numbers are the
number of pixels,
display the first "question". The
animated display will show successive
earth days, months or years.
ZX81 Programs
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causes the program to wait until a
number (the choice) is entered,
checks to see if the number entered is a
1, 2 or 3. If it is then the program
continues at line 245. If not, then a
reminder is PRtNTed (line 235) and the
program returns to the INPUT
statement at 225.
choose the period incrementing factor
(in days),
sets the period counter to one.
display the second "question". How
many days, months or years are to be
displayed?
makes the number entered at line 310
an integer if it was a decimal. It has no
effect if the number was already an
integer. Note that the number must be
greater than one for the program to
continue.
displays the third "question". What realtime
time delay is required between
successive "frames" of the animation?
Line 345 prevents the delay from being
zero seconds. Line 350 converts the
figure entered into a number used by
the PAUSE instruction.
DIMension 8 arrays (2 per planet) to hold
the pixel co-ordinates for PLOTting and
UNPLOTting. The first array in each case
holds X-coords, the second Y-coords.
calculate the positions that each of the
four planets will be at during the
simulation. Lines 450, 470, 490 and 510
calculate X pixel co-ordinates, whilst
Lines 455, 475, 495 and 515 calculate Y
pixel co-ordinates.
"black-in" the starting positions of the
planets.
sets the main loop counter to one.
increments the period counter,
display the positions of the planets
during the simulation,
checks to see if the required number of
days/months/years have been
displayed. If they have then the
program jumps to line 700.
"black-in" the positions of the planets
during the simulation,
increments the loop counter,
returns the loop.
PROGRAM BETWEEN LINES 700 AND 999 ONLY WORKS
WHEN THE MAIN SIMULATION HAS FINISHED.
18 ZX COMPUTING SUMMER 1982 i