ioccc/2013/morgan1

To build:

    make

NOTE: this entry requires the X11/Xlib.h header file and the X11 library to compile. For more information see the FAQ on "X11".

To use:

    echo "YYYY/MM/DD" | ./morgan1

where YYYY is the four digit year, MM is the two digit month and DD is two digit day of the month. The arrow keys control the shifting of the date as follows:

• Right : go forwards one day.
• Left : go backwards one day.
• Up : go forwards 20 days.
• Down : go backwards 20 days.

Try:

    ./try.sh

Use the same keys as described above to change the dates each time the program window is in focus.

Judges' remarks:

To verify the output, try echoing the date of an evening with a clear sky. Use binoculars and/or telescope as needed. In some cases you may need to wait for a better time of the evening. In a few cases, such as the South Pole in the summer, you may need to move to a different observing site.

Author's remarks:

Instructions

It displays the position of the 8 planets of the solar system around the Sun. The date given in the command line is in the format YYYY/MM/DD (year, month and day). The direction keys can be used to move in time :

• Left : go back 1 day.
• Right : go forward 1 day.
• Down : go back 20 days.
• Up : go forward 20 days.

It handles correctly all leap years and the switch between Julian calendar and Gregorian calendar (10 days missing between 1582/10/04 and 1582/10/15).

The scale is logarithmic to be able to display the 8 planets without having Mercury, Venus, Earth and Mars squeezed around the Sun.

The size is not proportional either.

Bonus

It also displays the position of 2 famous comets during their last approach:

• The first one around the third IOCCC.
• The second one between the thirteenth and fourteenth IOCCC.

Limitations

• ASCII character set is assumed.
• A C99 compiler is required (for some math.h functions).
• The position should be correct for years between (-1000; +3000)
• The approximations done in the formula are OK for displaying the position around the Sun but not for other applications like ephemeride or sky view from Earth (angles may be off by 1 or 2 degrees sometimes).

Obfuscation

• Reuse of 1 letter variables.
• Only the main() function with some for loops.
• Constants for orbital elements encoded in a string.
• Hexadecimal and decimal values used inconsistently.
• Comparisons order are not consistent.
• A lot of stuff is done in the different parts of the for loop.

Compilation warnings

With gcc 4.7.2 on Linux Ubuntu 12.10:

    morgan1.c:38:3: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    morgan1.c:40:34: warning: value computed is not used [-Wunused-value]
    morgan1.c:56:6: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
    morgan1.c:58:50: warning: value computed is not used [-Wunused-value]
    morgan1.c:61:5: warning: suggest parentheses around ‘-’ inside ‘>>’ [-Wparentheses]

With clang 3.1 on Linux Ubuntu 12.10 :

    morgan1.c:38:49: warning: '&&' within '||' [-Wlogical-op-parentheses]
    morgan1.c:38:27: warning: '&&' within '||' [-Wlogical-op-parentheses]
    morgan1.c:55:13: warning: '&&' within '||' [-Wlogical-op-parentheses]
    morgan1.c:56:14: warning: '&&' within '||' [-Wlogical-op-parentheses]

This is due to most of the parentheses and braces being removed when unnecessary.

To verify the display, you can look at one of these programs :

• xephem : https://web.archive.org/web/20130310133219/http://www.clearskyinstitute.com/xephem/
• homeplanet : https://www.fourmilab.ch/homeplanet/
• https://www.theplanetstoday.com