Apparent & absolute magnitude

I have made a simple program that calculates the absolute magnitude when you
give the distance and apparent magnitude. it can also calculate apparent
magnitude and distance. You can download it at
http://users.pandora.be/alex.duerloo/MAGNIT.EXE (41 Kb) If you want the
code, here it is, it's written in QBasic 4.5:

DECLARE FUNCTION LOGAR10! (X!)

10 DO
CLS
PRINT "Magnitude calculator by Alexander Duerloo (2003)"
PRINT ""
PRINT ""
PRINT "What do you want to calculate?"
PRINT ""
PRINT "1. The absolute magnitude of a star"
PRINT "2. The apparent magnitude of a star"
PRINT "3. The distance of a star"
PRINT "4. Exit"
PRINT ""
ch$ = INPUT$(1)
ch = INT(VAL(ch$))
IF ch 0 AND ch 5 THEN
EXIT DO
END IF
BEEP
LOOP
CLS
SELECT CASE ch
CASE 1
INPUT "What's the apparent magnitude of the star"; apmag
INPUT "What's the distance of the star (in light-years)"; dist
dist = dist / 3.26
abmag = apmag - ((-5) + (5 * LOGAR10(dist)))
PRINT ""
PRINT "The absolute magnitude of the star is "; abmag; "."
PRINT ""
SHELL "pause"
CASE 2
INPUT "What's the absolute magnitude of the star"; abmag
INPUT "What's the distance of the star (in light-years)"; dist
dist = dist / 3.26
apmag = abmag + ((-5) + (5 * LOGAR10(dist)))
PRINT ""
PRINT "The apparent magnitude of the star is "; apmag; "."
PRINT ""
SHELL "pause"
CASE 3
INPUT "What's the apparent magnitude of the star"; apmag
INPUT "What's the absolute magnitude of the star"; abmag
dist = 10 ^ (.2 * (apmag - abmag + 5))
PRINT ""
PRINT "The distance of the star is "; dist * 3.26; "light-years."
PRINT ""
SHELL "pause"

CASE 4
SYSTEM
END SELECT
GOTO 10



FUNCTION LOGAR10 (X) STATIC
LOGAR10 = LOG(X) / LOG(10#)
END FUNCTION