The starting and finishing points of astronomical calculations are usually angles, expressed as degrees, minutes and seconds of arc, or times, measured in hours, minutes, and seconds. Yet computers often cannot handle these quantities as they stand. They must first be converted to decimal degrees or hours before the computation can begin, and then the result converted back into the more-familiar minutes and seconds form. Consider, for example, the problem of finding the time corresponding to 1 hour 37 minutes before 3 : 25 in the afternoon. We first convert the times to their decimal forms, 1.616667 hours and 3.416667 hours, then subtract the former from the latter to get 1.800 hours, and finally convert back to minutes and seconds: 1 : 48 pm.

Your computer can deal with the problem of converting angles or times between decimal and minutes/seconds forms with the aid of the subroutine MINSEC. The direction in which the conversion is carried out is controlled by the switch SW(1). When SW(1) has the value +1, then the quantity represented by the input argument X (decimal degrees/hours) is converted into degrees/hours, minutes, and seconds which are returned by the output arguments XD, XM, XS respectively. XD and XM represent whole numbers while XS represents the seconds correct to two decimal places. The sign of X is returned by the string variable S$ which has the value ‘ + ’ unless X is negative when S$ becomes ‘―’. The real variable SN also carries this information since SN = +1 if X is positive, SN = 0 if X is zero, and SN = −1 if X is negative.