hp 25

more advanced than the hp-21, the 25 has two shift keys, orange and blue, the addition of statistics functions and its programmable.

the deg/rad switch of the 21 is transformed into a prog/run switch whilst deg/rad and now grad are on keyboard. the mode is, however, lost when switched off along with the memory.

49 step programmable, no subroutines, but a comprehensive set of conditional branches. 7 memories. branch targets are step numbers rather than labels. this made sense with limited memory.

it comes with a fur lined protective zipper pouch, owner's manual and a handy quick reference guide which can fit in the pouch for in-the-field use.

example#1: moon phase program

calculation of the phase of the moon from the date.

01 STO 1
02 3
03 R/S
04 x>=y
05 GTO 12
06 1
07 STO- 1
08 ROLL
09 1
10 2
11 +
12 1
13 +
14 3
15 0
16 .
17 6
18 *
19 INT
20 RCL 1
21 3
22 6
23 5
24.
25 2

26 5
27 *
28 INT
29 +
30 R/S
31 +
32 6
33 4
34 .
35 1
36 -
37 2
38 9
39 .
40 5
41 3
42 /
43 FRAC
44 8
45 *
46 .
47 5
48 +
49 INT

calculate the phase of the moon to the nearest segment where

0 or 8 = new moon
1 = crescent (waxing)
2 = first quater (half moon)
3 = gibbous (waxing)
4 = full moon
5 = gibbous (waning)
6 = second quater (half moon)
7 = crescent (waning)

Usage:

enter the year since 1900 (eg 2002 is 102) R/S, then the month (1= jan) R/S, then the day R/S.

Example: what is the phase of the moon for sunday, december 1, 2002.

F PRGM (position to start)
102 R/S
12 R/S
1 R/S

answer is: 7 soon to be a new moon.

example#2: easter sunday program

given a year between 1900 and 2099, find the date of easter sunday.

01 sto 1
02 1
03 9
04 /
05 frac
06 2
07 0
08 9
09 *
10 2
11 0
12 4
13 -
14 chs
15 3
16 0
17 /
18 frac
19 3
20 1
21 *
22 2
23 8
24 x<>y

25 x>=y?
26 gto 29
27 1
28 +
29 rcl1
30 5
31 *
32 4
33 /
34 int
35 sto2
36 +
37 7
38 /
39 int
40 7
41 *
42 rcl2
43 -
44 +
45 1
46 -

Usage: enter the year (eg 2003) R/S

if the anwer <= 31, easter falls on that date in march.
otherwise easter falls in april. subtract 31 to give the april day.

eg. what day for 2003.

2003 R/S 51 (ie april) 31 - gives april 20.

this program took me some time to develop. for a long time i thought the problem too complex to fit in less than 50 steps. the final result takes only 46 steps and derives from clever use of the formula:

int easter(int y)
{
    /* 1900 - 2099 */
    int d;
    d =(204 - 11*(y%19))%30;
    if (d < 28) ++d;
    return d+27-(5*y/4+d)%7;
}