From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node199.html | 174 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 174 insertions(+) create mode 100644 src/slalib/sun67.htx/node199.html (limited to 'src/slalib/sun67.htx/node199.html') diff --git a/src/slalib/sun67.htx/node199.html b/src/slalib/sun67.htx/node199.html new file mode 100644 index 0000000..93a4839 --- /dev/null +++ b/src/slalib/sun67.htx/node199.html @@ -0,0 +1,174 @@ + + + + +Formatting angles + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: Vectors and Matrices +
+Up: Spherical Trigonometry +
+ Previous: Spherical Trigonometry +

+

+

+Formatting angles +

+SLALIB has routines for decoding decimal numbers +from character form and for converting angles to and from +sexagesimal form (hours, minutes, seconds or degrees, +arcminutes, arcseconds). These apparently straightforward +operations contain hidden traps which the SLALIB routines +avoid. +

+There are five routines for decoding numbers from a character +string, such as might be entered using a keyboard. +They all work in the same style, and successive calls +can work their way along a single string decoding +a sequence of numbers of assorted types. Number +fields can be separated by spaces or commas, and can be defaulted +to previous values or to preset defaults. +

+Three of the routines decode single numbers: +sla_INTIN +(integer), +sla_FLOTIN +(single precision floating point) and +sla_DFLTIN +(double precision). A minus sign can be +detected even when the number is zero; this avoids +the frequently-encountered ``minus zero'' bug, where +declinations etc. in +the range $0^{\circ}$ to $-1^{\circ}$ mysteriously migrate to +the range $0^{\circ}$ to $+1^{\circ}$.Here is an example (in Fortran) where we wish to +read two numbers, and integer IX and a real, Y, +with X defaulting to zero and Y defaulting to +X: +

+            DOUBLE PRECISION Y
+            CHARACTER*80 A
+            INTEGER IX,I,J
+
+      *  Input the string to be decoded
+            READ (*,'(A)') A
+
+      *  Preset IX to its default value
+            IX = 0
+
+      *  Point to the start of the string
+            I = 1
+
+      *  Decode an integer
+            CALL sla_INTIN(A,I,IX,J)
+            IF (J.GT.1) GO TO ... (bad IX)
+
+      *  Preset Y to its default value
+            Y = DBLE(IX)
+
+      *  Decode a double precision number
+            CALL sla_DFLTIN(A,I,Y,J)
+            IF (J.GT.1) GO TO ... (bad Y)
+
+

+Two additional routines decode a 3-field sexagesimal number: +sla_AFIN +(degrees, arcminutes, arcseconds to single +precision radians) and +sla_DAFIN +(the same but double precision). They also +work using other units such as hours etc. if +you multiply the result by the appropriate factor. An example +Fortran program which uses +sla_DAFIN +was given earlier, in section 1.2. +

+SLALIB provides four routines for expressing an angle in radians +in a preferred range. The function +sla_RANGE +expresses an angle +in the range $\pm \pi$;sla_RANORM +expresses an angle in the range +$0-2\pi$. The functions +sla_DRANGE +and +sla_DRANRM +are double precision versions. +

+Several routines +(sla_CTF2D, +sla_CR2AF +etc.) are provided to convert +angles to and from +sexagesimal form (hours, minute, seconds or degrees, +arcminutes and arcseconds). +They avoid the common +``converting from integer to real at the wrong time'' +bug, which produces angles like $24^{h}\,59^{m}\,59^{s}.999$.Here is a program which displays an hour angle +stored in radians: +

+            DOUBLE PRECISION HA
+            CHARACTER SIGN
+            INTEGER IHMSF(4)
+            :
+            CALL sla_DR2TF(3,HA,SIGN,IHMSF)
+            WRITE (*,'(1X,A,3I3.2,''.'',I3.3)') SIGN,IHMSF
+
+

+

+ +next + +up + +previous +
+ Next: Vectors and Matrices +
+Up: Spherical Trigonometry +
+ Previous: Spherical Trigonometry +

+

+

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +
+ + -- cgit