# Chapter 1 – Time (Local Sidereal Time)

## Local Sidereal Time

Our household clocks, which use a 24-hour period (or two 12-hour periods), reflect the daily passage of the Sun across the sky. The Sun is approximately due south at noon (approximately because the Earth’s orbit about the Sun is not a perfect circle but slightly elliptical). This means that the Earth’s orbital velocity will vary with the seasons whereas the Earth’s rotation on its axis is almost constant. The net result is the Sun gets out of synchronisation by plus or minus about 15 minutes from its noonday passage of the southern meridian.

The stars, in contrast, keep virtually perfect time. Because they are so remote, the Earth’s varying orbital velocity is of no consequence. The stars return to the same point in the sky (for any fixed location) four minutes earlier each day whether daylight blots them out or not. Sidereal time or star time is based, therefore, on a clock that runs four minutes fast on the household clock.

Knowing the sidereal time for any date and hour is important for astronomers so that they can plan ahead for their viewing sessions (weather permitting). The best viewing occurs when the region of interest of the sky is to the south of the observer in the northern hemisphere (to the north in the southern hemisphere) and at the greatest altitude. (Twinkling stars caused by a disturbed atmosphere are less prevalent away from the horizon.)

This short program calculates the (local) sidereal time against the prompted INPUTs. For example, if the sidereal time is conputed as 6hr 44m then Sirius – the brightest star in the sky – will be due south as this time coincides with the Right Ascension (explained in the next chapter) for Sirius.

9 REM **********************
10 REM Local Sidereal * Time
11 REM **********************
15 PRINT PAPER 6;”LST=stars RA due south:your site”
20 INPUT “Your longitude lll.l:Greenwich=0-(west)+(east) “; LINE l\$: LET l=VAL l\$
30 IF l\$(1)”-” AND l\$(1)”+” OR ABS l>180 THEN GO TO 20
40 PRINT PAPER 5;”Local Sid Time(LST)Long:”;l\$;CHR\$ 130;
50 PRINT PAPER 5;(“W” AND l\$(1)=”-“)+(“E” AND l\$(1)=”+”)
60 INPUT “Date yyyy,mm,dd”;TAB 5;y;TAB 10;mm;TAB 13;d
70 IF mm>12 OR d31 THEN GO TO 60
75 PRINT d;”/”;mm;”/”;y,
80 INPUT “GMT/UT:hh.mm”;TAB 7; LINE e\$: LET e=VAL e\$
85 IF e>24 THEN GO TO 80
90 PRINT “GMT=”;: LET t=INT e+((e-INT e)/60*100)
100 LET m=mm: IF m>2 THEN LET m=m+1: GO TO 120
110 LET y=y-1: LET m=m+13
120 LET j=INT (365.25*y)+INT (30.6001*m)+d+1720982
130 LET g=6.63627+6.570982e-2*(j-2443144)
140 LET ts=g-INT (g/24)*24
150 LET s=l/15+t+ts+t/1436*4
160 IF s>24 THEN LET s=s-24
170 IF s<0 THEN LET s=s+24
180 LET st=INT (s*100)/100
200 LET h=t: GO SUB 300: PRINT ‘,”LST=”;: LET h=st: GO SUB 300
210 PRINT : PRINT : GO TO 60
300 PRINT INT h;”h”;INT ((h-INT h)*60+.5);”m “,: RETURN
9900 REM ***********************
9990 SAVE “LSTime”

Example output from the Local Sidereal Time program.