🚀 go-pugleaf

robf@mcs213j.cs.umr.edu (Rob Fugina) writes:
>My roommate just asked me to design a clock for him.   It's  for 
>astronomical purposes...

>The  clock  has to count from 00:00:00 to 23:56:03,  then  reset 
>back  to  zero.  I figured it would be easy enough to  design  an 
>oscillator with a 555 to 100Hz, calibrated with a 15-turn pot and 
>an oscilloscope ...

>Power supply...how easy is it to make it AC line powered, with a 
>battery  backup?   This I'm not so worried about,  but  I'd  like 
>advice  on  whether it would be better powered by AC line  or  by 
>battery...i.e.,  how  long do you think the battery would  last? 
>...

There are plans for a digital sidereal clock in _Sky & Telescope_ 
magazine, July 1976, p. 59ff.  The clock itself is a MM5314 clock chip 
(all divide-by-six and setting functions included; requires external 
driver transistors for LED readouts).  The "sidereal rate generator" 
is a TTL circuit which counts 1461 pulses from the 60Hz power line, 
then inserts four additional pulses between the next 60-Hz pulses 
going to the MM5314's input.  4/1461 closely approximates the factor 
required to make the sidereal clock gain ~ 3m 56s per day (one 
day/year).

Alternaively, one could use an oscillator with a special crystal which 
runs slightly faster than the rate for a normal clock;  sidereal 
wristwatches have been advertized in _S&T_.  A clock which counts 
power-line cycles may err by several seconds at any time, but the 
long-term stability is excellent.  (A 555 oscillator is _not_ stable 
enough for a clock.  National Semiconductor makes an IC which 
generates 60 Hz from a 3.579545 MHz color-tv crystal.)

Better designs have been published subsequently, however, it's easier 
anymore to do sidereal time by running a program in your 
PC/Mac/whatever, which reads the internal clock and does a 
calculation.

The _S&T_ article also tells how to set a sidereal clock; local 
sidereal time depends upon your longitude.

The Earth rotates ~366 times per year, but since it goes around the 
sun once, one of those rotations is "unwound," resulting in only 
365.xxxx solar days/year.  The constellations appear to make one 
revolution per year.  We base our normal affairs upon the solar day; 
the sun is at the same meridian at the same time every day.  

A sidereal ("star time") clock is synchronized to the actual rotation 
of the earth; it gains one solar day per year over ordinary time.  
Astronomers use sidereal time because the same stars are at the same 
places every day at the same sidereal time, thus simplifying 
calculations.  On sidereal time, the constellations appear fixed while 
the sun goes around once per year.  Running our mundane affairs on 
sideral time would be inconvenient because the sun would be out at 
"midnight" part of the year.

--

Frank Reid     reid@ucs.indiana.edu