Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!news.cs.indiana.edu!iuvax!daemon From: commgrp@silver.ucs.indiana.edu (BACS Data Communications Group) Newsgroups: sci.electronics Subject: Re: Sidereal Time Clock Message-ID: <77901@iuvax.cs.indiana.edu> Date: 12 Dec 90 21:47:14 GMT Sender: daemon@iuvax.cs.indiana.edu Lines: 59 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