From: werme@alliant.com (Ric Werme)
Subject: Re: speed of light and age of the universe
Organization: Alliant Computer Systems Corp.
Date: Wed, 29 Jul 1992 13:20:31 GMT

In article <712351487.F00006@blkcat.UUCP> Craig.Wagner@p2.f120.n109.z1.fidonet.org (Craig Wagner) writes:
>
>  PZ> From: phz@cadence.com (Pete Zakel) Newsgroups: rec.org.mensa
>  PZ> Organization: Cadence Design Systems
>  PZ> 
>  PZ> Eh? Our measurement of a day is EXTREMELY precise.

Oh, it's not all that great.  Our measurement of *time* is, but the true
length of a day is based on visual measurements of the Sun's motion.  Kinks
in the Earth's orbit mess make that a pain, so what people really measure
is the length of a sidereal day, the amount of time between the stars being
in one position in the sky and the next time they occupy (nearly) the
same position.  This is done with special high power refractor telescopes
that can only see stars that are transiting (crossing an imaginary line that
runs from the North Pole to the Zenith (the point directly overhead) to the
South Pole.  Astronomers need this information to ensure they can point
telescopes at the same celestial object over time.

>Does anyone happen to know much more about the "wobbles"?  Wobbles
>implies to me that it may increase *or* decrease slightly from time
>to time.  Yet we only ever *add* leap seconds, as far as I can
>recall.  Is this a semantic issue, perhaps, or is there more involved
>than that?

Off the top of my head, here are the various things that affect the apparent
motion of Sun and stars.  These are kind of hard to rank by magnitude because
some are period and have zero long term impact, but I'll try anyway.

1) The Earth spins 366.2422 times per year.  (One definition of "year"
   the average time from one start of Spring (sun above equator and
   moving North) to the next.

2) The Earth orbits the Sun.  This reduces the number of sunrises in
   temperate and tropical zones to 365.2422.  1) & 2) combine to give
   us our calendar, including leap years.  The Gregorian calendar (the
   one most readers use) is designed for a year 365.2425 days long.
   There's another system that is more accurate, but it will be
   centuries before they diverge.

3) The Earth's elliptical orbit makes the Sun's apparent motion vary
   with a period of about one year.

4) The Earth's axial tilt makes the Sun's apparent motion vary with a
   period of about six months.  3) & 4) combine into something called
   the Equation of Time, and produces an error on sun dials of up to
   16 minutes.  This is one reason astronomers measure sidereal days
   instead of solar days.  I wrote an article on the Equation of Time for
   sci.astro, I'll post it here later.

5) There is an effect called nutation where the Earth's axis wobbles
   with an 18 year period.  This is due to precession of the Moon's
   orbit, which has an 18 year period.  (I.e. the point where the moon
   is directly above the equator would circle the Earth every 18 years
   if the Earth didn't rotate.)  This is a very nice, smooth wobble,
   but I don't think it has much to do with leap seconds.

6) Seasonal weather changes shift mass distribution on the Earth.  Ice
   accumulates near a pole in winter and melts in spring.

7) Earthquakes shift the position of the Earth's axis.  6) & 7) have
   been measured and I believe they are responsible for leap seconds
   being declaared at unpredictable times.

8) Ice ages and plate tectonics do similar things, but over longer
   periods of time.

9) Tides transfer rotational momentum to the Moon.  Kinetic energy in
   the tides slow down the Earth because we rotate faster than the
   Moon orbits Earth.  The tidal bulge is not perfectly lined up with
   the Moon, thanks to the Earth's rotation, so the bulge is
   accelerating the moon.  Ultimately the Moon will be in a higher
   orbit and the Earth will rotate in sync with the Moon.
   Archaeologists have found evidence for more more days per year and
   shorter lunar periods, but I forget the details.  9) is the reason
   we add leap seconds instead of taking them away.

10) Precession of the Earth's axis of rotation means Earth wobbles once
    every 24,000 years.  I forget if the precession is due to gravitational
    effects from other planets or just the Sun.  Never did get a good feel
    for precession in Physics classes.  This is what shifts the Zodiac and
    requires that astronomers make new star charts every 50 years or so.
    Also, it made Polaris the North star sometime near the point Europeans
    became serious mariners.  12,000 years from now Polaris will be wrong
    by 46 degrees!

Disclaimer:  I've probably missed something.  I'll probably hear about it.
-- 
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