From: edp@math.zko.dec.com (Eric Postpischil)
Newsgroups: rec.org.mensa
Subject: Re: speed of light and age of the universe
Date: 22 Jul 92 14:29:49 GMT
Reply-To: edp@math.zko.dec.com (Eric Postpischil)
Organization: Digital Equipment Corporation

In article <63995@hydra.gatech.EDU>, cegtitd@prism.gatech.EDU (Tim Dodd)
writes:

>)Not by me, certainly.  I'm willing to trust the experts in this area
>)whoJoccasionally convince the world of the necessity to throw in a
>"leap second"
>)to keep everything timed correctly.  I've been told that this is
>necessary
>)because the earth's spin rate is decreasing.  What I find interesting,
>though,
>)is that there doesn't seem to be any regularity to the necessity of
>doing so. 
>)(i.e. it's not every "n" months, but appears to be something almost
>random)  
>)While this seems odd to me, it does tend to reassure me that it's not
>related to
>)an inaccuracy in the length of our "24 hour" day.    <pondering: 
>perhaps we
>
>Actually, this is incorrect.  The inaccuracy is caused by the
>discrepency
>between the length of the sidereal year and that of the calender year.
>Correction for this difference is administered every four years by
>"leap
>years".  Further correction is also required for strict precision, but
>I
>don't remember how often or how much.

Bull!  First, just consider a terrestrial year (as opposed to a sidereal
year):  How many days must pass before the sun is observed to return to
a given position (which can be measured relative to background stars)? 
The answer is a fractional number of days, about 365.2422.  That
fraction is the reason for leap years.

A sidereal year is how long it takes for the Earth to return to the same
position as seen by an observer off the Earth, say somebody looking
"down" from "above" the Solar System.  This time will be equal to that
of a terrestrial observer seeing one year go buy:  The Earth returns to
the same position in its orbit at the same time the Sun is observed in
the same position against the background stars, as seen from Earth.

A sidereal day is different from a terrestrial day because a sidereal
day is one rotation of the Earth as seen by an observer off the Earth,
whereas a terrestrial day is one rotation of the Sun as observed from
Earth -- but since the Earth moves in its orbit, the terrestrial
observer does not see the Sun come around at the same time the Earth
rotates back to the same orientation in space.  However, the difference
in these individual days does not affect the position of the Earth in
its orbit, so it does not affect the length of a year.  Further, the
difference of nearly 4 minutes between a sidereal day and a terrestrial
day is much larger than 1 day every four years.  (Four times as large;
it would amount to one day a year, exactly.)

The true reason for leap seconds is that Earth's rotation is not
constant.  Today's atomic clocks are very accurate, and their
time-keeping is more regularly than Earth's rotation.  In addition to
tides due to the Moon and Sun, the Earth is pulled or pushed in its
orbit by gravitational influence from other planets.  The rotation of
the Earth might even be affected by biological actions --  I've heard
the rotation can be affected by trees carrying water and other material
off the ground to their limbs when they are in full foliage.

These factors can increase or decrease Earth's spin and movement along
its orbit, so the occasional adjustments made to Universal Time
Coordinated can be either positive or negative.


                                -- edp (Eric Postpischil)
                                "Always mount a scratch monkey."
                                edp@alien.enet.dec.com
Newsgroups: rec.org.mensa