Re: [LEAPSECS] what should a time standard encompass?

From: Ed Davies <ls_at_edavies.nildram.co.uk>
Date: Sat, 25 Jan 2003 17:20:40 +0000

Ken Pizzini wrote:
> For any day of the year one can find some locus of points on the
> earth's surface where local-apparent-noon-to-local-apparent-noon will
> have a duration of one mean solar day.

Aren't local solar days longer than mean solar days everywhere around
perihelion?

I have to admit I only half understand the equation of time. I'm
quite happy with the effect of the eccentricity of the Earth's orbit.
The Earth moves quicker at perihelion so has to rotate further to
get any particular meridian back facing the sun.

What makes my brain hurt is trying to understand the effect of the
inclination of the Earth's rotation axis to the ecliptic. As the
Earth rotates steadily (at least steadily enough for this discussion
if not for more general leapsecond consideration) the plane of
a meridian sweeps across the ecliptic at a variable rate but I find
it really difficult to visualise the effect.

If the Earth's orbit was circular, would all local solar days be
the same length or would the inclination still cause variation?

In other words, is the second cause of wobbles in the equation of
time directly the inclination or is it because the inclination
changes the sweep rate of the meridonial plane along the ecliptic
and hence changes the effect of the extra rotation required at
perihelion (and less than mean rotation required at aphelion,
of course)?

This is all rather peripheral to leapseconds as such but worth
understanding if only to remind ourselves how disconnected all
modern time standards are from the apparent motions of the sun.

Ed Davies
Received on Sat Jan 25 2003 - 09:21:55 PST