# [time-nuts] UTC and leap seconds

Tom Van Baak tvb at LeapSecond.com
Fri Jun 11 11:14:06 EDT 2010

```> I was wondering, why we assume that  Earth's rotation is slowing down, instead
> that clocks are speeding up?
>
> Antonio I8IOV

Hi Antonio,

Good question.

If all you had is one clock; then it is the time.

If you have two clocks you can see them drift apart (if
you can't, then you either aren't looking close enough
or not waiting long enough). Note this doesn't tell you
which one is better, but it does give you an idea how
accurate they aren't.

It takes three or more clocks before you get an idea of
which ones are more stable than the others.

So what "clocks" do we have?

The earth's rotation about its axis is a clock; we call its
period a day. The earth's revolution about the sun is a
clock; we call that period a year. The moon's revolution
about the earth is a clock; we call that period a month.
We all have quartz clocks; many of us have rubidium
or cesium clocks as well. Many other clocks exist, both
cosmic and microscopic.

With so many clocks it becomes possible to intercompare
them to determine how stable each one is; to see which
ones are fast or slow, or which ones are speeding up or
slowing down, etc.

The result of these comparisons show the earth day has
more drift and is less stable than the earth year. And both
of these two clocks are more unstable than atomic clocks.

You can see for yourself the ADEV of the earth:
http://www.leapsecond.com/museum/earth/

When you compare these phase, frequency, and stability
plots to other clocks you'll see why we know the "problem"
is with the earth "day" and not with other clocks in the sky,
or clocks in your lab, or all the clocks that make up UTC.

/tvb

```