Monday, October 16, 2017

Select Questions-Answers From All Experts Astronomy Forum (2. Variations In Earth's Rotation)

Question -
I heard that due to friction the Earth’s rotation rate is slowing down.  If so, what
is the source of friction?  How does it slow down the rotation?

First, the investigation of Earth’s  rotation is not a simple matter. The
reason is that the Earth's shape departs from a perfect sphere. Thus, for example,
one cannot simply calculate ONE moment of inertia - say 0.4MR 2  for a
sphere - but must reckon separate axial and equatorial moments since
the Earth is oblate, e.g. the polar diameter is less than the equatorial.

To be specific, the equatorial moment of inertia is roughly 1/300 of the
axial moment. This may seem like an insignificant difference - but when
one is dealing with extremely small measures, times, it emerges as
important.

Second, the changes in Earth's rotation are not uniform across the board-
though true, there is a general long term trend to slow down - arising
from the tidal breaking of the Moon.

Let's go into this a bit more - prior to me giving examples of how the
Earth's rotation can get faster.

The Earth rotates faster than the Moon moves in its orbit. Because the
tides are linked to the more slowly moving Moon, they act by friction as a
brake on Earth's rotation, gradually slowing it down.
(It is estimated by 0.0007 seconds per century)

The angular momentum lost by the rotating Earth in this process is
transferred to the Moon's angular momentum. Thus, the Moon is accelerated
in its orbit, causing it to slowly spiral outwards, away from Earth. The
day and month are thus lengthening at different rates.

Calculations have actually been retro-worked to show how the length of
month differed when the Moon was much closer to Earth in the past. For
example, when the Moon was only 16,000 km away (10,000 miles) the month
was approximately seven mean solar days long.

Similar calculations based on the conservation of angular momentum also
allow us to project into the future. Thus, about three billion years
hence the day and month will be equal - about 47 of our present days
long -  and the Earth will always turn the same face towards the Moon.

Now, let's get back to exceptions to day lengthening. These mostly arise
from sporadic tectonic events such as earthquakes- or more recently the
Indian Ocean tsunami, and the massive sea quake that incepted it.

Recent computations of the seismic moment arising from the generating
quake have determined that the length of day briefly decreased by
about 2.68 micro-seconds. (This would be analogous to a spinning ice
skater briefly pulling her arms in closer to herself).

Then too, cumulative earthquakes over magnitude 5 (of which there have
been 21, 600 since 1977) have an overall tendency to make the whole planet
rounder and more compact in all directions, thereby shortening the length
of day.

It is also theorized that mammoth solar flares - by expanding the
atmosphere- can also contribute to a change in the rotation rate. How
exactly remains to be worked out, but no doubt friction (creating drag)
between atmosphere and planetary surface might play a role.

All this is to try to make you aware that variations in the Earth's rotation
arise from multiple sources and can alter in either the positive
or negative direction. Again, the general trend is for slowdown - owing
to the reasons given earlier.