THE SOUTHWORTH PLANETARIUM
207-780-4249   www.usm.maine.edu/planet
70 Falmouth Street   Portland, Maine 04103
43.6667° N                   70.2667° W
Altitude:  10 feet below sea level
Founded January 1970
Julian date: 2458731.5
2019-2020:  III
               "We love the Universe.  It's great!"


THE DAILY ASTRONOMER
Thursday, September 5, 2019
Spinning Water Off Earth

The Southworth Planetarium is approximately half a mile from the Atlantic
shore.  Its close proximity to the sea is one of the many reasons our
humble hobbit-hole of a subterranean star dome theatre is steeped in
ancient enchantment. (Apart from the fact that it was constructed on the
remains of an 11th century encampment assembled by bewildered Vikings.)
 Being near the ocean also affords us many opportunities to both admire the
0.0000000000001% of the cosmos within naked eye sight and to indulge in the
odd experiment or demonstration.*

Today, in fact, we're embarking on a minor field trip down to the sea to
answer an intriguing question a patron once asked. Don't worry. As my
attempts at rhapsody and poetical imagery tend to be tedious, I decided to
wander down here blindfolded.  Vision limitations not withstanding, I can
still do precisely what I intended.   See this green tennis ball? (I think
it's green.)    Watch as I submerge the ball into the DAMN COLD water and
then bring it back up.    Notice what happens when I spin the ball?  The
water flies off in all directions!   That brings us to today's question.

*A patron once asked, "Since Earth is moving at 1000 miles an hour**, why
don't the oceans fly off into outer space like water does off a wet ball?"*

It is a brilliant question that has a simple answer, fortunately:  gravity.
  The planet's pull on the water, as well as on us and the atmosphere and
everything else on or just above its surface easily overpowers the
'centrifugal force" that might otherwise cause everything to fly off into
space.   Even though we're moving at hundreds of miles per hour -the entire
Earth isn't rotating at 1,000 miles per hour (see the ** footnote.)- Earth
is spinning relatively slowly relative to its size.   In fact, it requires
a day to complete one rotation.

[image: 9.jpg]
Earth is spinning quickly by terrestrial reckoning. However, it completes
a rotation once a day and the spin rate is not sufficient to cause the
water to
scatter into space.

The tennis ball spins very quickly.  Not at hundreds of miles an hour, but
for centrifugal force, it is the number of revolutions per unit time that
matter.    The ball can complete many rotations within a few seconds.
Moreover, the ball doesn't have the gravitational attraction necessary to
counteract the tendency of the water to disperse away from it as it
rotates.

Look out at the water (I assume I am pointing in the right direction)   Be
secure in the knowledge that the liquid on which all life depends might be
moving on a spinning Earth, but will, itself, ever be spun out into space.




*Cumulatively, centuries of time have been invested discussing (arguing
about) the difference between an 'experiment,' and a 'demonstration.' The
difference is a matter of certainty and uncertainty.   Let's assume that a
professor offers extra credit to anybody willing to climb to the top of the
building and release one of the pieces of faulty equipment (circa 15th
century) of which he has grown so fond.   That act would not be an
experiment.  It would be a demonstration, as the result is a foregone
conclusion.  Now, let's just pretend the same professor says, "I just
concocted a brilliant ointment that turns the epidermis into a hard rubber.
   Extra credit for anyone who will let me coat them with this ointment and
then throw them off the top of the building."   That event would not be a
demonstration.  It would be an experiment to determine if the ointment will
cause the falling body to bounce on impact.  As elated as the professor
might be by his invention, he is not entirely sure that the ointment will
lend the skin the rubber-like properties that would enable a falling person
to survive.  (That he didn't offer to coat himself and then jump off the
building gives us insight into his confidence level.)

**Earth is not rotating at 1000 miles an hour everywhere!     The
tangential rotation speed of the planet at the equator is 1036 miler per
hour.  However, that speed is reduced with increasing distance away from
the equator.   The actual equation is 1036 x cosine of the latitude.    The
graph below shows Earth's rotational speed at different latitudes.

[image: 19dmwicax90ybpng.png]