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.

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.