Time and Tide wait for no Physics Teacher

The study of Physics is the study of our Universe, and how the things within it interact with one another.  The Universe is a complex place, and therefore its description is likewise complex.  To begin to understand its workings, we often strip a situation down to its simplest state, and add back in complexities as our understanding increases.  Unfortunately, this can be confusing to the student, as it asks them to imagine a situation which they *know* to be untrue (i.e. “what if there were no friction or air resistance”)

experiment

http://xkcd.com/669/

In their simplest explanation, high and low ocean tides are caused primarily by the gravitational force the Moon exerts on the Earth, and the fact that the force is a bit less on the side furthest from the Moon (this is also what is being referred to when someone mentions “being ripped apart by tidal forces” either in science fiction or science fact.)

The Sun, of course, also exerts tidal forces on the Earth, though due to the much greater distance between the bodies, the differences in forces are smaller; the tidal effects of the Sun are most noticeable in conjunction or opposition with the effects from the Moon.

tide06a_450follow this link to see the animation: NOAA: Tides

“Together, the gravitational pull of the moon and the sun affect the Earth’s tides on a monthly basis. When the sun, moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very low, low tides — both commonly called spring tides. One week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces moderate tides known as neap tides. During each lunar month, two sets of spring and two sets of neap tides occur (Sumich, J.L., 1996).”

from oceanservice.noaa.gov

So, Saturday September 10, at approximately 5 pm: the Sun is setting in the west (as it does), and a half moon is high in the sky, nearly at its zenith. We should be nearly at a high neap tide, with low tide occurring around midnight, and another high tide about 6 in the morning, yes?

Nope.

gulf-shores-icww-alabama

Spring tides. Low tide at 6:30pm. Only one high and one low tide per day. This is *not* the way the tides are supposed to work, and it’s North America’s fault…though South America certainly isn’t helping any.

The Continents mess things up.  Due to Earth’s rotation, they push the water ahead of them so the tidal bulges precede the moon rather than lag behind.  Coastal irregularities put a local spin on a global phenomenon.  The Bay of Fundy in Nova Scotia has the world’s highest high tides. The Gulf of Mexico experiences only one high and one low tide per day.

tide07d_240“This map shows the geographic distribution of different tidal cycles. Coastal areas experiencing diurnal tides are yellow, areas experiencing semidiurnal tides are red and regions with mixed semidiurnal tides are outlined in blue.”

from NOAA (follow link for additional explanation)

 

Why does the Gulf have only one cycle of tides per day?  I dunno. Why do Spring and Neap tides occur at opposite times to what they ‘should’?  No idea.  But someone knows.  It might be something that’s readily understandable with a smallish amount of research – or it may take years of specialized study.  For me, it’s enough right now to know that the anomaly is understood.