I am sending this one early...as I want it to arrive before 11:22 a.m. Youll see why. THE SOUTHWORTH PLANETARIUM 207-780-4249 usm.maine.edu/planet 70 Falmouth Street Portland, Maine 04103 We love the image of the Roman God Neptune that adorns a downtown building. The only issue is its completeness: that the ocean gods entire body is set before us. It is lovely and a tribute to the artist who crafted it. However, we shouldnt see all of him....in fact, perhaps we shouldnt see him at all. Instead, a greater representation might be a mammoth golden obelisk protruding out of the sea that we can view only from a distance. Because, Neptune, the presider over the worlds oceans, wouldnt trouble himself much with a small port city. Maybe once, in 17th century, when Portland was taking form, he might have passed through en route to one of the myriad ports that dot the continents contours. To make him truly grand, we should retain only a phantom image of the uppermost part of the trident that he carried when he happened upon us in the distant past and, by necessity, could not return for centuries to come. THE DAILY ASTRONOMER December 4, 2013 The King Tide Seems a rather grandiose title for a water flow, doesnt it? The King Tide, refers to the highest high tide of a given year. The 2013 King tide occurs today at 11:22 a.m. in Portland, Maine. While we shouldnt expect the Universal deluge to inundate our shores, we will experience an unusually high tide, as evidenced by a greater than usual encroachment of water. However, before we can explain the King Tide, we have to contend with explanations of tides, themselves. And, yes, we understand that tide talk is the astronomical equivalent of visiting a Victorian dentist without a bracer. Nevertheless, despite the pain, we proceed. Earth generally experiences two high tides and two low tides a day,* The tides result from a differential gravitational force (pay no attention to that drilling sound.) We know that all massive objects exert a gravitational force on all other massive objects, in accordance with Isaac Newtons Universal Gravitation Law. Moreover, the magnitude of the gravitational force between all massive objects diminishes with the square of the distance. (If you double the distance between two objects, the force is reduced to a quarter; triple the distance, the force is one ninth of the original value.) Now, if all massive objects were point masses, as described in elementary physics texts, then the matter would be simple: the forces would be equal along each mass and the math would treat us sweetly. Of course, the physical world doesnt have point masses. Instead all massive objects occupy a specific volume, whether theyre volleyballs or planets. Differential gravity arises from this inconvenient breadth. Regard Earth and the Moon. Both bodies exert a force on one another. However, the magnitude of the force varies within each body because different regions experience a different gravitational force. The portion of Earth closest to the Moon feels a greater tug than the planets center because gravity falls off with the square of the distance, which is 6,371 km (3,959 miles). Regard Earth and the Sun. Yet, again, both bodies exert a force on each other. And, likewise, the gravitational force the Sun exerts on Earth varies along its volume. The part of Earth closest to the Sun (noon) experiences less gravity than the part farthest away (midnight.) However, the Sun is much farther away from us than the Moon. The Suns mean distance is 93 million miles; the Moons is 240,000 miles, so the tidal force the Sun causes will be less than the Moon (44% as great, actually.) This is because the force difference between the near and far sides of Earth relative to the Sun is not as profound as the difference the Moon induces. Both the Sun and Moon contribute to the high tides, although, as mentioned previously, the latter is more influential than the former. Each day, as Earth rotates beneath the Moon, the planet bulges along the Earth-moon line.** This bulge region has a high tide. Yet, so, too does the point on Earth that is diametrically opposite of the first bulge. Here, our intuition fails us. Though it might be easier to understand why the area just under the Moon has a high tide because of the differential gravity, one is at a loss to know why the most distant region also experiences one. Think of this: the pull of the Moon, and to a lesser extent, the Sun, exerts a bulging effect on Earth, similar, in principle, to the effect of squeezing the planet into an oblong shape. When we regard this issue physically, we can think of Earths center as being stationary, with the two opposite points along the Moon-Earth line pulled away. The water is drawn toward both these bulges, while the two areas perpendicular to it experience low tides. The water amount is constant, so an excess in one region must create a deficiency in another. To add a little more complexity, we remind the reader (yes, we know you havent forgotten) that the angle between the Sun, Moon and Earth is always changing. During New Moon, the Moon is between the Sun and Earth, so both Moon and Sun are puling in the same direction and their gravitational influence is combined. At full moon, Earth is between the Moon and Sun. Theyre pulling in different directions, of course, but theyre both operating along the bulges. The tides occurring around new and full moon are known as spring tides. The lowest high tides happen during first and last quarter (quadrature), when the Moon and Sun are operating perpendicularly and therefore diminishing the tidal effect. Now, to make life even more interesting, the Moons distance from Earth and Earths distance from the Sun are both constantly changing. This continuous change happens because both Earth and the Moon travel along elliptical orbits, which we can envision as ovals. If the orbits were perfect circles, the distance would be constant. Of course, it isnt. During every orbit the Moon reaches a point of least distance, called perigee. Just as Earth, itself, also has a closest point, called perihelion. When the Moon reaches perigee, the tidal forces are enhanced because the differential gravitational force varies with the cube of the distance. The same effect occurs, to a lesser extent, when Earth reaches perihelion because at that time the Sun is closest to us, Here is where we watch the gears turning: Every so often, the Moon will reach perigee around the time it is either at full moon or new moon. This coincidence is called astronomical high tide, a term also applied merely to the tides corresponding to the full and new moon. (Although the term spring tide is more apt.) Perigee doesnt generally correspond to the full or new moon because the period between successive perigees, called an anomalistic month, is about 27.5 days, whereas the phase cycle, called a synodic month, is 29.5 days. However, when they correspond, the tides will be quite high. Now, imagine, you had the perigee moon corresponding with the new or full moon on or around the time when Earth reaches perihelion. Earth reaches perihelion during the first week of January. The 2014 perihelion date is January 4th. Today, the Moon is at perigee and it was new at 7:21 p.m. Monday, December 2nd. Meanwhile, were close to perihelion. These three factors contribute to produce this mornings King Tide. If you watch the waters tipping over their usual boundaries, realize that youre seeing the combined machinations of the Sun, Moon and Earth as they alter themselves constantly relative to each other. *Sometimes, a high tide will occur just after midnight, so a certain day might have just one high tide and two low. Or, vice versa. **The Moon is not exactly aligned with this bulge, to be more accurate, but lets not extract too many molars.
Posted on: Wed, 04 Dec 2013 19:42:02 +0000
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