The ebb and flow of the tides is such a regular occurrence that it can be forecast for years in advance. These changing water levels have been observed, studied, analyzed and even predicted for thousands of years prior to the age of computer generated weather forecasts. As residential living creeps closer and closer to the shoreline it is more than just ship captains that start paying attention to the ebb and flow of the tides. Residents and city planners notice these changes too and, in some cases, are waging war against these unstoppable rising tides. Most days these high water levels go unnoticed but certain times of the year, along with certain types of weather, the seawalls and pumps are no match for the approaching seas.
These times of the highest of tides are called king tides and they can be predicted. The reason being is due to the forces that cause the tides and, unlike the weather, are a little more stable in the near and long term. All objects in the universe exert forces on other objects near them and in the case of the tides, these objects are the Sun and Moon. As the Moon takes its 27 day orbit around the rotating Earth it is constantly exerting a force on the Oceans. This creates a bulge that travels around the globe and gives each region it encounters a high tide. One of two in fact since the Earth’s rotation creates a similar bulge on the opposite side of the globe.
So that is the simple explanation of the tides. Forces generated by the Moon, Sun (to a lesser degree than the Moon) and the Earth’s rotation create these high and low tide cycles. Two of each occur per day and each occurs 24 hours and 50 minutes later the following day. Tide gauges can track these levels and it seems pretty simple and consistent, until you take a closer look. Ask anyone near a tidal zone and they will tell you that not all tides are created equal. As the distance between the Earth and Moon vary along with the relative positions of each, then so do the tides.
These subtle changes come in many ways. Some happen monthly, while others only twice a year. All can increase or decrease a tide height individually, or combine their effects and create the highest tide of the year: the King Tide.
The Sun and Moon can work with or against each other to influence the tide. A new or full Moon means the forces of each are working together and tide levels increase (both high and low). This period is known as a spring tide, not for the season, but for the fact that tide levels seem to spring higher and lower than normal. A half Moon will will put the forces of each perpendicular to one another and result in lower tide ranges. The forces are no longer working together and thus the tides will not be as severe.
The orbit phase is not the only change that can influence the tides. The distance between the Earth and Moon itself varies throughout the entire orbit. There are two points in a lunar month when the it is at its closest point (Perigee) and its farthest point (Apogee) from Earth. Since the force exerted by the Moon also depends on its distance, this can influence the tidal ranges. If a new or full Moon occurs on or around Perigee, then the added forces will increase the already high and low spring tide. A full Moon at perigee is also know as a Supermoon and is 14 percent larger and shines 30 percent brighter than a full Moon at apogee.
Finally there is the relative position of the Sun and Moon to the Earth. This can be described as the point on the Earth where either is directly overhead. These points are measured in angles away from the equator and reported as the declination angle. This can be visualized with the solstices (the Sun overhead the southern and northern most point) and the equinox (the Sun passing directly overhead of the equator). The Moon also varies its relative position to Earth but can also appear to pass directly over the equator.
On or around the equinoxes there will come a point when the Sun, Earth and Moon practically form a straight line, or each appears to be directly overhead of the equator. With the forces all along a similar path it can actually create higher than normal tides as opposed to other times of the year.
A King Tide typically occurs with a full Moon near the equinox and, should this occur near Perigee, then minor to moderate coastal flooding is likely to occur. With coastal communities established and an ongoing demand for beach front property continues, residents or prospective residents need to be aware of these coastal flood threats, especially the rare but predictable King Tide.