Here, we see the relationship between the tidal cycle and the lunar day. High tides occur 12 hours and 25 minutes apart, taking six hours and Note: This animation is shown from the perspective of a viewer in the northern hemisphere.
These bulges of water are high tides. High tide left and low tide right in the Bay of Fundy in Canada. Image credit: Wikimedia Commons, Tttrung. Photo by Samuel Wantman. As the Earth rotates, your region of Earth passes through both of these bulges each day. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide. This cycle of two high tides and two low tides occurs most days on most of the coastlines of the world.
This animation shows the tidal force in a view of Earth from the North Pole. As regions of Earth pass through the bulges, they can experiences a high tide. Tides are really all about gravity, and when we're talking about the daily tides, it's the moon's gravity that's causing them. As Earth rotates, the moon's gravity pulls on different parts of our planet. The moon's gravity even pulls on the land, but not enough for anyone to tell unless they use special, really precise instruments.
When the moon's gravity pulls on the water in the oceans, however, someone's bound to notice. Water has a much easier time moving around, and the water wants to bulge in the direction of the moon.
How do tides happen? Why do some areas get more dramatic tides than others? And why can't the sea level just stay constant everywhere, all the time? Today we're going to look at the physics and idiosyncrasies of planet Earth's tides. Check out the diagram below.
In the picture, you will notice that our planet is sitting inside a blob of ocean water that's kind of shaped like a rugby ball. There's a bulge in the ocean on each side of the planet. Observe that one bulge is protruding from the half of the globe that's facing the moon while the other is located on Earth's opposite end. Why do these bulges exist? In a nutshell, they're primarily caused by the moon's gravitational pull upon the Earth. That force can have two separate components.
It can pull matter "vertically," by which we mean perpendicularly to the Earth's surface. And it can also pull things "horizontally" — i. Now, the spot on the globe that sits right beneath the moon at any given time is called the sublunar point. Meanwhile, the spot on the other side of our planet that is directly opposite the sublunar point is known as the antipodal point. It's no coincidence that the ocean bulges are highest right over those two spots. At the sublunar point and the antipodal point, the moon's gravitational pull lacks a horizontal component — something that is also missing at the two corners of the world that are located 90 degrees away from these spots.
Those four areas are unique in that regard; every other location on Earth experiences a horizontal force that pushes water molecules in the ocean toward either the sublunar point where the moon's gravitational force is at its strongest or the antipodal point where the moon's gravitational pull is at its weakest. The tide generating force due to the Sun is 0.
Spring tides occur when the lunar and solar semi-diurnal tides interfere constructively. Using the simplistic analogy of tidal bulges — this is when the lunar tidal bulge and the solar tidal bulge are superimposed upon one another.
This occurs when the Sun and the moon are aligned in space at either new moon or full moon. Spring high tides are higher and spring low tides are lower than average.
Neap tides occur when the moon is at its first or third quarter. Now the lunar tide and solar tide cancel each other out, leading to a smaller tidal range than average. The spring-neap cycle causes tides to build to a maximum and fall to a minimum twice each month.
The regularity of astronomical forcing, combined with the geometry and friction of the real oceans result in spring tides occurring between one to two days after new or full moon. For any specific location, high water at spring tides occurs at approximately the same time of day: for example, at Liverpool spring high tides are always around midday and midnight. Neap means low. Tides can be predicted far in advance and with a high degree of accuracy. Tides are forced by the orbital relationships between the Earth, the moon and the Sun.
These relationships are very well understood and the position of the celestial bodies can be forecast very accurately into the future. However, as sea levels rise , the periodicity and range of the tide will be altered due to different bathymetry underwater depth and topography the physical features of an area.
Therefore predicting tides a long way into the future could be less accurate. Storm surges are short term sea level changes caused by the weather winds and atmospheric pressure that also affect tidal predictability. Storm surges can only be forecast with the same time horizon as weather forecasting about two to five days.
The predictability of planetary motion means that we can also reconstruct tides in the past. For instance, we know that the disastrous flooding of the Bristol Channel on 30 January New Style occurred at 9am — exactly the time of high water. This, combined with records of high winds, allows us to rule out a tsunami as the cause of the disaster. Tidal knowledge also explains the phases of fighting in the famous Battle of Maldon 10 August New Style : the ebbing tide allowed Vikings to cross a causeway in the River Blackwater in Essex where they then slaughtered the Anglo-Saxon Brythnoth and his men.
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