How does the tide work

That's why in some places, the difference between high and low tide isn't very big, and in other places, the difference is drastic. The tidal force causes water to bulge toward the moon and on the side opposite the moon. These bulges represent high tides. If the moon's gravity is pulling the oceans toward it, how can the ocean also bulge on the side of Earth away from the moon?

It does seem a little weird. It's all because the tidal force is a differential force—meaning that it comes from differences in gravity over Earth's surface. Here's how it works:. On the side of Earth that is directly facing the moon, the moon's gravitational pull is the strongest.

The water on that side is pulled strongly in the direction of the moon. On the side of Earth farthest from the moon, the moon's gravitational pull is at its weakest. At the center of Earth is approximately the average of the moon's gravitational pull on the whole planet. Arrows represent the force of the moon's gravitational pull on Earth. To get the tidal force—the force that causes the tides—we subtract this average gravitational pull on Earth from the gravitational pull at each location on Earth.

The result of the tidal force is a stretching and squashing of Earth. This is what causes the two tidal bulges. Arrows represent the tidal force.

It's what's left over after removing the moon's average gravitational pull on the whole planet from the moon's specific gravitational pull at each location on Earth. These two bulges explain why in one day there are two high tides and two low tides, as the Earth's surface rotates through each of the bulges once a day.

The Sun causes tides just like the moon does, although they are somewhat smaller. When the earth, moon, and Sun line up—which happens at times of full moon or new moon—the lunar and solar tides reinforce each other, leading to more extreme tides, called spring tides. When lunar and solar tides act against each other, the result is unusually small tides, called neap tides.

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.

The tidal force generated by other planets is negligible. The nearest approach of Venus to Earth is more than a hundred times further than the moon. The tidal force is approximately 0.

The next most significant effect is from Jupiter, with a tidal force of 0. Even if all the planets aligned such that their effects combined the additional force would be insignificant. In UK waters, high tides occur approximately every 12 hours 25 minutes. It takes 24 hours and 50 minutes a lunar day for the same location on Earth to re-align with the moon. When the highest point in the wave, or the crest, reaches a coast, the coast experiences a high tide. When the lowest point, or the trough, reaches a coast, the coast experiences a low tide.

Imagine the ocean is shaped like a football pointing at the moon. The point facing the moon is formed because the gravitational pull of the moon is strongest on whichever side of the Earth faces it. Gravity pulls the ocean towards the moon and high tide occurs. The bulge on the far side of the Earth is caused by inertia. The water moving away from the moon resists the gravitational forces that attempt to pull it in the opposite direction.