However, the alignment of the moon and sun relative to Earth, the presence of the continents, regional geography, and features on the seafloor, among other factors, make tidal patterns more complex. Around the world, there are three basic tidal patterns: semidiurnal, mixed, and diurnal. When both high tides are about equal to each other, and the low tides are also roughly equal, the pattern is called a semidiurnal tide.
If the two highs and lows differ substantially, the pattern is called a mixed tide. Where there's only one high and one low tide a day, it's called a diurnal tide. It takes six hours and Unlike a hour solar day, a lunar day lasts 24 hours and 50 minutes. This occurs because the moon revolves around the Earth in the same direction that the Earth is rotating on its axis. The king tides occur when the Earth, Moon and Sun are aligned at perigee and perihelion, resulting in the largest tidal range seen over the course of a year.
So, tides are enhanced when the Earth is closest to the Sun around January 2 of each year. They are reduced when it is furthest from the Sun, around July 2. Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.
Also, the moon's orbit around the Earth is tilted by about 5 degrees from the plane of Earth's orbit around the sun. Thus, the maximum tidal bulge will usually be either above or below the equator. Sometimes, some places on Earth experience only one of the two tidal bulges in a day, producing only one high tide.
We study tides for safe navigation, recreation, and coastal development. Commercial and recreational fishermen use their knowledge of the tides and tidal currents to help them improve their catches. Depending on the species and water depth in a particular area, fish may concentrate during ebb or flood tidal currents.
Tide heights at intermediate times between high and low water can be approximated by using the rule of twelfths or more accurately calculated by using a published tidal curve for the location.
Tide levels are typically given relative to a low-water vertical datum, e. The moon's gravity pulls the ocean toward it during high high tides. During low high tides , the Earth itself is pulled slightly toward the moon, creating high tides on the opposite side of the planet. The Earth's rotation and the gravitational pull of the sun and moon create tides. In many places there are two high tides and two low tides every day-the model semi-diurnal tides.
Tides are the daily rise and fall of sea level at any given place. The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer.
High tide left and low tide right at Bay of Fundy on the Gulf of Maine. The Bay of Fundy has the greatest tidal ranges on Earth at To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much but the water is pulled by the gravity and a bulge is created.
This bulge is the high tide beneath the Moon. This creates a second high tide bulge on the opposite side of Earth from the Moon.
Since so much water is pulled into the two high tides, low tides form between the two high tides Figure below. As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides every day. The tidal range is the difference between the ocean level at high tide and the ocean at low tide Figure below. The tidal range in a location depends on a number of factors, including the slope of the seafloor. Water appears to move a greater distance on a gentle slope than on a steep slope.
So when the Sun and Moon are aligned, what do you expect the tides to look like? Waves are additive so when the gravitational pull of both bodies is in the same direction the high tides add and the low tides add Figure below.
0コメント