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Tides are the rise and fall of sea levels on a regular basis. They are caused by the combined gravitational forces of Moon and Sun and the rotation of the Earth. Most places experience two high tides and two low tides each day. The times and amplitude of the tides are influenced by the alignment of the Sun and Moon, the pattern of tides in the deep ocean, the shape of the coastline, and the depth of the ocean floor near the shore.
Tides produce oscillating currents known as tidal streams. The moment that the tidal current ceases is called slack water(slack tide). The tide then reverses direction(turning). Slack water usually occurs near high water and low water. But there are locations where the moments of slack tide differ significantly from those of high and low water. The two high waters on a given day are typically not the same height (daily inequality); these are the higher high water and the lower high water in tide tables. The two low waters each day are the higher low water and the lower low water. The daily inequality is not consistent and is less when the Moon is over the equator.
Tidal changes are the net result of multiple influences that act over varying periods. These influences are called tidal constituents. The primary ones are the Earth’s rotation, the positions of Moon and the Sun relative to Earth, the Moon’s altitude above the Earth, and bathymetry. In most locations, the largest constituent is the principal lunar semidiurnal. Its period is about 12 hours and 25.2 minutes(average time separating lunar zeniths) and is the time required for the Earth to rotate once relative to the Moon.
The semidiurnal range varies in a two-week cycle. Around new and full moon when the Sun’s tidal force reinforces the Moon’s. The tide’s range is at its maximum(spring tide). When the Moon is at first or third quarter, the solar gravitational force partially cancels the Moon’s and the tide’s range is at its minimum(neap tide). Spring tides result in higher than average water, low waters that are lower than average, slack water time that is shorter than average and stronger tidal currents. Neap tides result in less extreme tidal conditions. There is about a seven-day interval between springs and neaps.
The shape of the shoreline and the ocean floor change the way that tides propagate, so there is no simple, general rule that predicts the time of high water from the Moon’s position in the sky. Coastal characteristics such as underwater bathymetry and coastline shape mean that individual location characteristics affect tide forecasting. As you can see, tides are not just the simple moving in and out of water at the shore.
We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.