Tides Background

Tidal Forces

Notice, that astronomical tides do not include weather and atmospheric effects. Also, changes to local conditions (sandbank movement, dredging harbour mouths, etc.) affect the tide's actual timing and magnitude.

Type of tides

Based on the number of high and low tides and their relative heights each tidal day, tides are described as diurnal, semi-diurnal or mixed. When the moon is directly over Earth's Equator, its associated tidal bulges are centered on the equator. In theory, all locations on the planet except at the highest latitudes would rotate through the two tidal bulges and experience two equal high tides and two equal low tides per tidal day; this is known as a semi-diurnal tide. Semi-diurnal tides have a period of 12 hrs and 25 min, and theoretically have a wavelength of more than half the circumference of Earth.

Different types of tides occur when the moon is either north or south of the equator. Whereas semidiurnal tides are observed at the equator at all times, most locations north or south of the equator experience two unequal high tides and two unequal low tides per tidal day; this is called a mixed tide and the difference in height between successive high (or low) tides is called the diurnal inequality. When the moon is above the Tropic of Cancer or Tropic of Capricorn, the diurnal inequality is at its maximum and the tides are called tropic tides. When the moon is above or nearly above the equator, the diurnal inequality is minimum and the tides are known as equatorial tides. When the moon and its associated tidal bulges are either north or south of the equator, most points at high latitudes in theory would be impacted by one tidal bulge and would experience one high tide and one low tide per tidal day. This so-called diurnal tide has a period of 24 hrs and 50 min.

The separate sets of ocean bulges related to the moon and sun act at times together and at other times in opposition. About every two weeks, the positions of the sun, moon, and Earth forma straight line (Figure A). At these times of new and full moon phases as viewed from Earth, the lunar- and solar-related ocean bulges also line up (and add up) to produce tides having the greatest monthly tidal range (that is, the highest high tide and lowest low tide); these are called spring tides. Between spring tides, at the first and third quarter phases of the moon, the sun's pull on Earth is at right angles to the pull of the moon (Figure B). At this time, tides have their minimum monthly tidal range (that is, unusually low high tide and unusually high low tide); these are called neap tides or fortnightly tides. Furthermore, the moon orbits Earth in an ellipse (rather than a circle) so that the moon is closest to Earth (stronger tide-generating force) at perigee and farthest from Earth (weaker tide-generating force) at apogee. The moon completes one perigee-apogee-perigee cycle once every 25.5 days.