Tide and Time
Hull, constantly changing and completely in touch with its maritime past and future
The time is readable from both sides of the Tide and Time Clock, regardless of which way the arrow is facing. Viewed from either the River Humber or Hull’s Castle Street it provides an eye opener for visitors arriving in Hull on the North Sea ferries or travelling by road from the M62 motorway. It’s a real talking point and shows Hull as an innovative city that is proud of its name.
Tide and Time Views
An Explanation of Tides
Since the dawn of history the rise and fall of tides have been studied by people living near the oceans.
The tide is a periodic motion of the water in the oceans, including open sea, gulfs, and bays. This movement results from the gravitational attraction of the moon and the sun on the
water and the earth itself.
At most shores throughout the world, there are two high and two low tides during a cycle coincident with the 24 h 50 min it takes the earth to make one rotation relative to the moon.
When the moon is directly over a given point on the surface of the earth, it exerts a powerful pull on the water, which therefore rises above its normal level. Water covering the portion
of the earth farthest from the moon is subject to a fainter pull than the underlying earth, so that another dome of water is formed in those areas. At both crests, the condition known as
high water occurs, while along teh circumference of the earth perpendicular to the direct-opposite tidal axis, low water phases prevails.
Accompanying the vertical rise and fall of water are various horizontal or lateral movements commonly known as tidal currents or tidal streams, which are very different from the common ocean currents. In confined areas, a tidal current flows for about 6 h 12 min in an upstream or shoreward direction, it then reverses and flows for approximately the same time in the opposite direction. During the period of reversal, the water is in a state of rest, known as slack water. A current flowing toward the shore or upstream is called a flood current; that
flowing in a direction away from land or downstream is known as an ebb current.
The moon, being much closer to the earth than the sun, is not only the greatest influence on out tides, but its phases also greatly affect their character. The tide-rasing force of the sun is only about half of that of the moon. As the moon orbits the earth, it aligns with the earth and sun semimonthly, during the full moon andnew moon phases. At such times, we have extremely
high and low tides. These conditions are known as spring tides.
Spring tides. the earth is shown in schematic cross-section. Blue represents water and green the rigid earth.
Between these phases, when the moon is in its first and third quarters, it's at right angles to the sun. At these occations the height of the waves is subject to the opposing forces
of the sun and moon, resulting in a minimum range between high and low tides. These conditions are named neap tides. the work spring in this context has nothing to do with the season with the same name. Both spring (an outflow of water) and neap (Anglos-Saxon for 'scanty') come from Old English.
The Orbit of the Moon
The orbit of the moon is not exactly circular; it is rather slightly elliptical in shape, with the Earth located at one of the foci of thi sorbit. So, the Moon's distance from the Earth changes along its orbit from a minimum value of about 363,300 km to nearly 406,000 km.
(The minimum and maximum distances of the Moon from the Earth are termed perigee and apogee, respectively; these terms are of Greek origin.)
Consequently, the apparent size of th eMoon changes noticeably from perigee to apogee. The mean apparent diameter of the Moon is approximately 31 minutes of arc. As the sky is spherical in shape (shaped like a dome), the apparent sizes of the celestial objects and the distances on the sky are measured in degrees, minutes of arc and seconds of arc.
The Moon orbits the Earth once every 27.32 days. thi speriod is known as the sidereal period of the Moon as it is measured relative to the stars. That is, in this period, the Moon completes one orbit around the Earth, i.e., it keeps the same face towards the Earth as it rotates about its axis once every siderial period. The lunar hemisphere directed away from the Earth is termed the farside of the Moon. Our Sister Planet
The Moon is our nearest cosmic neighbour and our faithful companion that follows planet Earth whereever it goes through its cosmic voyage.
The Moon is a cold rocky planet measuring 3,476 km across. The Moon does not shine light of its own; it merely reflects sunlight falling on its surface. Amazingly, the Moon reflects only about 6% of the sunlight that strikes its surface. This reflectivity is close in value to that of coal or blackboard.
Our planet is much larger than the Moon. Earth, with a mass of approximatly 6 x 1024kg, is over 80 times more massive than the Moon.
The Earth's diameter (about 12,756 km) is over 3 times larger than that of the Moon. The volumes of the Earth and the moon are approximately 108.3 x 1010 km3 and 2.2 x 1010km3, respectively, i.e., Earth is about 50 times larger in volume than the Moon. (If Earth was hollow, there would be enough space inside Earth for 50 Moon-sized planets.)
Interestingly, the ratio between the sizes of Earth and the Moon is similar to that between the basketball and the baseball.
Our Sister Planet
Spring tides. The Earth is shown in schematic cross-section. Blue represents water and green the rigid earth.
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