Besides knowing how to forecast the weather, Sailors must also have
knowlege of Tides and Tidal Currents. Tides are the rise and fall of
ocean water levels on the Earth's surface. Tide levels (High Tides
and Low Tides) vary depending on the proximity between the earth,
moon, and sun.
Tides are the result of two factors, Gravity (The gravitational effects
of the Moon as it revolves around the Earth), and Inertia (Centrifugal
force as a result of the Earth’s rotation in relation to the sun).
Gravity and inertia maintain the orbits of the sun, earth and moon.
As a result of the elliptical motion of the moon and earth, the
force of gravity and inertia are at different points on the earth’s
surface each lunar day. A Lunar Day is 24 hours and 50 minutes long
and begins at 12:00 Midnight.
A single rotation of the earth’s surface in relation to the sun takes
24 hours, while it takes the earth an additional 50 minutes to complete
a single rotation in relation to the moon. Because the orbits between
the moon and the earth are elliptical, the moon is closest to the earth
every 24 hours and 50 minutes. Lunar tidal effects (i.e., Tidal
Sequence) directly depend upon whether the body of water on earth is
nearest or farthest away from the moon.
The position of the moon and sun in relation to the earth is
significant regarding Tidal Sequence. When the moon and sun line
up together and are relatively close to the earth, the combined
gravitational effects produce Spring Tides. Spring Tides occur
approximately every two weeks during the New and Full moon.
Conversely, when the gravitational influence of the sun and moon
is reduced, the tides reduce in strength until they reach their
weakest point at Neap Tides which occur approximately every two
weeks at first-quarter moon and at third-quarter moon.
Thus, a typical Tidal Sequence begins with Spring Tide followed each
lunar day by lesser tides leading to Neap Tide approximately one week
later; which, in turn the tides become stronger until the next week
later when it becomes the subsequent Spring Tide. During the Tidal
Sequence, each lunar day consists of a tidal pattern that produces
two high tides and two low tides (Semidiurnal) that repeats itself
every 24 hours and 50 minutes. Due to the elliptical motion of the
earth and moon, this tidal pattern occurs approximately 50 minutes
later each subsequent day.
On some of the earth's oceans, the tidal sequence occurs only once;
that is, one High and one Low (Diurnal) Tide per Lunar day. On seas
at other locations of the planet, the tide sequence is mixed; that is,
some days it's semidiurnal and other days it's diurnal.
During a Tidal Sequence, each Lunar Day consists of the rise and
fall of ocean water called Tidal Range. Tidal Range consists of
High (Flood) Tide and Low (Ebb) Tide. Tidal Range measures are
derived by subtracting the minimum water height at Low Tide from
the maximum water height at High Tide.
High (Flood) Tide occurs when the ocean water rises and floods into
shore until the maximum water height is attained. After the Flood
Tide peaks, there is a brief inactive period of time when the tide
remains stationary before flowing back out to sea and that water
measure is called Slack or Slack Water. Low (Ebb) Tide occurs
when the water flows back out to sea.
