Caption: The tidal forces of the Moon and Sun create the tidal bulges of the World Ocean.

As illustrated, spring tides are the largest tides and neap tides the smallest tides.

Features:

1. The tidal force is the stretching force due to varying EXTERNAL gravitational field.

   The tidal force is actually a combination of an ordinary force and an inertial forces both of which originate with the EXTERNAL gravitational field. We will NOT go further into this somewhat complex topic.

2. The tidal force combined with the Earth's self-gravity, and pressure force, and centrifugal force gives the Earth its shape.

3. Because the Moon is much closer than the Sun, it has a more rapidly varying gravitational field and that turns out to dominate over the overall stronger gravitational field of the Sun.

   Hence the tidal force of the Moon is stronger than that of the Sun.

4. When Moon and Sun are aligned with the Earth (i.e., new moon or full moon), their tidal forces add and tides are largest as aforesaid.

   These tides are spring tides where "spring" has the meaning of the jump, burst forth, rise, etc.

   Coastal storms that coincide with spring tides are particularly dangerous and prone to cause flooding.

5. When Moon and Sun are at a right angle as seen from Earth (i.e., first quarter moon or last quarter moon), their tidal forces counteract each other, but Moon's wins, but the tides are smallest as aforesaid.

   These tides are neap tides. "Neap is an Old English (AKA Anglo-Saxon) word meaning without the power" (Wikipedia: Range variation: springs and neaps, slightly edited).

6. Because of the Earth's rotation (counterclockwise as seen from the north celestial pole (NCP)) relative to the observable universe (with period sidereal day = 86164.0905 s = 0.997269566 days = 1 day - 4 m + 4.0905 s (on average)), the tide is continuously deforming and reforming water bodies. However, Moon also orbits the Earth (counterclockwise as seen from the north celestial pole (NCP)) relative to the observable universe with mean lunar sidereal month 27.321661547 days (J2000) ≅ 27.32166 days (to 7 digits) ≅ 27.3 days Thus, the Moon's rotation partially cancels the Earth's rotation prolonging the time between high tides to ∼ 12 hours and 25.2 minutes in most locations. (Wikipedia: Tide: Principal lunar semi-diurnal constituent). So sailors have to have tide tables handy so as to know when the tides will be and how high/low they will be. For example, see Sven.

7. As well as the water tide, there is also an Earth tide---but we never notice the continents going up and down by a ∼ 1 meter about twice a day.

   There's an atmospheric tide too and furthermore:

   There is a tide in the affairs of men,
   Which, taken at the flood, leads on to fortune;
   Omitted, all the voyage of their life,
   Is bound in shallows and in miseries.
   
   --- Brutus (85--42 BCE) to Cassius (before 85--42 BCE), Act IV, Scene 3 (scroll down ∼ 70 %) in Julius Caesar (c. 1599), William Shakespeare (1564--1616).

8. See Tide videos below (local link / general link: tide_videos.html).

   EOF