A tide is a periodic change in ocean level driven mainly by the differential gravitational forces of the Moon and Sun, together with the Earth's rotation. Their tidal forces act across the extended planet; a uniform acceleration shared by the whole Earth does not by itself generate a tide. Ocean-basin resonance and coastal geometry modify the local response.
The tidal range is the difference in sea-surface height between a specified high tide and the neighboring low tide, measured relative to the same datum. A high-water level of and low-water level of give a range of .
For a sinusoidal tide , the range is , not the amplitude . Interference between lunar and solar tidal contributions produces larger spring ranges and smaller neap ranges. Coastline geometry, water depth and resonance modify the local response, so the tidal range cannot be inferred from astronomical forcing alone.
The Bay of Fundy is a coastal basin in eastern Canada notable for large tidal ranges. Its geometry and the dynamical response of the connected bay system strongly amplify the incoming semidiurnal tide.
The important physical idea is forced motion of a basin: water depth, shape and friction determine its natural response, while the external astronomical tide supplies the drive. Large range is not a direct local measurement of an unusually strong gravitational field. Range also varies with location and the spring-neap cycle, so one quoted maximum does not characterize every observation.
A semidiurnal tide has two principal oscillations per day. The dominant lunar constituent has a period of approximately 12 hours 25 minutes; the solar semidiurnal constituent has a period of 12 hours.[ref]
About Harmonic Constituents by National Oceanic and Atmospheric Administration. Examples M2 and S2
A spring tide has an enhanced high-to-low tidal range because the Moon and Sun are approximately aligned as seen from the Earth, so their principal tidal contributions reinforce. Such alignments occur twice in a lunar phase cycle.
Articles by others on the same topic
Tide can refer to several different things, depending on the context: 1. **Oceanography**: Tide refers to the regular rise and fall of sea levels caused by the gravitational forces exerted by the Moon and the Sun, combined with the Earth's rotation. Tides are typically observed in cycles of approximately 12 hours and can significantly influence coastal ecosystems and activities. 2. **Laundry Detergent**: Tide is a brand of laundry detergent produced by Procter & Gamble.