Tide
A tide is the periodic rise and fall of sea level caused by the gravitational forces exerted on Earth by the Moon and the Sun, combined with Earth's rotation. The Moon's gravity creates a tidal bulge on the side of Earth nearest to it and another on the far side, resulting in two high tides and two low tides in most locations every approximately 24 hours and 50 minutes. Tides are essential for coastal ecosystems, navigation, fishing, and are a significant source of renewable tidal energy.
Key Formula
Tidal force F = (2 * G * M * m * r) / d^3
LaTeX: F_{tidal} = \frac{2GMmr}{d^3}
| Symbol | Meaning | Unit |
|---|---|---|
| F | Tidal force | N |
| G | Gravitational constant (6.674×10⁻¹¹) | N·m²/kg² |
| M | Mass of Moon (or Sun) | kg |
| m | Mass of water element | kg |
| r | Radius of Earth | m |
| d | Distance from Earth center to Moon | m |
Worked Example
Problem
A coastal town experiences a high tide of +3.2 m and a low tide of −0.8 m. Calculate the tidal range, and determine the time of the next high tide if the current high tide occurs at 06:00.
Solution
Step 1: Calculate the tidal range: Tidal range = High tide − Low tide Tidal range = 3.2 m − (−0.8 m) = 4.0 m Step 2: Determine the time interval between successive high tides (semidiurnal cycle): The semidiurnal tidal period ≈ 12 hours 25 minutes = 12.42 hours Step 3: Calculate next high tide time: Next high tide = 06:00 + 12h 25min = 18:25
Answer
Tidal range = 4.0 m. The next high tide will occur at approximately 18:25 (6:25 PM).
Types of Tides and Their Characteristics
| Tide Type | Cause | Frequency | Tidal Range | Example Location |
|---|---|---|---|---|
| Spring tide | Sun-Moon alignment (new/full moon) | Twice monthly | Maximum | Bay of Fundy, Canada |
| Neap tide | Sun-Moon at 90° (quarter moon) | Twice monthly | Minimum | Most coastlines |
| Semidiurnal | Dominant lunar forcing | 2 per ~24.8 hr | Moderate | Atlantic coasts |
| Diurnal | Mixed solar-lunar forcing | 1 per ~24.8 hr | Moderate | Gulf of Mexico |
| Mixed semidiurnal | Unequal successive tides | 2 unequal per day | Variable | US Pacific coast |
| Tidal bore | Incoming tide in narrow channel | Once or twice daily | High | Severn Estuary, UK |
Interactive Tools
NOAA Tides and Currents
Real-time tide predictions and historical tidal data for US coastal stations
Open ToolPhET Gravity and Orbits
Simulation exploring gravitational forces including lunar tidal effects
Open ToolKhan Academy: Tides
Video lesson on how gravitational forces from the Moon and Sun create tides
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Ocean Wave
An ocean wave is a periodic disturbance at the sea surface in which energy is transferred through the water without the net transport of water itself — water particles move in circular or elliptical orbits as the wave passes. Most ocean waves are generated by wind transferring energy to the water surface through friction and pressure, with wave size depending on wind speed, duration, and fetch (the distance over which wind blows). Ocean waves are critical to coastal erosion, sediment transport, navigation safety, and marine ecosystems.
Ocean Current
An ocean current is a continuous, directed movement of seawater generated by forces acting upon the water, including wind, the Coriolis effect, temperature and salinity differences, and tidal forces. Surface currents, driven primarily by wind, affect the upper 10% of the ocean, while deep-water currents are driven by density differences related to temperature and salinity. Ocean currents play a vital role in regulating global climate by redistributing heat from the tropics toward the poles and influencing weather patterns on nearby landmasses.
Coastal Upwelling
Coastal upwelling is an oceanographic phenomenon in which wind-driven surface water is pushed away from the coast, causing cold, nutrient-rich water from deeper ocean layers to rise and replace it at the surface. This process is driven by the combined effects of prevailing winds blowing parallel to the coastline and the Coriolis effect, which deflects the surface water offshore — a process described by Ekman transport. Coastal upwelling regions are among the most biologically productive ocean areas on Earth, supporting major fisheries such as those off Peru, California, and West Africa.
From Old English "tid" (time, season, period), related to Old High German "zit" (time). The word specifically referring to the rise and fall of sea level emerged in the 13th century. The gravitational explanation of tides was first given by Isaac Newton in his "Principia Mathematica" (1687).