The moon and sun create tides primarily through their gravitational pull on Earth's oceans.
The Primary Force: Gravity
Tides are the result of gravitational forces exerted by celestial bodies, most notably the Moon and the Sun, and the Earth's rotation. While Earth's own gravity holds the planet together, the gravitational pull from the Moon and Sun affects the water on Earth's surface, causing it to bulge in certain areas.
The Moon's Dominance
Despite being much smaller than the Sun, the Moon is the primary driver of tides. This is because the Moon is significantly closer to Earth, and the gravitational force weakens rapidly with distance. The Moon's gravitational pull is roughly twice as strong as the Sun's in creating tides.
Creating the Bulges
The Moon's gravity pulls most strongly on the side of Earth facing it, causing the water on that side to bulge outwards. A less intuitive bulge also forms on the opposite side of Earth. This happens because the Moon's gravity pulls the solid Earth more strongly towards it than the water on the far side. This differential pull essentially leaves the water on the far side "behind," creating a second bulge. As the Earth rotates through these two bulges, different areas experience high and low tides.
The Sun's Role
The Sun also exerts a gravitational pull on Earth's water, contributing to the tides. Its influence is significant, although secondary to the Moon's. The relative positions of the Sun, Moon, and Earth determine the magnitude of the tides.
Spring and Neap Tides
The alignment of the Sun and Moon creates variations in tidal ranges:
- Spring Tides: Twice a month, when the Earth, Sun, and Moon line up (during new and full moons), their gravitational power combines to make exceptionally high tides, called spring tides, as well as very low tides where the water has been displaced. The combined pull accentuates both the high and low tidal bulges.
- Neap Tides: When the Sun is at a right angle to the Moon (during first and third quarter moons), their gravitational pulls partially cancel each other out. Moderate tides, called neap tides, result, with less difference between high and low tide levels.
Here's a simple comparison:
| Tide Type | Alignment of Sun, Earth, Moon | Gravitational Effect | Tidal Range | Occurs During |
|---|---|---|---|---|
| Spring | Aligned (180°) | Combined gravitational pull | Maximum | New Moon, Full Moon |
| Neap | Perpendicular (90°) | Opposing gravitational influence | Minimum | First Quarter, Third Quarter |
The rotation of the Earth relative to these gravitational bulges is what causes locations on the coast to experience the rhythmic rise and fall of tidal waters, typically resulting in two high tides and two low tides approximately every 24 hours and 50 minutes.
