Lunar and Solar Tides

Tides cause daily changes in water levels in many coastal areas. Factors such as local topography and weather contribute to the timing and height of tides, but the primary reason for tides is the gravitational attraction between liquid water on the Earth and the Moon. All objects on Earth experience tidal forces. However, the effect is most pronounced with water because, as a liquid, it is more easily deformed by gravity when compared to solid objects.

The gravitational attraction between two objects depends on the mass* of the objects and the distance between them. The closer they are to each other, the stronger the gravitational is attraction between them. Because of this, water closest to the Moon experiences more gravitational force from the Moon than water further away. Water furthest from the Moon experiences the least force.

Differences in gravitational force with distance causes two bulges of water to form. Water closest to the Moon gets pulled more forcefully, creating a bulge towards the Moon. Water furthest from the Moon bulges away from the Moon. To make sense of this second bulge, think about it in relation to the rest of the Earth. The Earth is closer to the Moon than this water is, and the Moon pulls the Earth towards itself with greater force than it pulls the water, leaving a bulge of water behind.

The Earth rotates through these two water bulges as it spins on its axis. If the Moon was stationary and the only movement was the rotation of the Earth, there would be two high and two low tides every 24 hours. The Moon is not stationary. During the 24 hours it takes the Earth to complete one rotation around its own axis, the Moon moves a small amount in the same direction. As a result, a spot on Earth has to rotate 50 minutes longer to 'catch up' to the Moon. This period*icity means that on average:

  • From a vantage point on the Earth, the Moon takes 24 hours and 50 minutes to complete one full orbit.
  • There is a high or low tide once every 6 hours and 12 minutes.
  • High tides occur about 12 hours and 25 minutes after the previous high tide. Same for low tides, they occur about once every 12 hours and 25 minutes.
  • Every day, high and low tides occur about 50 minutes later than they did on the previous day.

Gravity from the Sun also influences water levels on the Earth. The solar-tidal bulges are about half the size of those caused by the Moon. Like the Moon, gravitational attraction to the Sun creates one bulge towards the Sun and one away from it. Unlike the Moon, solar tide*s do not vary from day to day. If there were no Moon, the daily tidal period would be exactly 24 hours. High tide would be at Noon and Midnight, and low tide at 6 PM and 6 AM every day.

The orientation of the Moon and the Sun vary in relation to each other over the lunar cycle. The different phases of the Moon are a manifestation of this variation. This change in orientation also causes tidal heights to vary predictably over the monthly cycle.

  • The largest tidal ranges are called Spring tide*s. These occur during full and new Moons when the gravitational influence of the Sun and the Moon line up with each other.
  • The smallest tidal ranges are called Neap tide*s. These occur during first and third quarter Moons when the gravitational influence of the Sun and the Moon are at right angles to each other.

The illustration above explores how the Earth-Moon-Sun orbital system causes daily and monthly tidal patterns. The illustration uses a simplified system that assumes tides occur on a perfectly spherical, water-covered Earth that rotates around its own center of gravity. Additional factors influence real tidal patterns, but given the overwhelming influence of the Moon, a simplified model that includes only the Moon provides substantial explanatory power for daily tidal patterns. The addition of the Sun and the interactions between the Sun and the Moon explain monthly tidal patterns.

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