The rise and fall of sea levels, known as tides, is a predictable phenomenon primarily driven by the gravitational forces exerted by the Moon and the Sun on Earth’s oceans. The vertical difference between the high water level and the low water level is called the tidal range. This range is not constant and varies significantly over the course of a lunar month due to the changing astronomical alignment of the three celestial bodies.
Defining Spring Tides
The term for a tide that has a large range is a Spring Tide. It is characterized by the highest high tides and the lowest low tides of the lunar cycle. The name “Spring” does not refer to the season, but rather is derived from the concept of the tide “springing forth” or surging, indicating a powerful flow. This maximum range occurs twice each lunar month, resulting in a high water mark considerably higher than average and a low water mark that drops significantly below average.
The Mechanics of Maximum Tidal Range
A Spring Tide is created when the Earth, the Moon, and the Sun are aligned in a straight line, a configuration known as syzygy. This alignment happens during both the New Moon and the Full Moon phases. When the three bodies are in line, the gravitational pull of the Sun and the Moon combine their forces to act in a similar direction. This combined pull creates a greater gravitational force on Earth’s oceans, which enhances the tidal bulge.
The resulting effect is an increase in the height of the high tide and a corresponding deepening of the low tide. This reinforcement of the tidal forces leads to the greatest difference between the two water levels, resulting in the maximum tidal range. The largest annual tidal ranges often occur when a Spring Tide coincides with an equinox.
The Smallest Tidal Range
The opposite phenomenon to a Spring Tide is a Neap Tide, characterized by the smallest tidal range. Neap Tides occur when the difference between the high and low water marks is at its minimum. This period results in high tides that are lower than average and low tides that are higher than average. The reduced range is due to a different astronomical alignment of the Sun, Earth, and Moon.
A Neap Tide occurs when the three celestial bodies form a right angle, a configuration known as quadrature. This happens during the Moon’s first and third quarter phases. In this perpendicular arrangement, the gravitational pull of the Sun partially works against the gravitational pull of the Moon. The Sun’s pull attempts to create a tidal bulge in a different direction than the Moon’s. This “destructive interference” results in a weaker overall tidal force and the most moderate tidal fluctuations of the month.