A double planet, tides, and the importance of edges

A Double Planet

It is hypothesised that the moon developed out of a interplanetary collision between the young Earth (over 4,000 million years ago) and a huge asteroid, potentially the size of Mars. Vast amounts of matter and energy were released, which later gathered together through gravitational forces to form the moon [1].

Scientific simulations suggest that at the time of its formation, the Moon was much closer than it is today (only around 22,000km away). And the Moon continues to enlarge its' orbit away from the Earth at a rate of 3.78cm per year. In the very early years, the Moon stopped to rotate around it's axis, due to the interactions between the tides of the Earth and Moon, and the transfer of energy via friction (known as tidal locking). This interaction also slows the rotation of the Earth, and also means that we always see the same side of the Moon.

The Moon is now orbiting at an average distance of 384,000 km. It is an unusually large satellite (Earth to Moon ratio 81:1), orbiting unusually close to Earth (their barycentre is 1750km below the Earth’s surface), and is close to Earth’s ecliptic plane. Predominately for these reasons, some astronomers state that it is in fact a Earth-Moon system - a double planet.


The gravitational attraction between the Moon and the Earth is the major cause of Tides. The Sun, although has a lesser influence, causes one much smaller tidal cycle per day, and when the three planetary bodies line-up, known as spring tide - when the Earth is influenced by both the Sun and Moon gravitational pull - tides are about 20% greater than average. When the Moon is perpendicular to the Sun, the high tides are at their lowest, known as neap tides. As a note 'spring tide' has noting to do with the season - instead it comes from the concept of "springing forth" - and occurs twice a month (as do neap tides), irrespective of the season.

During the Moon’s elliptical orbit of Earth (approximately every 27.3 days), it’s gravity pulls strongest at the side of the rotating Earth surface closest to it. This creates a rising "bulge" as the rock moves slightly (only a few centimetres), and the oceans move even more (as they are fluid) towards the moon (a few metres), creating a high tide. Whilst on the opposite side of Earth - the far side, the gravitational attraction (acceleration) is less as it is further away; and so here, inertia (the tendency to move in a straight line) is greater than the gravitational force, meaning that the ocean tries to keep going in a straight line, creating a bulge (and a high tide) in the opposite direction. Two low tides are consequently occurring at right-angles to the moon, at the mid-point between the high tides. As Earth rotates around its' axis, it rotates through these bulges, which causes two lunar tides per day (in fact, tides are closer to 12.5 hours apart, as the Earth rotates around its' axis, the moon is also rotating around the Earth).

Tidal Power

Tidal power continues to be of interest, and reality, for the generation of renewable energy. Although, not without its' complications, the main appeal is its constancy, which means that no storage is required - and its invisibility (under water)...  Various technologies are being used and developed, however most principally drive turbines, transforming the kinetic energy in the rising or falling, or back and forth of tidal cycles. One example is the Annapolis Royal Generating Station in Nova Scotia, which has the highest tides in the world (greater than 15 meters), due to the magnifying effect of its funnel shaped bay.

Rich ecosystems on the edges

Tides also help stir up chemicals, spread nutrients, and contribute to ocean currents, which help moderate global temperatures by transporting heat away from the equator towards the poles. And it is also believed that tides helped provide the necessary ecological testing ground between sea and land, for early species evolving out of the sea.

Seaweeds, many micro algaes, crabs, shrimps, mussels and sea snails, for instance, all live in these intertidal ecology zones. These abundant 'edges,' are where forms of life have co-evolved to take advantage of the challenges in changes in depth, such as desiccation (drying out) and submersion. The ability for species to be able to cope with desiccation, also helps to form distinct vertical zones of specific species. Many of the life in these areas have formed biological rhythms in tune with the tidal cycles, such as gestation and egg hatching; and such links to womens' menstrual cycles (about one lunar month) may hint at our common decent from marine ancestors.


 [1] Harding, Stephan (2009 Second Edition) ‘Animate Earth: Science, Intuition, Gaia.’ Green Books, Cambridge, U.K.

Figure 1: The interactive image above shows a idealised view of the tides, designed to illustrate the main concept. However, the Sun and Moon gravitational effects, also interact with other influences, such as the non-circular orbit of the Moon - when closer together (perigee) stronger than average tides, or when further away (apogee) - which also effects its attitude in the sky, the varying landscape of the Earth's surface under the oceans (known as bathymetry); and the varying shapes of the coastlines. And so, some coastal areas experience semi-diurnal tides (two nearly equally high and low tides per fay), diurnal tides (one high and low tide per day), or mixed tides (two uneven tides per day - one high and one low). However, tide measurements can be taken at specific locations, and can be highly accurate and predictable - even although they may differ between locations.