The search for terrestrial extrasolar planets in the habitable zone of stars suggests that these planets may not be all that rare. However, examination of the solar system points to the fact that planets with a satellite similar to the Moon are probably a lot less common. It is massive compared to Earth, and while larger moons exist in the solar system, they also belong to much more massive planets, like Jupiter and Saturn. In fact, I’ve seen people refer to the Earth-Moon system as a binary planet.
While the sight of the Moon in the night sky looks serene now, current theory has it that the Moon resulted from a collision between early Earth and a Pluto-sized object.
What would Earth be like without the Moon? What is the function of the Moon? Do we need it? Does life need it? Life on Earth evolved under the nightly glare of this huge satellite.
Aside its cultural value in mythology and human time-reckoning, the Moon performs two major functions:
– it is the main cause of the tides (not the waves, as people sometimes claim)
– it stabilises Earth rotation
I’ve often heard science fiction writers say that a planet without moon would have no tides. But is that true? Any fisherman or surfer knows that the height of highest tide in a day rises and falls over a period of roughly two weeks. For example, around Sydney spring tide is +2m, and neap tide only about +1.3m. Why the difference? Well, the tides are caused by the Moon’s gravity. During neap tide, which falls in between full moon and new moon, the force of gravity exerted on the ocean by the Moon is perpendicular to that exerted by the Sun. Yes, the Sun causes a tide that is approximately 45% of that caused by the Moon (This page has good information on calculations of tides). Hence, if we had no moon, we would still have the solar tides, except they would always coincide with a day-night cycle, which would have profound effects on communities of intertidal organisms.
Out of the two main functions of the Moon, the second is probably the most important, even though we do not notice the effect. A stable rotation is essential for evolution of life. If Earth were to wobble too much on its axis, we would have various parts of the Earth pointing to the Sun at irregular times. Thus, seasons would be irregular, and all living things have evolved with stable seasons. Imagine if one place on Earth would, over the course of twenty thousand years, go from a polar climate to a tropical climate. It would be a major impediment for the development of most species of plant or animal which are adapted to a fairly narrow temperature and water availability range. For this reason, people suggest that a planet with higher life needs a moon, but the stabilising function could probably be performed just as well by multiple moons or rings.