What if the Earth had no moon (part 2)

A while ago, in this blog post, I mused about what the earth would be like with no moon. You may hear the Moon blamed for things as diverse as reproductive cycles and people’s moods, but in that post, I argued that if all of a sudden, we’d find ourselves without a moon, not a great deal would change. The Sun causes tides, albeit smaller ones, and any stability issues of the Earth’s axis would not change overnight.

But this is not a realistic scenario. The Moon isn’t going to just disappear (the subject of Death Star-like blasting a planet apart is a subject for another blog post, but let’s just say it’s physically nigh impossible). The real question I should have asked is: what would life on Earth be like if Earth had never had a moon? If the Pluto-sized object that may or may not have collided with us to form the Earth and Moon as we know them had missed the Earth, and sailed straight on, to eventually burn up in the Sun.

The situation would look vastly different. The Earth-Moon system is vastly different from other known planets in terms of relative size. Only the largest moons of Saturn and Jupiter are similar in size to our Moon, but those planets are of course much more massive. The Earth-Moon system could well be referred to as a double planet. Of course Earth and Moon affect each other, and this influence comes in the form of gravity. While the gravity of the Moon affects all surfaces of the Earth equally, it’s only the oceans that can react to this gravity. Yeah, that’s how the tides are formed.

Imagine the Earth as an oblong bubble of water, with bulges both on the side where the Moon is at and the opposite side. This bulge orbits the Earth at the rate the Moon does. But the Earth itself rotates inside this envelope of water, and this creates friction at the place where water and solid surface meet. In effect, the water is forever trying to keep up with the planet. Friction creates warmth, and yes, loss of speed.

So it is that over the four-plus billion years of the Earth’s lifetime, the rotation speed of the Earth has slowed from an eight-hour day to our current twenty-four hours. Is still slowing, in fact.

Of course life on a planet with days of eight hours would be very different. We’d experience vastly more powerful weather and especially wind systems. With a rotation speed like that, there would probably not be much opportunity for much North-South weather movement, but we’d have strong bands of air movement, much like Jupiter (which has a daylength of ten hours). We’d have similar ever-lasting cyclonic systems.

And what would it do to the seashores, having the tides jump up twice in eight hours?

Or to the biology of animals evolving with that kind of daylength? Would we all have nervous tics from seeing the Sun whizz by?


What if the Earth had no moon? Part 2

In a recent post, I talked about the answer to the question: what if the Earth had no moon? The question I answered in that post really should have read: what if we suddenly took the Moon away? Or, alternatively, what functions does the Moon fulfil today? And the answer was: not terribly much.

Recently, I read the book What if the Moon didn’t exist, by Neil F. Comins, and it goes into far more detail. Today, the Moon appears in the sky like benign companion. It is pretty, but not terribly vital to us. It causes tides, but the Sun also does that. It provides a handy measure of the passage of time, and it gave us the concept of a month. However, we don’t actually need it. Even the fact that the human fertility cycle and a moon cycle are similar seems pure coincidence. If the Moon was so important to fertility, then all animals would have developed a cycle based on it.

The picture, though, looks very different when you consider the formation of the Earth-Moon system. The Moon formed from a collision of a Pluto-sized object with the early Earth. Right, there goes the peaceful reputation.

In answering the question: what if the Moon didn’t exist, the book takes the history of the Earth back far enough so that this object (remember we’re no longer allowed to call it a planet) missed Earth and left the solar system or found its own orbit around the Sun.

In this case, life on Earth would have been hugely different from the way we know it.

One of the most important things the Moon has done over those billions of years since the collision is to slow the rotation of the Earth. It does this through tides. The Earth rotates, and the Moon pulls whatever part of the Earth facing it. If this part is water, it will rise towards the Moon. But because the Earth rotates, the water is always trying to catch up with the Moon’s current position. Over many, many years, this slows the Earth down. How much? At the time of the Collision, the Earth made a full rotation in just eight hours. Without the Moon, the Sun would have slowed the Earth down, but not by far as much, probably only to about ten hours.

If the thought of ten-hour days doesn’t drive you to distraction, think about what it would have done to the weather. Current weather patterns are determined by many things, including differences in temperature and land shapes. The prevailing movement of weather from west to east is caused by the fact that the Earth rotates inside its own atmosphere, and the atmosphere is not rotating at the same speed. If the Earth had ten-hour days, those west-to-east patterns would be much stronger. In short, we would have a lot of strong wind, and violent weather, and with both weather and ten-hour days combined, a very different planet.

That is not to say that every planet without a moon the size of ours would rotate fast. Mars has a similar day, and Phobos and Deimos are puny captured asteroids that barely deserve the term ‘moon’. Venus has no moons at all and its day is longer than its year. Clearly, a planet does not need a moon to slow its rotation to 24 hours,. Neither does thist answer the question as to whether a random planet in the habitable zone needs a large moon. Earth needed the Moon, but another planet may not.