12
$\begingroup$

Consider a planet identical to Earth except that it has no moon.

  1. How long is the complete day-night cycle on this planet? I've heard that having no moon would speed up the planet's rotation, resulting in a day-night cycle of 6 hours. Is this true?
  2. If so, then what would the conditions be so that this planet could have a 24-hour day-night cycle without a moon? (For example, would having a larger mass make it rotate slower?)
  3. What would the seasons look like on this planet? Would there be seasons at all? I've heard that having no moon means that there would be no changing seasons. Instead, certain areas of the planet would be locked in a single season, and that season would vary depending on the area.
  4. Would the temperature of this planet be different compared to Earth because of its lack of a moon?
  5. Are there any other notable things (besides the tides, the darker nights, and the lack of eclipses) that should be mentioned?
$\endgroup$
  • 1
    $\begingroup$ where did you heard all that? (no, no, same, no, no.) $\endgroup$ – njzk2 Jul 7 '16 at 16:03
  • $\begingroup$ @Devsman So... you're saying that Earth with a moon is the exact same as Earth without a moon? $\endgroup$ – Summer Jul 7 '16 at 18:09
  • 4
    $\begingroup$ @Summer Well no. The Earth without a moon doesn't have a moon. $\endgroup$ – Devsman Jul 7 '16 at 18:46
  • $\begingroup$ Venus is a counterexample for day length. So is mars, as the two moons are recently capured asteroids and temporary. But on average having a satelite slows rotation. $\endgroup$ – JDługosz Jul 7 '16 at 20:38
  • $\begingroup$ @Devsman Yes... but surely there are more differences between Earth-with-a-moon and Earth-without-a-moon besides the fact that one has a moon and one doesn't. Anyway, the answers I got below pretty much answered my question, so thanks anyway. $\endgroup$ – Summer Jul 8 '16 at 20:14
26
$\begingroup$

Removing Earth's moon now would not speed up its rotation. The tides caused by the moon have slowly reduced the speed of Earth's rotation over billions of years, but the current length of the day is a result of the original speed of rotation and the Moon's (and the Sun's) tides.

Read the Wikipedia page on the Giant impact hypothesis, which is our best idea at present for how Earth acquired the Moon (summary: Splat!). If that didn't happen to Earth (as it didn't to Venus), we wouldn't have a large moon, but that doesn't tell us anything much about the length of the day. That would be determined by whatever speed Earth ended up rotating at after it accreted out of the Sun's protoplanetary disc. So an Earth-without-moon can have whatever length of day you like, and nobody can say you're wrong.

The seasons are not driven by the moon. They're a product of Earth's orbit and its axial tilt, and if those things were the same, the seasons with be pretty much the same without a moon. In the same way, the moon doesn't have any drastic influence on Earth's temperatures.

$\endgroup$
  • 2
  • 2
    $\begingroup$ Good points except the part "the moon doesn't have any drastic influence on earth temperatures". The moon controls the tides. Meaning water would be stagnant and gross. It would be thicker and contain lots of garbage and pretty much prevent evolution as currently theorized. The tides also create the underwater currents that drastically impact the global temperature. Hotter places would be hotter, colder places would be colder. Much hotter and much colder. Weather would be much more extreme. Sorry Nemo. No EAC for you. $\endgroup$ – danielson317 Jul 7 '16 at 14:24
  • 2
    $\begingroup$ @danielson317 The sun also causes tidal movement of oceans. That's why the pattern of the tides is such a complicated cycle. Without the moon, the tides would be much weaker, but there would be tides. $\endgroup$ – Todd Wilcox Jul 7 '16 at 15:58
  • 9
    $\begingroup$ @danielson317: the Sun causes tides, about half the strength of the moon's ones. While the absence of lunar tides would change things, it's unlikely to be as radical as you're suggesting. The Coriolis force due to the Earth's rotation is also important in driving ocean currents, as are temperature differences. $\endgroup$ – John Dallman Jul 7 '16 at 16:01
  • 3
    $\begingroup$ @danielson317 I suggest you visit the Great Lakes or the Mediterranean Sea. Both areas of water are virtually tideless - and both are famously full of marine life. Busted, dude. $\endgroup$ – Graham Jul 7 '16 at 16:03
17
$\begingroup$

The one item I feel is missing from the last responses that you might want to consider...the moon does act as a gyroscope for the earth. The result is that the procession of the earths axis is regulated. Without the moon, the earth's axial tilt could vary much more significantly over a much shorter time period (still millions of years - short for earth, pretty long for most book plots!). So on such a world, we could find a sun directly over the equator - essentially a planet without seasons and days and nights of equal length year round. Or a world tilted "on its side" where the sun was directly over the pole at solstice (24 hour day for one hemisphere and 24 hour night for the other) and directly over the equator at the equinox; a world with dramatic seasons, to say the least.

Since current lunar theory holds the moon is a part of the original primordial earth that was ejected during a massive collision, your moonless earth could be more massive than our earth. A more massive earth could be more volcanic and have more active geology in general due to a higher core temperature. Additionally, earths magnetic fields could be significantly stronger due to a more energetic core leading to different conditions in your earths radiation belts - impacting aurora's, radio communications, orbital equipment.

The moon is pockmarked with craters, many quite significant. Some percentage of the objects that impact the moon could instead have impacted the earth. Asteroid extinction events might be more common on your earth, driving a different evolution...?

The moon sweeps earths near orbit. Small bodies, such as asteroids would be difficult for the earth to capture due to the moons presence. Without a giant moon, your earth would be more likely to capture a satellite from the asteroid belt or even generate a satellite as the result of an asteroid impact. Examples are the moons of Mars. Your earth could actually have several such small captured satellites on odd crossing orbits of different periods.

If your earth is indeed more massive, your atmosphere would be more dense. Depending on how massive, your atmospheric composition might be different, with different gasses, more gases that we see only in trace amounts. But I am not the person to do that sort of chemistry and calculations. Similarly, this could have an impact on surface temperatures and weather, but that is the topic for another questions if and when you decide more precisely on the evolution of your earth and its atmosphere.

$\endgroup$
  • 1
    $\begingroup$ The moon does not act as an effective shield for the Earth. It does absorb some impacts that would have "hit" the Earth, but not the majority of them. The vast majority of space rocks that encounter the Earth burn up in its atmosphere. $\endgroup$ – Martin Carney Jul 7 '16 at 18:35
9
$\begingroup$

How long is the complete day-night cycle on this planet?

This depends on the angular velocity, and little else. The effect the moon has is that it slowly slows the earth over time, because of the friction caused by tides and the movement of the moon's mass away from Earth. So knowing the length of the day/night cycle would require knowing when in Earth's life you're talking about.

What would the seasons look like on this planet? Would there be seasons at all?

Seasons are caused by a variety of factors, but the primary one is the tilt of the Earth and the relation of that tilt to the Sun. Loss of the moon would affect this, because it would affect tides and therefore affect the meteorological effects of the seasons, but it would not stop there being warmer and cooler parts of the year in various places.

Would the temperature of this planet be different compared to Earth because of its lack of a moon?

Not significantly, likely.

Are there any other notable things

It is likely that tides and moonlight affect a great number of species in a wide variety of ways from navigation to life cycles (werewolves) to how they get about (fish returning to spawning grounds, turtles to the ocean). Thinking about how humans work with the tides we can already see how a great number of things would be different if the seas were (relatively) stable.

$\endgroup$
4
$\begingroup$

Not much would change without the Moon. It is often suggested that the Moon was key for life on Earth but there are nothing concrete to back that up (for example, see here: http://www.astrobio.net/news-exclusive/earths-moon-may-not-critical-life/).

For instance, Earth's axial tilt is indeed stabilized by the Moon at Earth's current spin rate. However, after formation Earth spun much faster (the exact rate was determined by the last couple giant impacts on Earth). At a faster spin rate, the Moon is not needed to stabilize Earth's spin. And newer work suggests that a Moon-less Earth would still have a pretty stable spin axis. So, the Moon never actually made much of a difference in terms of stabilizing Earth's spin axis. However, Earth-Moon tidal interactions did cause the Moon to be pushed farther from the Earth and the Earth's spin rate to slow down (making the day longer)

The day-night cycle is only affected by the Moon in terms of the length of the day (which would be much shorter, probably around 4 hours long, without the Moon). The seasons don't care about the Moon, and neither does the surface temperature.

There are a lot of mysteries related to the origin of the Moon (e.g., http://nautil.us/blog/the-genetics-of-the-earth-and-moon). And there are a lot of opportunities for imagining life on moons in science fiction (like Pandora: https://planetplanet.net/2014/11/18/real-life-sci-fi-world-6-pandora-from-the-movie-avatar-the-habitable-moon-of-a-gas-giant-planet/). But I don't think the Moon disappearing would have that big of an effect on the Earth.

$\endgroup$
3
$\begingroup$

I think other answers give you a pretty good idea of what the differences would be between a moonless Earth and our own. But there are a few points that I think might be highlighted, that seem to be missing.

As others mentioned, the Moon has no deal with Earth's seasons, aside from estabilising its axis. But the impact that created the Moon - if this is what happened - or even the capture of a small-size planet, if we believe otherwise - could have had whatever consequences a fiction writer wants. So you could position the South Pole of your fictional Earth in Africa - or whatever continent it would result in its place from the Moon not being created/captured - and explain that away by stating that what put Antarctica in the South Pole was the creation/capture of the Moon. The same goes for the length of the day: while the existence of the Moon slows Earth's rotation very slowly, the acquision of the Moon might have brutally reduced the rotation speed in one coup - or, on the contrary, accelerated a tide-locked Earth.

(Note that a moonless Earth would be more different from actual Earth if the Moon was indeed created by an impact; it would be unlikely that in that case the outline of continents or oceans would be remotely similar. If you don't want to rebuild our geography too much, it would be probably better to postulate that scientists are wrong, and the Moon was a neighbouring dwarf planet that got gravitationally captured into Earth's orbit.)

More importantly, though, the tidal effects of Moon affect all of Earth's liquid parts - most obviously the oceans, but also whatever liquid or semi-liquid parts of its mantle and core. This creates heath of itself - imagine the amount of attriction it generates - and also tends to stir the inner parts of our planet, making the decantation of heavier elements towards the core slower. In other words, a moonless Earth would quite probably have a bit less internal heath, resulting in reduced vulcanism, perhaps a deeper crust, even to the point that tectonic plates would weld into each others, and quite probably would have a crust less rich in iron and heavier than iron elements. And really heavy, radioactive elements would be even rarer, which would result in an even cooler interior, as part of the Earth's internal heath comes from radioactive decay inside it.

Nights would be darker. Would we have to adapt to this, too? And would other animals become more - or less - dangerous from the total absence of light at night, without a cycle of darker/lighter nights?

It would be interesting to see what impact the absence of the Moon would have upon human knowledge, supposing that your moonless Earth is inhabited by humans (but would it be, or, as Asimov suggests, reduced radioctivity would have slowed evolution too much?) Would realising Newton's laws be made more difficult without a close example of an orbit? Would time recording be too difficult for early men without a relatively easy-to-count 29 day cycle? Would we be able to realise the regularity of years/seasons without months and weeks to mediate/facilitate the calculus?

... and, of course, there's the pressing question of whether would there be such a thing as "romantic love" in a Moonless Earth.

$\endgroup$
  • 1
    $\begingroup$ A couple comments. First, continents took time to form and they were not there yet at the time of the Moon-forming impact (note that the only version of the capture idea that works also involves an impact). And tides affect the entire Earth, although the crust deforms to a much lesser degree than the oceans. Earth's internal heat flux is dominated by the decay of long-lived radionuclides (mainly Uranium-238). Tides contribute less than 1% to Earth's internal heat flux. And tides from the Sun are almost as strong as tides from the Moon. So, removing the Moon would not affect volcanism. $\endgroup$ – Sean Raymond Jul 8 '16 at 8:02
0
$\begingroup$

Without the moon we'd have no tides, or at least very weak tides (the sun does exert some tidal force). Tides are actually quite useful: they serve to keep smaller bodies of water like bays and estuaries from getting stagnant. And that's not even addressing the fact that many marine animals (as pointed out by Hurda's comment) use the moon for navigation, among other things. Some animals, such as jellyfish, time their spawning according to the lunar cycle. I'm no marine biologist, but I think it's safe to say that ocean life in general would look quite different if we had no moon.

$\endgroup$
0
$\begingroup$

The lack of a moon, or even more dramatically, the lack of a moon and any other planets close enough to be seen with the naked eye, would probably retard the development of mathematics and physics. These observations were critical, for example, in our capacity to generalize gravity to the Newtonian and the general relativistic state, and made it necessary to use precision observations to make complex calendars work.

$\endgroup$
0
$\begingroup$

To answer a question not entirely covered by the other answers, the day-night cycle would indeed be shorter than that of Earth -- assuming that the planet had no moon to begin with -- as the Moon was the main factor slowing down Earth's rotation from 4 hours a day to 24. Without the Moon one day would be closer to 6 hours.

The planet having a larger mass would make it rotate faster, not slower, in general; the best way to ensure a 24 hour day/night cycle is to have the planet be closer to the sun (i.e. having a dimmer K-type (yellow/orange) sun pushes the habitable zone closer in).

However, as Luis Henrique's answer mentioned, the Moon would be needed to ensure a stable axial tilt with an Earthlike day/night cycle. This would mean that at some points in the precession cycle (lasting several million years) that the planet would have small axial tilt, resulting in barely any seasons, and at other points the tilt would be extreme resulting in extreme seasons.

The temperature of the planet would not be significantly affected; though it can be set at various values higher or lower than Earth's by placing it at slightly different orbits.

There would still be tides due to the sun; and if it is indeed closer to the sun than Earth is, the solar tides would be stronger (given a planet in a corresponding position to Earth and appropriate mass/luminosity relations, they are roughly proportional to the solar luminosity to the power of 1.2).

Now, if instead the Moon disappeared from Earth, it would be a completely different case.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.