What Would We See If The Moon Rotated Every 24 Hours?

If the Moon rotated as fast as the Earth, would we only see one side of the Moon?
Framed by the Earth's horizon and airglow, the full moon floats in the blackness of space in this photo from the Expedition 10 crew on board the International Space Station. Image credit: NASA

Framed by the Earth's horizon and airglow, the full moon floats in the blackness of space in this photo from the Expedition 10 crew on board the International Space Station. Image credit: NASA

Originally posted on Forbes!

The Earth rotates around its own axis once every twenty-four hours. The Moon, on the other hand, rotates once around its own axis every 28 days, and once around the Earth in that same 28 days. The end result of this combination is that the same side of the Moon is always facing the Earth. As the Moon moves to be directly above a different portion of the earth, its face also turns at exactly the same rate, so that only one hemisphere of the Moon is ever visible from our home here.

If the Moon turned at any other rate (either faster or slower), we would eventually see all sides of the Moon, and what is currently the lunar far side would be a much more familiar sight to us. If we spun up the Moon to one rotation every 24 hours, how dramatic would this be?

I’m assuming that we’re not changing the Moon’s orbit here - so that the Moon would still orbit the Earth once every 28 days. This means that the rising and setting of the moon would happen in the same way as they do now - slightly later every day, and the phases of the moon would remain the same, because the phases are simply the combination of the Moon’s location in its orbit around the Earth, and what fraction of the near side of the Moon is illuminated by the Sun. So we would still have a new moon and a full moon about once per month. What would certainly change is which portion of the moon is illuminated.

Speeding up the Moon’s rotation so that it spins once every 24 hours is a pretty dramatic change. That means the Moon has to rotate the full 360 degrees of a circle in 24 hours, which puts us at 15 degrees of rotation every hour. That may not sound like a lot, but over the course of an evening, which we’ll say is an average of 12 hours (half of our Earth’s 24), that means that the Moon has rotated by 180 degrees. A full moon could rise with the familiar near side facing us, and by the time it sets, 12 hours later, we’d be looking at the unfamiliar jagged territory of the lunar highlands - what is currently the lunar far side. In a six hour period, you’d expect the Moon to rotate by 90 degrees. If you were in a half-moon phase, where only half of the Moon’s face is illuminated, you would expect that illuminated portion to change completely, twice over, every time the Moon rose above the horizon.

However, if the Moon truly did rotate once every 24 hours, the two sides would probably look much more similar to each other than they do now. Part of the intense cratering of the far side of the Moon is because it is constantly facing “outwards” towards space, and it’s an easier target for interplanetary fragments of rock to hit, than the somewhat protected, Earth-facing side. If the Moon rotated faster, these meteoroids would have a pretty even chance of hitting any face of the moon, and the cratering would probably be more evenly distributed.

It’s fun to think about how this kind of situation might have influenced our calendars - since our months are roughly based on the lunar cycle, perhaps we would have used the appearance or disappearance of certain features of the moon as a smaller unit of time. But we certainly wouldn’t have grown attached to one side of the Moon - what we see now as the near side would be just as normal to us as the far side.

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Can A Star Ever Turn Its Spin Backwards?

Can a star reverse its rotational direction during some time in their life, and if so, how would it affect any planets around it?
This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

This artist’s impression of the water snowline around the young star V883 Orionis, as detected with ALMA. Image credit: A. Angelich (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)

Originally posted on Forbes! 

The stars in the night sky all have their preferred direction of rotation, which depends directly on the exact way that the cloud of gas and dust that the star formed out of collapsed. If there was slightly more random motion in a clockwise or a counterclockwise direction, as the cloud of gas collapsed, the star would have magnified that hint of rotation, spinning up the same way that a figure-skater does, pulling in their arms and legs.

Once spun up this way, another piece of fundamental physics comes into play - inertia. Inertia tells us that objects in motion tend to stay in motion unless there’s something else that’s causing that object to slow down. On Earth, that something else can come in many forms - we have the mass of the Earth, whose gravity will pull objects down towards the surface, an atmosphere to move through, which will slow objects moving through it, or quite simply mountains and buildings which objects can bounce off of and away from.

If you’re in space, these sorts of Earthly obstacles which can serve to stop a moving object aren’t around. There are many fewer things which could serve to stop an object’s motion - without a thick atmosphere to move through, the planets and our spacecraft continue at their current speeds without any impediments. The most common method of slowing down (or speeding up) an object in space is by traveling near another large object (ones that are of a similar mass to yourself are the most effective) and letting the force of gravity alter your path.

These requirements for an external force hold both for motion as we normally think about it (a forward or sideways motion) and for spin. So if we think about spinning a bicycle tire, that wheel will continue to spin until the forces of friction in the axle (primarily) will slow it down, or until you clamp down on the brakes. An object which is spinning in space has no friction-containing axle around which to spin, and so if it’s isolated, without any external objects which can act as a braking force, that object should continue to spin as it is, ad infinitum. This is basically the situation that stars find themselves in. Stars do not reverse their spins as a standard part of their lifetimes.

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

Planet formation begins with a brilliant young star at the center of what’s called a protoplanetary disk. Collisions within the disk form rocks that act as planetary building blocks. They settle into orbit around the star, creating gaps in the disk. Image credit: NASA's Goddard Space Flight Center Video and images courtesy of NASA/JPL-Caltech

A reversal in the rotation of a star is extremely difficult to accomplish without something external to the star punching it backwards in the other direction, to slow down each rotating particle that makes up the star. If you could, using magic, reverse the rotation of our own star, without changing anything else, the planets surrounding our Sun wouldn’t be influenced at all - the orbits of the planets are determined by the gravitational pull of the Sun, which hasn’t changed if we haven’t changed its mass, plus the planets’ own velocities.

What sort of objects could act as a brake in space? The is easiest is always a good old large-scale collision. This is how we think the Earth formed the Moon, how Uranus got tipped onto its side, and Venus got tipped completely upside down. So if we wanted to reverse the spin of the Sun, we’d have to hit it with something pretty catastrophic in order to both stop the rotation of the gas & plasma which currently makes up the Sun, and reverse the direction of the spin. Any impact on that scale would definitely impact the planets. If it was another star that hit our own, we’d have gravitational and temperature-related chaos even before the impact, setting aside whatever cataclysm of energy would be unleashed during the collision.

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Do all planets rotate around their stars?

Do all planets rotate as they go around their stars? Do they all rotate in the same direction (e.g. clockwise or anticlockwise?) Or does it just depend on what started them rotating in the first place?
image

Originally posted @ Medium!

Before we can expand our thinking out to “all planets”, the easiest wayto start looking at planets and how they rotate is to look at our ownsolar system, which we can investigate in far more detail (and far moreeasily) than anywhere else in the Universe. What we see in our own solarsystem is that all of the major planets are rotating around their own internal axis.

We’re wellacquainted with the rotation of the Earth, even if we haven’t thoughtabout it in this way — the Earth’s rotation is what creates our days. Asa result, we all know how long it takes for the Earth to rotateonce — 24 hours. But 24 hours isn’t the rule within our solar system; infact, of all the other planets, only Mars rotates at a similar speed.Mars completes one rotation in 24 hours and 40 minutes; nearly identicalto our home planet.

Venus,our planetary sister gone horribly awry, rotates much slower than theearth — one rotation takes 243 days and 26 minutes; this makes it theslowest rotator in our solar system. Mercury comes in second slowest,and rotates once in 58 days, 15 hours, and 30 minutes. The gas giantsall seem to have similar rotation speeds, all of which are faster thanany of the inner, rocky worlds. Jupiter rotates once every 9 hours and55 minutes. Saturn recently had its rotation speed re-measuredto be 10 hours and 36 minutes. Uranus rotates once every 17 hours and14 minutes, whereas Neptune rotates once every 16 hours and 6 minutes.

Mosteverything in our solar system rotates in the same direction — the samedirection as the Earth. If we had a bird’s eye view of our solarsystem, where we’d flown into space “up” via the North Pole, and lookedback down, most of the planets would be rotating counterclockwise — orfrom the West towards the East.

You can usually remember which way the earth is rotating by thinking about thetime zones; the further east you go, the earlier it is — they’re pushedtowards daylight sooner than the west.

Sowhy does almost everything rotate in the same way? A hint of the answerlies in the Sun. The sun also rotates, and it rotates in the samedirection as the majority of the planets — counterclockwise from ourbird’s eye view. Since the Sun is also following our consistent rotationpattern, we’re going to have to make our way back to the formation ofthe solar system in order to make sense of this.

Our sun formed out of a cloud of gas and dust — enormous molecular clouds arethe only places where, under the persistent pull of gravity over time,material gets dense enough to begin to collapse and form a star (orten). However, during the collapse of the cloud of dust and gas, anyinitial motion in the cloud becomes quite important.

It’svery unusual for objects in the universe to be completely still inrelation to each other (in fact, they would have to be at absolute zeroto prevent any motion), so there’s a little bit of stirring about withinthe cloud that is always present. As the cloud collapses, the averagedirection of motion of the cloud is kept — and as gravity pullseverything closer to the center, the conservation of angular momentumcauses that average motion to speed up. You can do this experiment athome if you have a chair that spins. If you start yourself turning alittle bit with your arms and legs extended, and then pull your legs andarms in quickly, you’ll find yourself spinning much more rapidly thanyou were to start out. Pulling your arms in plays the same role thatgravity is playing in the collapse of the early solar system.

Because therotation speed is so much faster than it was initially, the gas formsinto a flat disk that’s all rotating in the same direction. If you’veever played with a ball of bread dough or silly putty, and spun it onyour finger, you’ll find it flattens out into a thin disk fairlyquickly. The star at the center is therefore being formed in anenvironment where everything is already rotating — as the star collapseseven more, its rotation will increase even more. All the proto-planetsare also forming out of gas which is rotating, so it makes sense thatthey too will absorb the rotation of the disk they formed in. So inprinciple, we expect most planets to form with a rotation that’sconsistent with the star at the center of their solar system.

However,“most everything” isn’t everything — we’ve got two notable exceptionswithin our solar system to the counterclockwise rotation rule: Venus andUranus. Uranus rotates 90 degrees off from everything else. If youconsider the plane of all the planets’ orbits around the sun as a flatsurface, most planets spin as though they were a coin spun on its edge,flicked counterclockwise. Uranus, on the other hand, spins like a beadrolled along the ground, instead of spinning vertically. Venus is evenodder- it spins clockwise. As far as we can tell, this means Venus issomehow upside down.

Whathappened to these planets? The picture I painted above indicates thatthey should have formed with a rotation aligned with every other planetin our solar system. In fact, they probably did form with a rotationthat matched. However, the very early solar system was a violent place,filled with many more proto-planets which were inclined to collide withother objects. Some of these collisions were likely with the earlyversions of the planets which exist today. A particularly bad collisionor series of collisions could cause such an energetic punch to the otherplanet that the planet was effectively tipped over onto its side (inthe case of Uranus) or upside down completely (in the case of Venus.)

Toexpand to the Universe outside of our own solar system; physics is thesame everywhere in the Universe, so solar systems should form in thesame way everywhere. However, we wouldn’t expect every planet to rotatein the same direction as its parent star, since — like our own solarsystem — the planets may have had a very collision-intensive early life.

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