Rotation period.?

Question

I need a down and dirty explanation of rotation periods applied to Astronomy. What properties and factors are involved in it, how exactly do they affect it and any available formulas to determine a rotation period. I need this to be kinda simple though, I just need a functional understanding of this so I can think of plausible solar systems for a game I'm designing.

Answer 1

After reading some of the other answers, let me begin by drawing a distinction between rotation and revolution. Revolution is the motion of the planet around the sun. Rotation is the motion of the planet around its axis.

The rotation of a planet is influenced by three things:
1 initial angular momentum from the formation of the planet
2 additions or subtractions to the angular rotational rate due to impacts by other bodies,
3 perturbations resulting from gravitational influence from other bodies.

In the first case, the planetary nebula which formed our solar system had an initial rotational state. That is the state which caused the planets to all revolve around the sun in the same direction. But it also influenced the rotation of the planets as they formed. Some scientists think that the coreolis affect came into play during that time.

In the second case, as the earthmoved in its orbit, it was occasionally struck by large rocks, small asteroids, etc. Depending on how they hit the planet, they would have a tendency to either add a little extra rotation or subtract a little rotation. In some cases, very massive impacts may even have knocked a planet off kilter (Uranus, for example) so that it rotates on its axis sideways.

Finally, gravitation from other objects can affect rotation of a planet. The earth/moon system exerts a gravitational tug on Venus every time that planet passes us in its orbit. Over a period of a few billion years, the result of that little tug has slowed Venus' rotation not only to a standstil, but has even started Venus rotating backwards very slowly.
Likewise, the gravity of the moon pulls on the tides of the earth on a regular basis. The tides exert friction against the sea bed and act like a set of brakes on the spinning earth. This is known as "Tidal Braking" and over a few million years has actually slowed the rotation of the earth somewhat.

As for rotational periods, there are two primary types.
A siderial period is the amount of time it takes for a planet to turn 360 degrees. An example would be how long it takes for the star Aldebaran to be seen from transit (directly overhead) one night to the next.
A synodic period is the amont of time it takes for a planet to rotate until a particular heavenly body returns to the same position as it was at the beginning of the period. An example of this would be the moon transiting at, say 10pm one night and then transiting at around 10:50pm the next. The reason for this is because during the earth's rotation, the moon has actually moved a bit more along in it's orbit, and the earth has to rotate a little extra to catch up to where the moon has moved to.

Hope that helps.

Answer 2

Any ball of rock or gas can rotate at darn near any period you want, for instance, giant Jupiter rotates about once every 12 hours, and dinky Mercury rotates once every 115 days. If the ball is too small or the rotation is too fast it could break up (and before that flatten out into a disk noticibly). Just balance centripetal acceleration against gravity to find this breakup speed.

v^2/r <= GM/r^2

where G is the gravitational constant (which you can look up online) and M is the total mass, v is the velocity of the rotation and r is the radius of the ball.

Rotational velocity V = 2*PI*r/P
where P is period. You can combine those to get Keplers law except r is the radius of the orbit now. (yeah, essentially the planets orbit the sun at breakup velocity, which is why they are not PART of the Sun.)

Keplers law is what you need to use if you are interested in the orbital periods of the planets. The mass you use in all cases is the total mass of the system, though in the case of planets around a star you can generally ignore everything but the star.

Answer 3

Rotation period P is related to distance A from the Sun by Kepler's 3rd law P^2 = A^3. If the orbit is not circular, A is the semi-major axis of the ellipse.

Answer 4

In astronomy, a rotation period is the time an astronomical object takes to complete one revolution around its rotation axis.For objects that are not spherically symmetrical, the rotation period is in general not fixed, even in the absence of gravitational or tidal forces. This is because, although the rotation axis is fixed in space (by the conservation of angular momentum), it is not necessarily fixed in the body of the object itself.

Answer 5

Period is the inverse of frequency. The link explains this for a sound wave. However, it is also true for a rotating object (or pair of objects?). If the frequency is 300 rpm (revolutions per minute) the inverse (period) is 1/300 minutes (one 300th of a minute).