Replies 2 Views 7K. Nancy October 28, Related Threads for: Direct vs. Angular Momentum - Precession. It only changed a small amount but it changed building of the giant dam in China, it changed the moment of inertia of the Earth. This slowing is incompatible with an equilibrium between gravitational and atmospheric tides. Astrobiology Circumstellar habitable Precession Retrograde Motion EP Earth analog Extraterrestrial liquid water Habitability of natural satellites Superhabitable planet.
Retrograde motion, or retrogression, within the Earth's atmosphere is seen in weather systems whose motion is opposite the general direction of airflow, i. Artificial satellites are usually launched in the prograde direction, since this minimizes the amount of propellant required to reach orbit by taking advantage of the Earth's rotation an equatorial launch site is optimal for this effect.
However, Israeli Ofeq satellites are launched in a westward, retrograde direction over the Mediterranean to ensure that launch debris does not fall onto populated land areas. If formed in the gravity-field of a planet as the planet is forming, a moon will orbit the planet in the same direction as the planet is rotating and is a regular moon. If an object is formed elsewhere and later captured into orbit by a planet's gravity, it can be captured into either a retrograde or prograde orbit depending on whether it first approaches the side of the planet that is rotating towards or away from it.
This is an irregular moon. In the Solar System, many of the asteroid-sized moons have retrograde orbits, whereas all the large moons except Triton the largest of Neptune's moons have prograde orbits. All retrograde satellites experience tidal deceleration to some degree. The only satellite in the Solar System for which this effect is non-negligible is Neptune's moon Triton.
All the other retrograde satellites are on distant orbits and tidal forces between them and the planet are negligible. Within the Hill sphere , the region of stability for retrograde orbits at a large distance from the primary is larger than that for prograde orbits.
This has been suggested as an explanation for the preponderance of retrograde moons around Jupiter. With the exception of Hyperion all the known regular planetary natural satellites in the Solar System are tidally locked to their host planet, so they have zero rotation relative to their host planet, but have the same type of rotation relative to the Sun as their host planet, because they have prograde orbits around their host planet.
That is to say, they all have prograde rotation relative to the Sun except those of Uranus. If there is a collision, material could be ejected in any direction and coalesce into either prograde or retrograde moons, which may be the case for the moons of dwarf planet Haumea , although Haumea's rotation direction is not known. Asteroids usually have a prograde orbit around the Sun.
Only a few dozen asteroids in retrograde orbits are known. Some asteroids with retrograde orbits may be burnt-out comets,  but some may acquire their retrograde orbit due to gravitational interactions with Jupiter. Due to their small size and their large distance from Earth it is difficult to telescopically analyse the rotation of most asteroids.
As of , data is available for less than asteroids and the different methods of determining the orientation of poles often result in large discrepancies. Most known objects that are in orbital resonance are orbiting in the same direction as the objects they are in resonance with, however a few retrograde asteroids have been found in resonance with Jupiter and Saturn.
Comets from the Oort cloud are much more likely than asteroids to be retrograde. Most Kuiper belt objects have prograde orbits around the Sun. The first Kuiper belt object discovered to have a retrograde orbit was KV Meteoroids in a retrograde orbit around the Sun hit the Earth with a faster relative speed than prograde meteoroids and tend to burn up in the atmosphere and are more likely to hit the side of the Earth facing away from the Sun i.
Most meteoroids are prograde. The Sun's motion about the centre of mass of the Solar System is complicated by perturbations from the planets. Every few hundred years this motion switches between prograde and retrograde. Stars and planetary systems tend to be born in star clusters rather than forming in isolation.
Protoplanetary disks can collide with or steal material from molecular clouds within the cluster and this can lead to disks and their resulting planets having inclined or retrograde orbits around their stars. The accretion disk of the protostar IRAS has parts rotating in opposite directions. This is the first known example of a counterrotating accretion disk.
If this system forms planets, the inner planets will likely orbit in the opposite direction to the outer planets. WASPb was the first exoplanet that was discovered to be orbiting its star opposite to the direction the star is rotating. In one study more than half of all the known hot Jupiters had orbits that were misaligned with the rotation axis of their parent stars, with six having backwards orbits. The last few giant impacts during planetary formation tend to be the main determiner of a terrestrial planet 's rotation rate.
During the giant impact stage, the thickness of a protoplanetary disk is far larger than the size of planetary embryos so collisions are equally likely to come from any direction in three dimensions. This results in the axial tilt of accreted planets ranging from 0 to degrees with any direction as likely as any other with both prograde and retrograde spins equally probable.
Therefore, prograde spin with small axial tilt, common for the solar system's terrestrial planets except for Venus, is not common for terrestrial planets in general. The pattern of stars appears fixed in the sky, insofar as human vision is concerned; this is because their massive distances relative to the Earth result in motion imperceptible to the naked eye.
In reality, the stars orbit the center of our galaxy. Stars with an orbit retrograde relative to a disk galaxy 's general rotation are more likely to be found in the galactic halo than in the galactic disk. The Milky Way 's outer halo has many globular clusters with a retrograde orbit  and with a retrograde or zero rotation. Several studies claimed to find a halo consisting of two distinct components. However, these findings have been challenged by other studies,   arguing against such a duality.
These studies demonstrate that the observational data can be explained without a duality, when employing an improved statistical analysis and accounting for measurement uncertainties. The nearby Kapteyn's Star is thought to have ended up with its high-velocity retrograde orbit around the galaxy as a result of being ripped from a dwarf galaxy that merged with the Milky Way. Close-flybys and mergers of galaxies within galaxy clusters can pull material out of galaxies and create small satellite galaxies in either prograde or retrograde orbits around larger galaxies.
A galaxy called Complex H, which was orbiting the Milky Way in a retrograde direction relative to the Milky Way's rotation, is colliding with the Milky Way. NGC is an example of a galaxy that has a bulge that is rotating in the opposite direction to the rest of the disk, probably as a result of infalling material.
The center of a spiral galaxy contains at least one supermassive black hole. Scientists have produced a theoretical framework for the formation and evolution of retrograde black holes based on the gap between the inner edge of an accretion disk and the black hole.
And where are you? Pamela Gay: I am in Lisbon, Portugal. Fraser Cain: And when do you leave there? So this is an epic 35 day trip of collaboration, communication and science education. Fraser Cain: Go space.
Pamela Gay: Go space. Fraser Cain: But if people wanna hang out with you, where is the next big opportunity? Dragon Con? Fraser Cain: Cool. Pamela Gay: I think you and I call that the back yard. Fraser Cain: The backyard. Well, for me, it is. Have you ever seen those? Pamela Gay: Yeah. Fraser Cain: You can find out sort of what area you live in, and so if you live in LA, you live in the red area and then orang and yellow and green. So just look for events.
Search for perseids and you should find that event and then just click yes and then you get a reminder in your calendar and start organizing your friends to go on a trip.
Just one night. Pamela Gay: There will be astrophotography involved on my end. Fraser Cain: Yeah. Yeah, me too. Actually, even video. They craft beautiful applications that are durable and reliable. For more information, visit them online at www. Drop them a note. Software is their craft. Fraser Cain: So the Earth is wobbling on its axis like a top. Thank you very much precession. So if I want to imagine precession in my brain, what am I looking at?
Pamela Gay: What you are looking at is the uneven torque on the Earth due to gravity. You have that same slight tilt. We call it the tilt of the planet, the axial tilt. Fraser Cain: Okay. Fraser Cain: Right. Pamela Gay: The side. Fraser Cain: Does the moon have any impact on the precession? So yeah. Those two forces are both out there pulling away on the planet Earth. If we could sort of speed up the process, what would we see the Earth doing as we were looking down at it?
Fraser Cain: Oh, okay. Fraser Cain: How much is 50 arc seconds a year? Pamela Gay: Well, it adds up to one full degree of motion every 72 years so it adds up fairly quickly. That means that in one normal human lifetime, all of the stars move twice the diameter of the moon across the sky. Fraser Cain: Like permanently? Like not permanently but I mean — Pamela Gay: For 25, years. Fraser Cain: Yeah, exactly. Because this has a real impact on the seasons. Fraser Cain: Not a lot but a little bit. So you talked about the stars and the stars are moving a couple of degrees over a human lifetime.
Pamela Gay: Well, a degree-ish over the course. Fraser Cain: A degree. Fraser Cain: What about the seasons? What about the days of the year? You figure on degree every 72 years. And it does not have to concern satellites. The fact is: it is a phenomena. So, with that, how is it exploited if at all in engineering design? Or is it just simply an observation that has no consequence in the real world.
Or this: An American football will enter direct precession. A Frisbee will enter retrograde precession. Knowing this, does it alter how it is thrown? Last edited: Oct 16, Related Classical Physics News on Phys. Filip Larsen Gold Member.
Spin around the symmetry axis of such elongated semi-rigid bodies is unstable see for instance the loss of Explorer 1 and will, due to dissipation of rotational energy, eventually end with spin around the inertial long axis. Spin of oblate bodies around the symmetry axis is stable.
This fact is rather important in satellite design and operation for maintaining rotational control. Filip Larsen said:. JTC said:. OK, I see that. But now may I ask this: WHY? I would think it is easiest to understand the reason for spin axis stability by regarding the rotational energy as previously mentioned.
Keeping rotational momentum constant it is fairly straight forward to find an expression for rotational energy for spin around the minor and major principal inertial axis of a rigid body and with some hand waving argue that rotation with maximum energy is unstable and minimum energy is stable since a free body can dissipate rotational energy but its momentum is fixed.
Understanding the exact mechanics behind this require more details of course, but it is included in many textbooks on mechanics like Goldstein's "Classical Mechanics". I doubt that it will make particular good sense to try understand rotational stability as a consequence of whether precession is prograde or retrograde.
Perhaps others here can provide some insight whether there is any useful relationship between precession and stability other that they correlate due to a common mechanical cause. Last edited: Oct 21, Want to reply to this thread?