Question arising from the clockwise sphere thread.
How did astronomers detect Uranus’s tilted rotation? Isn’t it pretty featureless from the top of the cloud cover? Does it have an equivalent of Jupiter’s Great Red Spot?
The rotation angle and rate were estimated by spectroscopic and photometric methods from terrestrial observatories but was not known definitively until the flyby of the Voyager 2 spacecraft where it was able to use its onboard instrument suite to directly measure the magnetic field and to observe the delicate ring system.
https://britastro.org/2017/a-short-historical-account-and-guide-to-regular-observation-of-uranus
That link says it was obvious by 1847 that it had an axial tilt of 98 degrees from observation of the satellites.
Satellites tend to migrate to low-inclination orbits due to forces from planetary oblateness. The effect is less powerful for planets close to the Sun (which has pronounced tidal forces on our moon), but it’s probably negligible for a planet as far away as Uranus.
I do wonder how much of this was understood back then, or if it was mostly observational. Earth and Mars have low-inclination satellites; so are the large inner satellites of Saturn and Jupiter. So maybe the conclusion was empirical in basis.
I think perturbation theory was known well enough by 1847 that it was clear why satellite orbits were low-inclination (the Moon’s orbit was very well studied by Laplace and others). I wonder when the shape of Uranus (its oblateness) could be observed - that would also be a clue to the orientation of the planet, but observing the oblateness depends on where Uranus is in its orbit
Interesting responses, thanks.
Follow-up question: what is a “low-inclination orbit” and how did it assist in the analysis ?
If you take the line an orbit qould trace around a planet, how closely does it line up to the equator?
A 0 inclination orbit goes above the equator, rotating around the planet on the same exact plane that the planet rotates around itself on.
A highly inclined orbit would be something like a polar satellite, that moves around the planet by passing over the poles.
Natural satellites tend to have low inclination orbits (because of certain facts about the formation and behavior of planets and other celestial bodies), so when it was found that Uranus’ satellites are tilted with respect to the solar plane, that was evidence that its rotation is as well.
Thank you, that’s helpful.
So if there’s a satellite around Jupiter or Saturn with a highly inclined orbit, is that an indicator that it was likely a captured satellite, rather than one that formed as part of Jupiter’s or Saturn’s formation?
That’s right.
Hyrokkin is a retrograde moon of Saturn, and is probably a captured asteroid. Triton is a large moon of Neptune, and is probably a captured Kuiper Belt Object (KBO).
Both have highly-inclined or retrograde orbits.
That’s a fairly likely explanation, but not the only one. It could have been smacked into a weird orbit by a collision, or it could have been pushed by a large neighbor.
I haven’t done the math, but I’m pretty sure the amount of energy required to switch a moon from a prograde orbit to a retrograde orbit would exceed the gravitational binding force of the body. In other words, hit a moon with enough energy to reverse its orbit and you won’t end up with a moon in a retrograde orbit, but with a ring system where most of the particles are still in a prograde orbit, a fair bit of the moon’s mass hit escape velocity and is no longer in any orbit, and virtually nothing in a retrograde orbit.
That’s a reasonably good guess except a large portion of the residual mass would probably fall into a lower orbit or even into the planet. The circumstances where a moon could go from a pronounced (not near-polar) prorgrade to retrograde orbit would be so unlikely (and would require repeated passes to progressively change inclination; a single impulse would almost certainly shatter both the moon and the impacting object into molten fragments) as to be essentially impossible. The reality is that virtually all of the “non-icy” moons of the outer planets are probably captured objects or residue of fragments from a collision between a native moon and a wandering object. Jupiter’s moon Io, for instance, is quite bizarre in its composition (which is nothing like any of the other Galilean moons) and was almost certainly a dwarf planet that started out somewhere within the current orbit of Mars and migrated outward. How it came to be captured into such a nearly perfect orbit (eccentricity of about 0.4%) is still a puzzler for planetary scientists and probably had something to do with resonances between Europa and Ganymede.
Stranger
I was thinking of retrograde rotation there, not retrograde orbit. But yes, good point.