*

Offline BillO

  • *
  • Posts: 583
  • Huh?
    • View Profile
Which planets can we visibly observe to rotate on an axis ?
« on: December 08, 2019, 03:49:08 PM »
This is a question asked by somerled in a topic that got locked in the Flat Earth Theory forum.

I thought it was a good question and should be addressed and this appears to be the right place to address it.

The easiest planet to see rotating is Jupiter.  Jupiter has a day of less than 10 hours, so on a long winter's night you can easily witness it do a full rotation.  You will need a decent telescope.  Let's make no mistake, your not going to be able to detect it rotating with binoculars or even a modestly priced 4".  However, with a well collimated and quality 6" scope or bigger it should be no issue at all to easily see enough detail (like the GRS) to see it rotate.

Mars is probably the next easiest to see rotating.  It's day is just a tad longer than earth, so you won't see it do a full rotation in one evening of viewing, but you could certainly see it rotate through 180 degrees or more.  And if you observe it every night over about 3 weeks, you can get to see the entire surface.  Again, a decent scope will be needed for this and if you're a skilled observer you could do it with an 8".

The next (and probably last) would be Saturn.  For this one you'll need at least 10" scope to see enough detail to detect rotation.  It's day is just under 11 hours, so it should be easy enough to see the entire surface in one (winter) night.

Although Venus can be fairly close to earth from time to time and easy to see, it appears featureless and detecting it's rotation is beyond what most amateurs can muster.  If you have a big scope (10"+) and a good set of filters, you might be able to see the notion of a bright spot, but that would be seeing the motion of the atmosphere, not the planet.  A day on Venus is about 5,800 hours.  So, even if we could see the surface features, it might be tough for the casual viewer to detect rotation.  The atmosphere, however, circles the planet in under 100 hours.

Mercury is tiny and far away.  It's day is also very long - nearly 1,408 hours.  I have access to an 18" scope and have never been able to even see details on Mercury, so I'm going to say it's not in the realm of reasonably easy to record rotation on Mercury for an amateur.

Uranus and Neptune are quite far off and even quite large high quality scopes have difficulty detecting any features, so again not in the realm of the casual viewer.

That being said, there are earth based instruments that can detect the rotation of all the planets.  It's just that you or me are likely never going to get to play with them.
« Last Edit: December 09, 2019, 12:23:10 AM by BillO »
Here a quack, there a quack, everywhere a quack quack.

Offline somerled

  • *
  • Posts: 319
    • View Profile
Re: Which planets can we visibly observe to rotate on an axis ?
« Reply #1 on: December 11, 2019, 02:42:54 PM »
Thanks for showing interest . Don't really know where else to start so here's what I've found out about Jupiter.

Great red spot (GRS) first seen 1664 by Robert Hooke and 1665 by Giovanni Cassini . Hooke placed the GRS in Jupiter's Northern hemisphere whereas Cassini views it in the Southern .
Note this is in the early days of telescopes with hand ground lenses , primitive by today's standards.
Cassini observed the GRS for nearly 50yrs always roughly in the same location . Astronomers then loose track of the GRS and it is not further observed until 1830 . All from https://www.universetoday.com/129968/big-great-red-spot/

So obviously are we observing just an atmosphere - how do we visibly observe a planet's rotation beneath an atmosphere?

I have a 4" reflector and 6"refractor - both give good views of Jupiter which always appears as a bright disc with two horizontal blueish bands . A friendly serious amateur astronomer with a 14" dobsonian and quality eye pieces has never seen the GRS in 40yrs of viewing the skies. He sees what I see when we observe . A lot depends on atmospheric conditions but if GRS was visible with rudimentary equipment in the 1660s why .

So far I haven't come across any info about how we visibly observe planetary rotation .

*

Offline BillO

  • *
  • Posts: 583
  • Huh?
    • View Profile
Re: Which planets can we visibly observe to rotate on an axis ?
« Reply #2 on: December 11, 2019, 05:35:14 PM »
The "rudimentary equipment" used by top-tier scientists in the 1660's was probably better than an inexpensive 6" amateur scope today.  Considerably better.  Good optics are not too hard to make, just expensive.

As for Jupiter and Saturn (and the other gas giants) they are essentially just atmosphere.  There are some that hypothesize a solid core, but there is no hard evidence of that except for the interpretation of some gravitational measurements of Jupiter winch could indicate a solid core of about 5% of it's mass.  However, it does seem that even these estimates are in flux, changing over time.  More likely is a progressive increase in the density of these planets from 0 through to very highly compressed and condensed gases and liquids similar to a malleable plastic, and everything in between.  So even if there was a solid core, where it began would also be in dispute.  The net net is that Jupiter does not have a solid surface.

So, when we talk about planets that are alt least 95% atmosphere and do not posses a solid surface, then the rotation of the atmosphere IS the rotation of the planet.  That is just the way it is, like it or not.

If you want to see a solid surface rotate, you'll have to get time on a better scope and observe Mars.

BTW, Hooke may not have observed the GRS at all, so don't be too adamant about your assertion.   It's is not always very visible and goes through phases where it might not be visible at all, but it has never been visible in the North.  The general consensus among far more qualified people than you or I is that his observation was of a different object.

In any case, the GRS, despite my original post, is probably not the best choice to see the rotation of Jupiter as it goes against the grain (rotates in the opposite direction), but it will show rotation nonetheless.
Here a quack, there a quack, everywhere a quack quack.

Offline somerled

  • *
  • Posts: 319
    • View Profile
Re: Which planets can we visibly observe to rotate on an axis ?
« Reply #3 on: December 13, 2019, 09:01:59 PM »
Cassini used a telescope built by Guissepe Campani - acknowledged best optical telescope builder of those days . Here is a test of one of his telescopes  .
 http://adsabs.harvard.edu/full/2004ESASP1278..133M

You can see that 11cm aperture scope has a resolution of 4.6 arcsec . Mag of 36x

My Celestron xlt 150 refractor gives a resolution 0.92 arcsec. Useful mag of 360x . Telescopes are better now . Technology improves , apart from rocket engines ( we can't put a man on the moon now) . Wasn't cheap either .

 I made no assertions about any of that history of the GRS , those are the facts stated in its history as taken from historical records and published in a mainstream science magazine .

Taking the movement of planetary atmosphere as a proof of rotation is assumption . Our atmosphere moves but no planetary rotation has been found by experiment

I've already upgraded my telescope eyepieces  - the figures I quoted were for the originals . Maybe  I'm just unlucky with viewing conditions . I will persevere but the original question still stands.


*

Offline BillO

  • *
  • Posts: 583
  • Huh?
    • View Profile
Re: Which planets can we visibly observe to rotate on an axis ?
« Reply #4 on: December 13, 2019, 10:25:14 PM »
I made no assertions about any of that history of the GRS , those are the facts stated in its history as taken from historical records and published in a mainstream science magazine .
Really?  Well, my guess is the author was a dolt and the magazine editors didn't bother with the whole fact checking thing.  The sole fact is that Hooke observed something on Jupiter.  That's it.  Whether it was the GRS we have come to know today, or not, can only be speculation.  The consensus of real astronomers is that it was not the GRS.  Just out of interest, who was the author and what was the magazine?

Edit:  I found the link to your GRS article.  To quote the author:

The first person to mention a spot on Jupiter was Robert Hooke, who described it in 1664, but he placed it in the northern hemisphere.

So, no.  Not even in that article is Hooke's discovery called the GRS.  Neither does the author call Cassini's spot the GRS.  Now whenever you call something a fact, I'll know to take it with a grain (or 10) of salt.  So the author was not necessarily a dolt and our credibility wains.


Yeah, telescopes have gotten better, but not the ones you are likely to have.  A $500 cheapie is not a great scope.  Quoting the diffraction limited resolving power of the aperture of your scope is a far cry from what you will actually see.  I'm just a once in a while thoroughly amateur astronomer but I own eyepieces that cost more than $500.  BTW, when I read that article about Campani it states that the 11.1cm scope had a resolving power of 1.2arcsec (expected) and a magnification of up to 223. 

When a planet is only atmosphere, then yeah, the rotation of the planet is the rotation of the atmosphere. ::)

At the risk of repeating myself, if you don't like the idea of gas planets, then go look at Mars.

And yes, the rotation of the earth has been both observed and measured from space.  In addition there is ample evidence from ground based observation which can only be explained by a rotating planet (and certainly cannot be explained by anything in FE).

Edit: Oh, this:
Hooke placed the GRS in Jupiter's Northern hemisphere whereas Cassini views it in the Southern .
Is wrong too.  Read that article again.   It clearly states Cassini's observation was in the northern hemisphere as well, and it's worth repeating, neither are called the GRS.

(space comma)
« Last Edit: December 14, 2019, 12:39:55 AM by BillO »
Here a quack, there a quack, everywhere a quack quack.