Talk:Ceres (dwarf planet)

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Revisiting Flattening[edit]

Since the definition of Dwarf Planet includes hydrostatic equilibrium (and thus an ellipsoid shape), I believe we should add flattening to Ceres's statistics. The last time I brought this up I was asked to show how I arrived at my numbers, step by step, so I shall do so here. I am not a math whiz, so please, feel free to correct me if you see a mistake! (Heck, I probably slaughtered the problem, but hey, gotta learn somehow, right?)

I am using the flattening formula, f = (a-b)/a. Ceres's polar and equatorial radii are listen in its article as: Equatorial radius 487.3±1.8 km Polar radius 454.7±1.6 km.

Using the values: a = 487.3 km and b = 454.7 km, I arrive at f = 0.066899.

I based the uncertainty in the flattening on the uncertainty in the radii, so I calculated the full range of possible flattening by adding and subtracting the max and min radii uncertainties and plugging the new values in to the flattening formula: a = 487.3 + 1.8 = 489.1 and b = 454.7 - 1.6 = 453.1, and thus f(upper value) = 0.073605 and a = 487.3 - 1.8 = 485.5 and b = 454.7 + 1.6 = 456.3, and thus f(lower value) = 0.060144

Then I found the mean value for f from the upper and lower values above (I'm assuming the uncertainty should balance): f = 0.073605 + 0.060144 / 2 = 0.0668745

Then I check to make sure of my balance and get my uncertainty number: 0.073605 - 0.0668745 = 0.0067305 (uncertainty +) and 0.0668745 - 0.060144 = 0.0067305 (uncertainty -)

Therefore I would list flattening for Ceres as 0.0668745 ± 0.0067305

I don't remember if that's exactly what I came up with last time, but my old number was removed (rightly so) pending review and arrival at the correct number. Am I at least boarded on the right train here? --Turboguppy (talk) 23:23, 29 April 2014 (UTC)

Much easier if you calculate the errors directly, once you know the way to do it. Check out here[1] and here.[2] (I'm not claiming those are great sources, but they're the first things that popped up when I searched, and they seemed okay.)
Basically, if you add a and b, you add the absolute errors. When you multiply a and b, you add the relative (%) errors. For a to the b power, you multiply the errors, etc. That's for things like calibration errors, not statistical errors. When dealing with standard deviations like we have here, you want to add the squares of the errors and then take the square root, like finding the hypotenuse of a triangle. Anyway, once you get the hang of it, and can derive a formula for the error for any given formula, you don't have to crank it out by hand and worry if you did it right.
This one[3] is a bit more rigorous, which might make it easier to use, depending on how you process this stuff. — kwami (talk) 01:17, 30 April 2014 (UTC)
Note that flattening alone is not enough for determining hydrostatic equilibrium, since it also depends upon spin, material composition, and internal differentiation models. E.g., see Saturn. So I don't think it's very relevant; however, the planets' articles seem to have it, so why not Ceres?
Anyway, I think the number should be f = 0.066899 +(0.073605-0.066899) - (0.066899-0.060144), which is 0.066899 +0.006706 -0.006755. (assumuming you did the calcs correct above) I.e., nominal +(max-nominal) -(nominal-min). But I don't know how these things are done, either, so check kwami's refs. And if you do some reasonable significant digits, it's just 0.067 ± 0.007 whichever way you do it.
Note that we'll have accurate info in a year or so, and the error bars should be small enough to ignore, so I think this is mostly an educational exercise, and hopefully fun. Tbayboy (talk) 04:05, 30 April 2014 (UTC)
If it depends on composition, then the object is not in HE and so not a DP. Since icy moons 50% larger than Ceres have turned out to not be in HE after all, I do wonder about Ceres. It might be difficult to tell, since its spin might not have changed much over its history.
BTW, the flattening comes out as ±7.4%, or 0.067±0.005. — kwami (talk) 07:01, 30 April 2014 (UTC)
The composition thing is true of any body. It's not so much the raw material as it is the distribution of different materials (the differentiation), which change the gravity slope. Heavy-core with light-mantle gives a different flattening than an evenly dense body, and the relative densities of the layers affects the degree of flattening (assuming a spin, of course). That's why it was so hard to figure out Iapetus for certain, since the error bars for some internal models went within HE until more accurate measurements pushed them all outside. Tbayboy (talk) 12:39, 30 April 2014 (UTC)
You're right, of course. — kwami (talk) 21:04, 30 April 2014 (UTC)
I am not sure that you can calculate the errors for flattening in such a simple way—by treating polar and equatorial radii as two statistically independent variables. They are not because they are model dependent and were obtained by fitting some shape model to the available observations. Calculating errors in this way would be original research. Unless some reliable sources are found, flatting can be only claimed to be around 7% but not more than that. Ruslik_Zero 13:19, 1 May 2014 (UTC)
Well, that throws a wrench in things. I doubt merely rounding would help. The axes are both given to ±0.36%, but if the flattening comes from the model, we can't infer precision from them at all. Wouldn't we need to say "0.067 (assumed by model)" or something? — kwami (talk) 17:19, 1 May 2014 (UTC)
How about forget the ± and use the approx: "~0.07"? Tbayboy (talk) 12:40, 2 May 2014 (UTC)
That might mislead readers into thinking that it is a measured value, or directly derived from measured values, rather than being generated by the model used to predict the shape of Ceres. If someone uses the flattening and spin rate to gain some insight into the structure of the asteroid, they're only recovering the structure assumed in the model.
I agree that if we had a measured value, that would be useful to include, but this is not. I think we should either note that it's artificial or omit it entirely, like we do unknown albedos. — kwami (talk) 16:48, 2 May 2014 (UTC)
On further reflection and after reading all the great posts here, I think I agree most with just leaving it off until we have an actual measurement, to avoid confusion. I'm grateful to everyone who helped me understand the problem better. Thank you!Turboguppy (talk) 06:32, 4 May 2014 (UTC)

Both an asteroid and a [dwarf] planet?[edit]

The introduction states that Ceres is both an asteroid and a dwarf planet. I thought it would be classified as either, not both. Is that correct or the intro needs updating? Cheers, BatteryIncluded (talk) 23:33, 25 January 2014 (UTC)

It's both, but it's not a planet.
The two classifications have nothing to do with each other, and 'asteroid' has no formal definition. — kwami (talk) 23:44, 25 January 2014 (UTC)
Thank you for the clarification, but this report from NASA defines the difference between an asteroid and a dwarf planet:
Plumes of water vapor are thought to shoot up periodically from Ceres when portions of its icy surface warm slightly. Ceres is classified as a dwarf planet, a solar system body bigger than an asteroid and smaller than a planet.
I'll just leave it on your desk. I am a biologist so I leave it to more competent editors. -BatteryIncluded (talk) 01:23, 26 January 2014 (UTC)
Actually, no. That claim is simply wrong: A dp is not smaller than a planet, for example. (I assume the authors know better but are dumbing it down so they don't have to explain what a dwarf planet is.) We've discussed this before, and have several sources, including some from NASA/JPL, which call Ceres both a dp and an asteroid. — kwami (talk) 02:10, 26 January 2014 (UTC)
OK. Thank you. BatteryIncluded (talk) 22:52, 26 January 2014 (UTC)

We should repeat the infobox information within the article[edit]

Information presented in the infobox, e.g. the pronunciation etc, should be repeated within the main article text. I usually never read infoboxes, and I know other people don't either. Furthermore, I may copy-paste or print an article without including its infoboxes, or I may zoom the page to 500-600% and make the infobox too big that it goes at the top so that I just scroll down without looking at it. In general, my experience is that if something is in the infobox but not the article text then I almost certainly won't be able to see it. It has happened to me that sometimes I try to find some information and I wikipedize it, I don't get an answer from the main article text so I then google it, but I later discover that the information I wanted was wikipedized into the infobox rather than the article text, so I think that infoboxes should only repeat what's already in the article. Cogiati (talk) 23:13, 28 January 2014 (UTC)

There's a lot of numerical data in astronomical articles that would be silly to put in the main text, as it would be in tables that would simply duplicate the info boxes. If you don't bother to read the entire article, then you can hardly expect to get everything out of it. — kwami (talk) 23:31, 28 January 2014 (UTC)
We should include the pronunciation after the dwarf planet's name in the lead paragraph, what do you think? Cogiati (talk) 23:48, 28 January 2014 (UTC)
We used to have it there (I put it there), but people decided to move all pronunciations to the box. — kwami (talk) 22:37, 11 April 2014 (UTC)

escape velocity[edit]

We had the escape velocity as 0.51 m/s², "calculated based on the known parameters". NASA has 1.855. Either NASA screwed up or we did, and if we did, there may be (probably are) other articles w the same error. — kwami (talk) 22:40, 11 April 2014 (UTC)

If you're talking about this NASA page, it's in km/h, not m/s. Looking at List of gravitationally rounded objects of the Solar System, .51 is about right (comparing it to similarly sized moons). Tbayboy (talk) 23:00, 11 April 2014 (UTC)
For a Ceres-sized object to have an escape velocity of 1.885 km/s, it will need a density of more than 25g/cm3!. Ruslik_Zero 12:57, 1 May 2014 (UTC)

Ceres...On Old Planet reborn?[edit]

this is Interesting...It has been a long theory of mine that the asteroid belt is the remains of a planet that was destroyed by some event.....the fact that the debris remains in a fairly stable orbit points to this being the case.... now With Ceres spinning in the opposite direction from the rest of the asteroid belt this would cause all the debris to eventually gather at one point. thus creating a "new" planet.....Billions of years of course but I believe this is what will eventually happen. — Preceding unsigned comment added by LordCaerdic (talkcontribs) 03:18, 11 June 2014 (UTC)