Talk:Phoebe (moon)

Semi-major axis

Am I reading that the diameter is 12955759 km (obviously not correct), or is there supposed to be some formatting that I'm missing? — Preceding unsigned comment added by Orokusaki (talk • contribs) 20:27, 9 February 2015 (UTC)

That's the orbital distance from Saturn. Specifically, the longest axis of the orbit divided by two, or semi-major axis. --Patteroast (talk) 01:22, 10 February 2015 (UTC)

Density

The density of 0.7 given in the article, also given at Solarviews, seems to be incompatible with being a captured carbonaceous asteroids. I'll take the (estimated) value of 2.3 given at NASA's Solar System Dynamics. -- Looxix 22:41 May 6, 2003 (UTC)

The first sentence "outermost known moon of Saturn" is inconsistent with the list of Saturn's moons.

Editted first paragraph to reflect new moons. --Patteroast 23:23, 17 Apr 2004 (UTC)

pronunciation

The moon itself of course is fee'-bee. The adjectival form for Phoebus is Phoebean fee-bee'-un per the OED; the change of gender shouldn't affect the adj. form, so I'll use Phoebean here as well. kwami 2005 June 30 04:45 (UTC)

Obliquity?

The figure quoted from http://exp.arc.nasa.gov/downloads/celestia/data/solarsys.ssc is extremely doubtful. That source dates back to April 2003, and Cassini got us a close look at Phoebe only in 2004. We need a recent source that actually gives the celestial coordinates of the bloody rotation pole. Urhixidur 00:38, 23 March 2006 (UTC)

One good source is http://www.hnsky.org/iau-iag.htm, but that is also suspect because of its date (2000-2001). The pole is stated to lie at right ascension 355.00, declination 68.70 (epoch J2000).

Ah, here it is: http://www.sciencemag.org/cgi/content/full/sci;307/5713/1237 (Cassini Imaging Science: Initial Results on Phoebe and Iapetus, Porco et al., Science, 25 February 2005: 1237-1242 DOI: 10.1126/science.1107981) --using Google's access, we read "We derived a spin-pole orientation of right ascension = 356.6°, declination = 77.9°" (a good match to the previous source, eh?). Urhixidur 01:00, 23 March 2006 (UTC)

To obtain the obliquity, what we need is Phoebe's orbital pole. http://aanda.u-strasbg.fr:2002/articles/aas/full/1998/04/ds5713/node5.html states that the J2000 Laplace plane at Phoebe's range from Saturn has its pole at R.A. 275.631°, dec. 68.031° (tilted 26.183° to Saturn's equator). The orbit of Phoebe is then given with respect to that plane, as longitude of the ascending node 233.037°, inclination 174.751°. This ascending node longitude is measured from the node of the reference (Laplace) plane on the J2000 Earth equator, not the usual ecliptic reference plane. We now have all we need to figure the orbital pole, and hence Phoebe's obliquity.

The ascending node of the Phoebean Laplace plane is easily located from the latter's pole: it lies at the pole's R.A. + 90° = 5.631°, dec. zero, with inclination 90° minus the pole's dec. = 21.969°.

Spherical triangle; the angles α, β and γ are called A, B, and C in the text

The spherical triangle we now need to solve has for sides:

• a, the segment of Earth's equator running from the Laplace plane ascending node to Phoebe's orbital ascending node;
• b, the segment of Phoebe's orbit running up from Earth's equator to the Laplace plane, and
• c, the segment of the Laplace plane running from Earth's equator to Phoebe's orbit. The length of c was given above as 233.037°.
• The angle A (opposite a) was also given above = 174.751°.
• The angle B (opposite b) was obtained earlier = 21.969°.
• The angle C (opposite c) will be equal to 180° minus the inclination of Phoebe's orbit.

Adding a to the Laplace plane's ascending node longitude (5.631°) will give the ascending node longitude of Phoebe's orbit. From those two values we can then work out the Phoebe orbital pole coordinates (R.A. = long. - 90°, dec. = 90° - incl.).

So we know A, B, c and need a and C. One identity is: Cos(C) = -Cos(A)Cos(B)+Sin(A)Sin(B)Cos(c) (hence C = 25.456°) and we then need only apply the law of sines to obtain a: Sin(a)/Sin(A) = Sin(c)/Sin(C), hence a = -9.792°.

Thus the (equatorial) longitude of the ascending node of Phoebe's orbit is -4.161°, and its inclination is 154.544°.

Thus Phoebe's orbital pole lies at R.A. 265.839°, dec. -64.544°. All that remains is to find the angle between that point and the rotation pole, at R.A. 356.6°, dec. 77.9°.

This time, our triangle is lacking just a third point, and we choose the Earth's north pole for this purpose (R.A. whatever, dec. 90°). Our new triangle is thus:

• a, the segment running between the Phoebean rotational and orbital poles;
• b, the celestial meridian running from the north pole to the Phoebean orbital pole; and
• c, the celestial meridian running from the north pole to the Phoebean rotation pole.
• The angle A (opposite a) is obviously given by the difference in R.A. between the two Phoebean poles = 90.761°.
• The angles B (opposite b) and C (opposite c) we do not care about.

b is the codeclination of the orbital pole = 154.544°. c is the codeclination of the rotation pole = 12.1°. The relation we use this time is Cos(a) = Cos(b)Cos(c) + Sin(b)Sin(c)Cos(A). Hence Phoebe's obliquity a = 152.14°.

Q.E.D.

Urhixidur 13:05, 23 March 2006 (UTC)

fr:Discuter:Phœbé (lune)

The close flyby was no coincidence

In the introductory section it says "Cassini's trajectory to Saturn and time of arrival were specifically chosen to permit this [close] flyby." But in the section titled Spacecraft flybys, it says "By a stroke of pure luck, Phoebe happened to be in the best part of its orbit to be photographed by the incoming Cassini probe." Obviously the second statement about a "stroke of pure luck" is incorrect and should be changed. —Preceding unsigned comment added by 208.73.31.50 (talk) 19:02, 9 May 2009 (UTC)

Split section to Phoebe ring.

Split section to Phoebe ring.--Daniel L. Barth (talk) 00:05, 8 October 2009 (UTC)

I agree.--Aaron Anaya (talk) 15:40, 7 October 2009

There is already an article called Rings of Saturn. Charvest (talk) 11:45, 8 October 2009 (UTC)

Spoken Wikipedia recording

I've just uploaded an audio recording of the article. Please let me know if I've mispronounced anything. :-) --Mangst (talk) 19:51, 10 January 2010 (UTC)

Retrograde orbit?

"Since the ring's particles are presumed to have originated from micrometeoroid impacts on Phoebe, they should share its retrograde orbit" <-from the Phoebe Ring section.

I'm just a little confused. If it does have a retrograde orbit, shouldn't that be included under the orbital characteristics section as well? —Preceding unsigned comment added by 72.11.83.130 (talk) 19:22, 4 March 2011 (UTC)

Dwarf planet

What source ever claimed Phoebe is a captured dwarf planet? The July version of this article called it a planetesimal. Oh I see: DN-boards1 says it was... -- Kheider (talk) 21:34, 3 October 2015 (UTC)

Your understandable sarcasm given DN-boards1's history aside, it's in the body of the article. It was 'spherical and hot early in its history' and it 'started with a nearly spherical shape, rather than being an irregular shape later smoothed into a sphere by impacts'. "Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet." There is even "Phoebe likely stayed warm for tens of millions of years before freezing up. The study suggests the heat also would have enabled the moon to host liquid water at one time." Liquid water at some point!--JorisvS (talk) 22:39, 3 October 2015 (UTC)

The source calls it a planet-like planetesimal that stalled out. They mention it was spherical, but I do not see them claiming HE. Many asteroids are partially differentiated. Seems iffy to call it a dwarf planet and not a planetesimal. -- Kheider (talk) 23:15, 3 October 2015 (UTC)

You're being even more dense than I was over Pallas. It was spherical and collapsed into a sphere, etc. It was once a DP. DN-boards1 (talk) 23:20, 3 October 2015 (UTC)

Your source does not specifically state that it was a dwarf planet and thus this borders on wp:synth. -- Kheider (talk) 23:25, 3 October 2015 (UTC)

What else could it have made a sphere in the first place? --JorisvS (talk) 00:25, 4 October 2015 (UTC)

The synthesis being made here is that if an object was nearly spherical and partially differentiated that it had achieved HE. The 2012 source does not mention HE or dwarf planets. It is hard to know exactly what Phoebes was like 4 billion years ago before impacts damaged it. -- Kheider (talk) 00:35, 4 October 2015 (UTC)

Kheider is raising a legitimate point. If we have a reliable source, then we can summarize what is in that reliable source. Otherwise we don't write it. Isambard Kingdom (talk) 00:40, 4 October 2015 (UTC)

Let's use common sense. An object is in a sphere. Therefore, it is in HE (or close to it, like Saturn's major moons). If a ball is round, it is in HE. There is no other way that an object could enter HE. DN-boards1 (talk) 00:50, 4 October 2015 (UTC)

When is something round? When is it only close to round? And are all round things in HE? Please provide sources. Thank you. Isambard Kingdom (talk) 00:53, 4 October 2015 (UTC)

(edit conflict)Well, it's not that simple. Large Iapetus is round, but it deviates rather significantly from HE. That said, AFAIK anything round needs to have been in HE at least at some point in its history. --JorisvS (talk) 00:59, 4 October 2015 (UTC)

And something close to round was maybe close to HE? A slippery slope in making such assessments, which is why we rely on sources. Isambard Kingdom (talk) 01:02, 4 October 2015 (UTC)

Then explain the logic, how could something like Phoebe have been round without ever having been in HE? We know that Phoebe was not an irregular object that just happened to be battered so that it became close to being round. --JorisvS (talk) 01:06, 4 October 2015 (UTC)

This is not about black and white logic, is it? Phoebe is only sort of round. And, we don't make inferences. We rely on sources. Isambard Kingdom (talk) 01:10, 4 October 2015 (UTC)

And for all I know, random collisions with smaller objects might result in a moon that is sort of roundish. But I'm not the one trying to make an assessment. Isambard Kingdom (talk) 01:16, 4 October 2015 (UTC)

The source does not even claim it has a rocky core. It just claims, "rock-rich material near the center". Looks like a failed/stalled dp to me. -- Kheider (talk) 01:19, 4 October 2015 (UTC)

@Isambard Kingdom: You're avoiding the question. How could it? [1]: 'The nearly oblate spheroid shape of Phoebe may retain characteristics of an early, relaxed object.'. [2]: 'Phoebe was spherical and hot early in its history'. And as I already said, we know Phoebe's roundedness is not the result of the impacts, see [3].

What I'm doing is suggesting that you stop making inferences. You might think you can do it for this chunk of a moon, but then the next one? You need to stop the OR. Isambard Kingdom (talk) 01:38, 4 October 2015 (UTC)

This isn't OR, this is common sense. There's a difference between OR and common sense. Something round is going to have at one point been in HE, that's how physics works. DN-boards1 (talk) 01:40, 4 October 2015 (UTC)

And something sort-of round was sort-of in HE? Is that the common sense you are advocating? Are you going to apply this common sense to lots and lots of other sort-of round objects? Isambard Kingdom (talk) 01:48, 4 October 2015 (UTC)

Sort-of-round? What the hell does that mean? It's either round or it's not. DN-boards1 (talk) 01:50, 4 October 2015 (UTC)

There is no such thing as a perfectly round planet. So where do you draw the line on nearly round? This is why we require sources. -- Kheider (talk) 01:56, 4 October 2015 (UTC)

This is the talk page, not the article itself. I'd like an answer how you think something could be round yet never have been in HE. Then we can properly discuss the merits of making a (legalistic?) distinction between them in the article. --JorisvS (talk) 01:43, 4 October 2015 (UTC)

I don't know the answer to your question. I'm advocating that you rely on sources. Isambard Kingdom (talk) 01:51, 4 October 2015 (UTC)

YOU have to provide a source stating that "sort-of-round" is even a thing (it's not). DN-boards1 (talk) 01:56, 4 October 2015 (UTC)

I do, I just also think about what those sources are saying so that we don't blindly parrot every idiosyncratic wording of a source, but phrase the things they are saying in a way that makes it more understandable (which is not OR). I don't see how you could have a round object that was never in HE. I don't know everything, so I'm open to hearing about other ways. This all said, in the article saying 'round' is preferable to saying the jargon wording 'in hydrostatic equilibrium', which is already exactly the wording of that one source anyway. --JorisvS (talk) 02:05, 4 October 2015 (UTC)

JorisvS, I respect your work here at Wiki. I'm just concerned about the broader project being undertaken by DN to try to catagorize planetoids and other bags of rocks on the basis of his assessment of "roundness" and other grey-shades. Slippery slope. Okay, way past my bedtime ....Isambard Kingdom (talk) 02:18, 4 October 2015 (UTC)

Isambard Kingdom, I'm a she. DN-boards1 (talk) 02:27, 4 October 2015 (UTC)

@Isambard Kingdom:, DN's project of strange OR classifications and other assessments like we've seen, so vastly different from the sources we have, has nothing to do with my position here. I will revert himher every such time. Way past my bedtime, too. --JorisvS (talk) 02:48, 4 October 2015 (UTC)

@Kheider: Differentiation is not a requirement for DPness. Callisto doesn't have much differentiation. --JorisvS (talk) 01:34, 4 October 2015 (UTC)

We also know that Callisto formed in the outer part of Jupiter's accretion disc, not in Solar orbit. -- Kheider (talk) 01:46, 4 October 2015 (UTC)

So? --JorisvS (talk) 01:47, 4 October 2015 (UTC)

Callisto is the outermost major moon. Between Jupiter, Io, Europa, and Ganymede I doubt there was much rocky material left for Callisto to form a notably dense core. -- Kheider (talk) 04:09, 4 October 2015 (UTC)

"Callisto is composed of approximately equal amounts of rock and ices." Callisto's density is 1.83 g/cm³, not much lower than Ganymede's and rather similar to Pluto's. Enceladus is partially differentiated at 1.61 g/cm³, and which is likely as high as it is through loss of icy material. It's easily sufficient for differentiation, it somehow just did not differentiate much. --JorisvS (talk) 10:32, 4 October 2015 (UTC)

In Jupiter's accretion disc, being further from Jupiter there would be less orbital energy during impacts, thus there would be less heat and less differentiation. -- Kheider (talk) 12:32, 4 October 2015 (UTC)

And formed more slowly, so heat from radioactive decay did not accumulate much. But the more reason that this has not bearing on DPness. --JorisvS (talk) 16:01, 4 October 2015 (UTC)

So the bottom line is we go by our sources. The 2012 source calls Phoebe a planetesimal, not a "planet, dwarf planet, or protoplanet". -- Kheider (talk) 18:25, 4 October 2015 (UTC)

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Where is/are the original photographic plate(s) used to discovered Phoebe in ?

Can anyone find the very first photographic plate(s) that Pickering used to discover Phoebe (1898)?

I cannot seem in find them in the JPL or NASA archives; while Ymir and many other irregular moons of Saturn have their discovery plates. IapetusCallistus (talk) 19:19, 1 February 2024 (UTC)