Talk:Absolute magnitude

Reorgantzaiion of paragraphs "computation" and "apparent magnitude"

The two paragraphs are redundant which makes unnecessary confusion. Now they are combined and reorganised, which should be more concise and clear. — Preceding unsigned comment added by Kiwakwok (talk • contribs) 08:04, 9 June 2016 (UTC)

brightness vs. luminosity

It would be helpful for the article to distinguish more clearly between brightness and luminosity. Astronomers generally use "brightness" to mean "apparent brightness", while "luminosity" refers to the intrinsic power emitted by an object. Absolute magnitude is a measure of a star's luminosity (usually in a specific filter band). A statement such as "The brighter the celestial object, the smaller its absolute magnitude" doesn't convey this distinction clearly. The term "intrinsic brightness" could be confusing in this context too, and should probably be replaced with "luminosity". Aldebarium (talk) 17:56, 5 January 2017 (UTC)

Comparison between planetary magnitude and stellar magnitude was just plain wrong: I blanked

I blanked the comparison between planetary mag and stellar mag, which said they differ by a constant. They can't be compared in this way, and the provided links did not support the idea. Absolute stellar magnitude sets only the distance of an emitter at 1 parsec. Abs planetary magnitude sets the distance to the object at 1 AU, AND the distance of the object from the Sun at 1 AU. The difference between the two measures will not be constant. Ordinary Person (talk) 13:24, 1 March 2019 (UTC)

Ideal diffuse reflecting sphere approximation

@Tomruen: has added the citation needed tag to the formula $p(\chi )={\frac {2}{3}}\left(\left(1-{\frac {\chi }{\pi }}\right)\cos {\chi }+{\frac {1}{\pi }}\sin {\chi }\right)$. I went ahead and added a rationale: No source is given for the formula. The only references I can find are [1] and [2]; the latter is a peer-reviewed publication. Nonetheless, both seem to borrow the formula from this Wikipedia article (copying it word for word with the same symbols used here, but without citing a source). Thus, those would be circular references. Renerpho (talk) 02:47, 8 January 2019 (UTC)

This presentation from today, given at the 2019 AAS meeting, is also apparently copying it from Wikipedia. Again, no source. Renerpho (talk) 02:57, 8 January 2019 (UTC)

I've now replaced the "citation needed" by "dubious - discuss" for both formulas in question. I've added the original source written by User:Tomruen in 1991,[3] and added by himself in 2004 in this edit. This is a non peer-reviewed, private publication. I've discussed the issue off-Wikipedia with Tomruen, and he suspects as much as I do that the formulas are either all or partially wrong (or at the very least constitute WP:OR). We both noticed the issue when we saw it being used in this presentation at the AAS meeting yesterday. I don't blame Tomruen, the edits in 2004 were made in good faith and forgotten since then. But the high relevance of the article, and the fact that the formulas were copied into peer reviewed and potentially highly relevant publications, make both of us believe that this should be rectified as soon as possible. Looking at the talk page sections above this, I see that the issue had been raised multiple times over the years. Sadly, it had never been addressed properly. I will go ahead and add a notification to the WikiProject Astronomy and WikiProject Physics talk pages, because we need help to resolve this. This possibly includes a need for someone reviewing the original source, to see if it contains errors. I am sure @Tomruen: will be happy to help. Renerpho (talk) 05:54, 8 January 2019 (UTC)

The phase integral p(χ) was a simple double integral I evaluated in college, but I have seen no other similar derivations. As a model it should be correct, but astronomical bodies are not perfect diffuse reflectors, so it can't be validated as accurate for any individual body. For actual bodies better formulas include the Slope parameter g, and that should be expanded here, mainly for asteroids. [4], [5]. Tom Ruen (talk) 06:23, 8 January 2019 (UTC)

• • Comment The H-G-magnitude-system is a valuable addition to the article, but is intended to model rough-surface shadowing. It is applicable to objects without an atmosphere. The gas giants Jupiter, Saturn etc. are better modelled as diffuse spheres. The general approach used in this article is sound, it's just that the calculations are unsourced. The formulas in their present form are very useful, making it strange that no other source on the topic seems to derive them. They are either correct (and creative), or too good to be true. If they are correct, we can still think about how to solve WP:OR. Renerpho (talk) 12:03, 8 January 2019 (UTC)
    • We solve WP:OR by removing the material until it can be supported by a reliable source. I have done so. This case is clear-cut, particularly given the statement above that the author of the material "suspects...that the formulas are either all or partially wrong".--Srleffler (talk) 01:26, 10 January 2019 (UTC)
      • Thank you! The situation is slightly less dramatic than I first thought, as the calculations turn out to be in essence factually correct. So, while we did had a clear-cut case of WP:OR, at least the information was not wrong. I've started below giving a couple of relevant sources. I don't have time at the moment to start rewriting the article, but I may later. Renerpho (talk) 08:25, 10 January 2019 (UTC)
  • Certainly there are formulas used out there. Maybe everyone just has their private tweaked formulae? Secondly, perhaps H-G magnitude system should be an article in its own right even if limited to asteroids, 422 google matches "H-G+magnitude+system", and 54 on Google Scholar "H-G+magnitude+system". Tom Ruen (talk) 21:26, 8 January 2019 (UTC)
• One formula is correct It took me a while to find a source, but the diffuse sphere formula is indeed correct. It is, for example, given in (Whitmell, 1907), where Whitmell cites (Mascart, 1893). Mascart appears to be the first who found it. This leaves only the formula for the apparent magnitude based on the phase integral unsourced. Renerpho (talk) 07:39, 10 January 2019 (UTC)
  • This means that all the formulas are in essence correct. The formula relating apparent magnitude to the phase integral follows from (7.38) in (Karttunen, 2016). It is not given in the exact form that appears in the article, but the formula in the article could simply be adopted (they mean the same thing). There is some work to be done to the article, to rebuild it in a form that isn't WP:OR, but at least it was never factually wrong. I suggest we use this opportunity to completely rewrite the section, also including the H-G magnitude system. I mentioned the book by Karttunen above because on the pages after it deals with the phase integral, it also presents the background for the H-G system (and other, more advanced, systems that may be relevant for an "alternatives" section). Renerpho (talk) 08:13, 10 January 2019 (UTC)
    • Thanks for finding some sources that can help. I've looked at H-G system, and less impressed, seeing its mainly for asteroids near opposition, phase angles under 20, and invalid for angles above 120. And as limited as the Diffuse reflector model is for accurate predictions for planets, clearly other models are best for atmospheres, and we yet have none publicly known, whatever are used privately. Tom Ruen (talk) 12:47, 10 January 2019 (UTC)
      • phase angles under 20, and invalid for angles above 120 - I think you had wrong expectations from the H-G system. Note that this range covers almost all asteroids that are readily observable. An asteroid with phase angle above 120° will be in twilight or daylight sky, hardly observable from the ground; and almost no main belt asteroid reaches phase angles above 20°. The power of the H-G model is, on the contrary, small phase angles below 2°, where the opposition effect kicks in. It models this very accurately, while a pure H system or a diffuse sphere model does not. Given the irregular shape of asteroids, it is clear that a model with just two parameters is limitted. If you look at the book by Kartunen linked above, you will find an alternative model with 3 parameters (the H-G1-G2 magnitude system) which is designed to model some of the more peculiar aspects of asteroid phase curves. It was however never widely adapted by the community. The problem is that even the 2-parameter H-G model is almost useless in practise because measuring the G parameter is incredibly difficult. Measuring G1 and G2 simultaneously is hardly possible, and so that model is used by virtually nobody. Renerpho (talk) 14:06, 10 January 2019 (UTC)

The reason why the H-G system fails for high phase angles is self-shadowing on the surface, which is very hard to model if you don't know what the surface actually looks like (which in turn is impossible to find out without sending a spacecraft). This is not at all relevant for clear atmospheres. It can be relevant for atmospheres that contain clouds/hazes, which again become very complicated. For high phase angles, atmospheres also tend to scatter light towards the observer, an effect that depends strongly on the chemical composition and temperature distribution of the atmosphere. You will not find a simple model that can handle those effects. To simulate them, you need to do a numerical simulation of the atmosphere itself - which is possible, but probably not what you're looking for. Renerpho (talk) 14:15, 10 January 2019 (UTC)

Unreadable

The current article is completely useless for non astronomers. I beg someone who knows what AM is to write a clear definition with examples. Thanks. — Preceding unsigned comment added by 189.9.0.124 (talk) 03:26, 12 January 2019 (UTC)

Thank you. We are currently working on rewriting the article. Renerpho (talk) 15:08, 12 January 2019 (UTC)

Removed text, for discussion

Here is the text that I removed, for discussion. Perhaps some of it can be sourced and returned to the article in improved form:--Srleffler (talk) 01:29, 10 January 2019 (UTC)

I replaced it with a draft for the section. I went ahead and started adding the commonly used models. Next would be the H-G system for asteroids (@Tomruen: do you want to start that part, under asteroids?), and maybe a section for comets that goes beyond a laconic "can't be done". There's still a lot of work to be done, obviously. Suggestions? Renerpho (talk) 13:30, 11 January 2019 (UTC)

This really looks great! I've never had any hope on comets. I'd like to plot the phase functions by planet to compare. Tom Ruen (talk) 14:13, 11 January 2019 (UTC)

Thanks. Comets are notorious, but rather than just saying that the m2 is a different scale, the article should at least define that scale. I can do that later. I think getting the asteroid section right is more important for now. Renerpho (talk) 14:49, 11 January 2019 (UTC)

I see some of this should be integrated into Phase curve (astronomy), unsure what is best, but they clearly go together. I see there's already some graphs there. Tom Ruen (talk) 15:23, 11 January 2019 (UTC)

Comet section added. I see you pointed out that $\chi$ is sometimes implied to be in degrees, other times in radians. Something needs to be done about that. Renerpho (talk) 15:48, 11 January 2019 (UTC)

I Converted function into degrees, probably better? Tom Ruen (talk) 17:28, 11 January 2019 (UTC)

Agreed. I've just added the necessary $^{\circ }$ symbol, and repaired the brackets. I also removed a wikilink to degree (angle) that appeared later in the text. Renerpho (talk) 21:56, 11 January 2019 (UTC)

I did some cleanup and made a few additional changes. If there are no objections, this can go back into the main article. Renerpho (talk) 02:06, 12 January 2019 (UTC)

Moved section back into the main article. The other sections of the article could need some work, too! Renerpho (talk) 22:16, 12 January 2019 (UTC)

Minor changes and Further Problem

I've slightly improved the Introduction and made the article consistent with the article page apparent magnitude. There are many inconsistencies between apparent magnitude and magnitude (astronomy), especially with history of the subject and common usage. e.g. The difference between visual magnitudes and the more theoretical photometric magnitudes. (To partial solve this another separate photometric magnitude article.) Furthermore, 'bolometric' needs to be better defined - relating true energy output against luminosity. The relevance of "As with all astronomical magnitudes, the absolute magnitude can be specified for different wavelength ranges corresponding to specified filter bands or passbands; for stars a commonly quoted absolute magnitude is the absolute visual magnitude, which uses the visual (V) band of the spectrum (in the UBV photometric system). " The problem is "absolute visual magnitude" is not defined at all.

Comment: @Lithopsian: There also needs to be an explanation of usage. Like most magnitudes, it is a unit of measure, meaning the expression is "4.83 magnitude" not "magnitude 4.83." The latter is better expressed as "magnitude of 4.83". Many astronomical articles have odd usage, like "magnitude 6.0" or "the components are magnitudes 4.0 and 5.1 respectively"[6] If magnitude is the unit measure of brightness, then the value must be followed by the unit measure. e.g, "6th magnitude' or "6.0v magnitude" or a "double star is 4.5v and 5.6v magnitude" must be correct. Using the plural of magnitude is also incorrect usage in this respect. Better usage would be "magnitude of 6.0"

I can find few examples of the reverse usage in the popular or astronomical literature. The claim "correct usage of magnitude as a unit," must be problematic? RfC?? (There is a need for self-consistency here.)

This is explained here[7], where the IAU officially says: "5.17 Magnitude: The concept of apparent and absolute magnitude in connection with the brightness or luminosity of a star or other astronomical object will continue to be used in astronomy even though it is difficult to relate the scales of magnitude to photometric measures in the SI system. Magnitude, being the logarithm of a ratio, is to be regarded as a dimensionless quantity; the name may be abbreviated to mag without a full stop, and it should be written after the number.." Also it says: "The use of a superscript m is not recommended. The method of determination of a magnitude or its wavelength range may be indicated by appropriate letters in italic type as in U, B, V. The photometric system used should be clearly specified when precise magnitudes are given." (This equally applies to absolute magnitudes.) Arianewiki1 (talk) 08:26, 19 May 2019 (UTC)