Talk:Umbra, penumbra and antumbra

Clean up 2006/05
This article has a whole heap of 'stuff' about the word umbra. Perhaps most of it could be put on an disambiguation page and the core of the article becomes umbra, with reference to shadows, as it once may have been intended? POds 03:58, 5 May 2006 (UTC)

Disambig?
As mentioned, it has a whole heap of meanings lumped onto one page. I tired seperating them out as best I could. However I feel that the main focus of this page is the use of Umbra in an astronomy/physics sense, as in dealing with light sources. Therefore, I propose a disambiguation page for Umbra, so that the other topics mention can be elaborated on as well. Stovetopcookies 04:41, 5 May 2006 (UTC)


 * I made the disambig page and moved everything there. Since this article is short, and because their definitions depend on one another, perhaps umbra and penumbra should be merged into one article (like deferent and epicycle)? --Fastfission 21:42, 5 May 2006 (UTC)

In law you would use penumbra so it is important to possibly seperate that out as well.

Sorry.
I am still new to this but I happend upon it when I was trying to point someone in the right direction. It is important because when you discuss law it is a term clarifying that the right is not in the constitution but in the penumbra.

merge umbra and penumbra
It looks like both these words have lots of uses, but they do also have very specific definitions, as parts of shadows. How about: Lisamh 03:39, 20 September 2006 (UTC)
 * 1) the definitions go to Wiki Dictionary
 * 2) we add a disambiguate page for penumbra (instead of the list of other meanings)
 * 3) the eclipse and sunspot specific uses of umbra, penumbra, and antumbra go to the articles on eclipses and sunspots

For: I am for the merge. These topics are very short, and besides, the figure illustrating these concepts shows both umbra and penumbra. Lunokhod 20:55, 7 January 2007 (UTC)

Merged
I've merged not only Penumbra, but also Antumbra, to Umbra. —Quarl (talk) 2007-02-28 10:41Z 
 * Why not move to, say, Umbra and penumbra? Seems to be more in line with what I've seen on other articles. Cctoide 18:02, 19 April 2007 (UTC)

question?
Could someone please address the legal use of the word penumbra. i.e. "If you are a history buff you will recall the confirmation hearings of Thomas, Scalia, and Robert Bork.  Specifically, the exchange of views concerning the US Constitution and its “penumbra of rights.”  The “penumbra” area is a dividing line for many constitutional scholars (and politicians)." —Preceding unsigned comment added by 141.197.12.183 (talk) 13:26, 24 September 2007 (UTC)

Diagram
It'd be really great to extend the diagram to the right (as it is currently oriented). That would allow for the antumbra region to be visible also. Additionally, orienting the diagram vertically, with Earth at the "bottom" might be intuitively easier to imagine.

My two cents on classification issues. The current set-up is probably best. All three ideas belong together as the diagram (if extended) would demonstrate. An article heading Umbra, penumbra and antumbra is just a little unwieldly imo, especially when it is clear these terms are all based on -umbra.
 * Umbra is the basic idea: the Sun's shadow in a total eclipse.
 * Penumbra is the name for the partial shadow of partial eclipse.
 * Antumbra the name for the special ring effect.

Extention of this article is possible in various ways. One that jumps to mind is that antumbra must happen because the Earth's orbit is elliptical. Hence when Earth is closer to the Sun (perihelion), the apparent size of the sun is larger.

Any recent historical example of an eclipse that cast a shadow over Europe or USA could be really helpful for people to get a feel of how the shadow moves across the surface of the Earth. The cities that fell under the umbra could be named, and the width of the penumbra could be described.

I wish I could provide these things myself, but I have not the skills or access to data. Alastair Haines 03:52, 29 September 2007 (UTC)


 * It'd be really great to extend the diagram to the right (as it is currently oriented). That would allow for the antumbra region to be visible also.
 * Done. The new diagram has been computer generated, so the shadow should be "correct".
 * Qarnos (talk) 07:00, 8 March 2008 (UTC)

Okay Dathe2025 (talk) 10:26, 3 October 2019 (UTC)

Okay Dathe2025 (talk) 10:26, 3 October 2019 (UTC)

Point source
I just changed the point source reference, as it said that a point source would cast an umbra and penumbra, but no antumbra. I don't see how you can get a partial shadow from a point source - either you can draw a line to it or not, there's no partial shadow. --198.103.172.9 (talk) 17:19, 23 May 2008 (UTC)

Penumbra just for light source?
The article states that a penumbra is usually used in reference to the shadow cast by celestial bodies, and that it is a feature of visible light. In fact, it applies to any EM radiation. It's an important principle in radiation physics. —Preceding unsigned comment added by Jonnylaney (talk • contribs) 12:20, 3 March 2009 (UTC)

Diagram
There should be a simple line diagram, showing the situation from the side, with also end-on drawings of the shadows cast on a plane within and outside of the umbra. The present artistic pictures should be scrapped. 94.30.84.71 (talk) 20:43, 16 September 2011 (UTC)

The "384000km" diagram is unnecessarily wide. 94.30.84.71 (talk) 20:43, 16 September 2011 (UTC)

It would be better to move legal and medical material to a separate section or sections, leaving the description of shadows unpolluted. 94.30.84.71 (talk) 20:43, 16 September 2011 (UTC)

I suggest tabulating the length, in km, of the solar umbrae of the Earth and Moon, and the umbra of the Moon using Earthlight. And also giving the approximate formula for when the bodies are much smaller than their separation, and also the exact formulae. 94.30.84.71 (talk) 20:43, 16 September 2011 (UTC)

Problem with diagram
The diagram shows the antumbra to be lighter than the penumbra. The text says the antumbra is darker than the penumbra. A contradiction? Most likely.

It appears that the diagram is deceiving. To my understanding, the area labeled "antunumbra" is not; it is the area where an antunumbra may be observed. The actual antunumbra is the shadow on the shadow-casting object itself, when the light-source is larger than the object, as with an annular eclipse.

The "diagram within a diagram," displays the actual antunumbra on the shadow-casting object.

Which raises a question... maybe two. Is the umbra the area of shadow, or is it considered to be what you see? Same Q for penumbra; because the "diagram within a diagram" labels the view of the semicircle of the light source as the penumbra, but the line/pointer indicates the penumbra is the area of shadow. Maybe they are both (view, as well as shadow) the penumbra, or different definitions of penumbra. 74.100.137.109 (talk) 15:58, 23 May 2014 (UTC) 74.100.137.109 (talk) 15:39, 23 May 2014 (UTC)74.100.137.109 (talk) 15:47, 23 May 2014 (UTC) 74.100.137.109 (talk) 15:58, 23 May 2014 (UTC)

the Moon's umbra

 * The "end" of the umbra is the point where the occluding body is equal in apparent size to the light source; for the Moon, this is roughly the semimajor axis of its orbit, since the angular size of the Sun and Moon are nearly identical.

This new sentence needs rewriting (and I'm just the Sloth to do it). The apparent angular size of the Moon (as seen from where?) is not the cause of the length of its umbra! —Tamfang (talk) 21:48, 24 May 2014 (UTC)

Moon is never in Earth's antumbra.
The image subtitle was wrong, in a lunar sclipse Moon can cross the umbra and penumbra zones, but never the antumbra one. — Preceding unsigned comment added by 135.245.248.72 (talk) 12:32, 18 May 2017 (UTC)

Incorrect statement: collimated beam (such as a point source)
Regarding this sentence "Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast." a point source is not a source of collimated light. If other agree I would suggest removing the part about point sources. — Preceding unsigned comment added by Alexbhandari (talk • contribs) 18:06, 24 March 2020 (UTC)
 * How about changing to something along the lines of "Only the umbra is cast when the light source is a point or collimated, assuming no diffraction" ? SlowJog (talk) 20:26, 21 December 2021 (UTC)

"Umbral eclipse"
This phrase appears (undefined) on many solar-eclipse pages. It would helpful if it could link to a section of this article, but the phrase doesn't appear here. Thmazing (talk) 13:35, 19 April 2024 (UTC)

Nature of light and Reflection at plain surfaces
Physics 197.239.13.47 (talk) 10:51, 7 July 2024 (UTC)