|WikiProject Astronomy||(Rated Start-class, High-importance)|
Visual vs Actual Diameters
First paragraph ended with: "For a spherical object at a large distance, the visual and actual diameters are the same." This statement is not correct given the definition of visual diameter earlier in the same paragraph. The difference between the actual and visual diameters of a sphere is directly proportional to the difference between the actual distance to the sphere and the distance to the projection plane. The magnitudes of these distances do not affect the difference between the actual and visual diameter so long as the ratio between actual distance and distance to projection plane is constant.
I should add that, while I removed that statement, I've noticed that the paragraph is still even more deeply flawed. It says essentially that angular diameter is visual diameter, and (emphasis mine) "The visual diameter is the diameter of the perspective projection of the object on a plane through its centre that is perpendicular to the viewing direction." True perspective projection is most accurately represented by the surface of a sphere. This is why we use angular measurements for apparent size in the first place, like how we use angular measurements to divide the Earth by longitude and latitude. Angular measurements do not project evenly along a plane. This further compromises the correctness of the statement I removed: "For a spherical object at a large distance, the visual and actual diameters are the same." If visual diameter is measured in angular units, and actual diamter is measured in units of length, the two values become entirely incomparable.
Should this page be categorized in Observational Astronomy?
- Not necessarily. Angular diameter is a geometric concept that applies to perspective, and not solely to astronomy — Preceding unsigned comment added by Octachoron (talk • contribs) 02:55, 8 June 2014 (UTC)
Sun diameter inaccurate?
The angular diameter of the Sun is quoted as 30'
Elsewhere, the entry for the Sun gives mean distance and diameter figures which would correspond to about 32'
This discrepancy should be resolved.
188.8.131.52 13:02, 18 April 2007 (UTC)Peter
- You are correct - I shall adjust. Jim77742 11:43, 9 September 2007 (UTC)
Me have question. I did the simple trig and I get theta = 2arctan(d/2D). Obviously, due to small angle approximation, it's close enough, but... I haven't found much (conclusive) evidence in either direction, however. Anyone mind checking? Jeff 01:26, 3 July 2007 (UTC)
- You are correct - I shall adjust. Jim77742 11:43, 9 September 2007 (UTC)
Sorry chaps, the correct formula for the angular diameter of a sphere is theta = 2arcsin(d/2D) and not the one currently shown. River6rat 13:49, 2 October 2007 (UTC)
Italic text== This article needs improving ==
* Betelgeuse: 0.049″ – 0.060″ * Alpha Centauri A: ca. 0.007″ * Sirius: ca. 0.007″
What's "ca" mean? Answer: Ca is an abbreviated version of "circa"; a latin word meaning around or about. See circa
And these paragraphs:
- This meaning the angular diameter of the Sun is ca. 250,000 that of Sirius (it has twice the diameter and the distance is 500,000 times as much; the Sun is 10,000,000,000 times as bright, corresponding to an angular diameter ratio of 100,000, so Sirius is roughly 6 times as bright per unit solid angle).
- The angular diameter of the Sun is also ca. 250,000 that of Alpha Centauri A (it has the same diameter and the distance is 250,000 times as much; the Sun is 40,000,000,000 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle).
- The angular diameter of the Sun is about the same as that of the Moon (the diameter is 400 times as large and the distance also; the Sun is 200,000-500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450-700, so a celestial body with a diameter of 2.5-4" and the same brightness per unit solid angle would have the same brightness as the full Moon).
How is this really relevant? I find it confusing and not very useful. I would like to remove them. But I'd prefer comment before I do.Jim77742 11:43, 9 September 2007 (UTC)
"For a disk-shaped object at a large distance, the visual and actual diameters are the same." This does not make sense as a disk shaped object like a galaxy will have an extremely small apparent diameter (a grain of sand held at arm's length) and a huge actual diameter (~100,000 ly). 9:23, 1Jul10. —Preceding unsigned comment added by 184.108.40.206 (talk)
Angular Diameter of planets from Earth
Should this be appended to say that it is the angular diameter as viewed from Earth when the various bodies are closest to the Earth in their orbits? Otherwise this can be minorly misleading. Jacotto (talk) 02:39, 4 April 2008 (UTC)
- It is not for when objects are closest to Earth. Look at Venus (closest) and Jupiter (largest) for two great examples. They are given as a range. Planets that are very far from the Earth simply do not vary enough to bother with a range. Only a professional telescope under the best of circumstances will resolve Ceres or Pluto as a small sphere. -- Kheider (talk) 03:27, 4 April 2008 (UTC)
When formulas are used, should there either be explanations (or examples) of each mathematical term? Or else, links to Wiki math pages that explain them? Not all of us are math wizards -- so terms such as "arctan" and "arcsin" are lost on us.
If you enter "arctan" or "arcsin" in the search box and hit Go, you're immediately transported to an explanation. In general it is not practical to explain all maths notation used; some people may not even know what a square root is, or exponentiation. And if you're not a "math wizard", then most likely the explanation at Inverse trigonometric functions won't do much for you; however, you can use that for all practical purposes these formulas are equal to δ = d / D. To get degrees instead of radians, multiply by 180o/π ≈ 57.3o. --Lambiam 23:04, 10 August 2008 (UTC)
Atmosphere lens effect
- No, the Sun isn't wider when it's closer to the horizon. It's flattened top to bottom by refraction. Refraction is greater closer to the horizon so the lower limb of the Sun is lifted more than the upper limb causing that flattening. There is also a perceptual illusion when the Sun or especially the Moon is close to the horizon, known as the "Moon Illusion", which makes astronomical objects look larger near the horizon, but this is entirely in our heads. There is no magnification by the atmosphere. In fact, the Sun and Moon are actually slightly smaller (when measured objectively) when they are on the horizon, due to both the flattening from refraction and the slightly greater distance. 220.127.116.11 (talk) 18:51, 6 April 2014 (UTC)
Comments about Figure 2
The caption for Figure 2 entitled "angular diameter" is more than a little strange. Comparison to a "football" is meaningless without giving the distance to the football. Also, Pluto's orbital radius averages about 40 AU. But a tennis ball is NOT 40 times smaller in diameter than a football. It doesn't matter whether "football" refers to a soccer ball or a "pigskin" viewed from the end. The figure is helpful enough without the nonsensical statements in the caption. — Preceding unsigned comment added by 18.104.22.168 (talk • contribs) 20:34, 9 March 2009 (UTC)
Football and tennis ball
The caption of the second diagram reads as follows:
Angular diameter : The sun appears the size of a football when seen from Earth. But when it is seen from Pluto (The dwarf planet - plutoid) its size will be like a tennis ball.
This is meaningless unless some indication is given of the distance at which the ball is held. I haven't checked—it's raining today, and I don't have a football anyway—but I'm pretty sure that the angular diameter of a football held at arm's length (whose arm?) is larger than that of the sun.