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irradiance


Definition/Summary
Irradiance is the power per area of the radiation received by a surface.

It is measured in SI units of watts per square meter ([itex]W.m^{-2}[/itex]), or cgs units of ergs per square centimeter per second ([itex]erg.cm^{-2}.s^{-1}[/itex], often used in astronomy).

Equations
surface area of star:
[tex]4\pi R^2[/tex]
total power radiated (Luminosity):
[tex]L\ =\ 4\pi R^2\sigma T^4[/tex]
power per area at distance d (Irradiance):
[tex]I\ =\ \frac{L}{4\pi d^2}\ =\ \frac{R^2}{d^2}\sigma T^4[/tex]
([itex]\sigma[/itex] is the Stefan-Boltzmann constant, [itex]5.6710^{-8} \, W\cdot m^{-2}\cdot K^{-4}[/itex])

Scientists

Recent forum threads on irradiance
efficiency, power, irradiance and speed problem
How to face a data processig to apply "swinging door algorithm" to a irradiance time
Computing irradiance at a point
far-field irradiance corresponding to the electric dipole moment
Spectral Irradiance question
 
Breakdown
Physics
> Astro Cosmo
>> Astronomy

See Also

Images

Extended explanation
Other similar units:

"Ir-" means "on", so irradiance relates to radiation on (received by) a surface, while radiance, and radiant intensity emittance excitance or flux, relate to radiation from a surface.

Commentary

gabe69 @ 12:48 PM Jul11-11
Since these two formulas are equal, the sum can be solved for the irradiance:

4piR2σT4 = 4pi(irradiance)(distance[ctex]^{2}[/ctex])
R2σT4 = (irradiance)(distance[ctex]^{2}[/ctex])
Irradiance = (R2σT4)/ (distance[ctex]^{2}[/ctex])

For any extra solar planet that you have these figures, star radius, star temperature, and planet distance,
you can calculate the irradiance. If you also have the eccentricity you can calculate the irradiance variation from perihelion to aphelion.
If Earth's Average irradiance is 100%, at perihelion it is 103.43%, at aphelion it is 96.74%.
Venus' average irradiance is 191.30%, at perihelion 193.93%, at aphelion 188.73%.
Mars' average irradiance is 43.11%, at perhelion 52.45%, at aphelion 36.06%.

False habitablility claims are easy to refute with these irradiance figures and formulas.
Example, the planet named HD 38283 b has been touted as possibly having habitable moons. In fact the average is 101.06%, but with an eccentricity of 0.41 the perihelion irradiance is 290.32% and the aphelion irradiance is 50.83%
Other examples:

Planet Perihelion Average Aphelion
Name Irradiance Irradiance Irradiance
HD 28185b 93.69% 81.03% 70.77%
HD 108874b 119.47% 103.33% 90.25%
HD 155358c 155.02% 105.26% 76.11%
HD 221287b 136.29% 115.36% 98.90%
HD 45364b 167.83% 116.07% 85.02%

I don't believe any of this needs to be deleted.

gabe69 @ 04:06 AM Jul10-11
Who is the nitwit that removed all the exoplanet examples, the practically vandalized the explanation, how do you revert that???

tiny-tim @ 10:41 AM May12-11
gabe69, you'll find it easier if you use the preview facility in any thread choose a thread with no replies (I like the Science Jobs ones), click "QUOTE", and do your preview there (remembering to click "Refresh" on your browser each time, to get the LaTeX working properly). That'll save you time, and prevent the Edit Log going into several pages.

gabe69 @ 11:40 PM May9-11
I don't understand the importance of autolinking, this is the first I have done anything here, but since no one else seems to know these basic formulas or bothers to add it to scientific publications I thought I would share it. Isn't there a way to use the title with sort of split tags, with commas or something.
I did as you suggested and added the general definition. I suspect that it still could see some improvement overall.

tiny-tim @ 03:38 AM May4-11
But, gabe69, you've done all this work, and nobody will ever see it (once it leaves the "ten latest" list) if it doesn't get autolinking. Just chuck in a general definition of "irradiance" (which btw I couldn't give from memory), change the title back, and everything fits!

gabe69 @ 02:44 AM May3-11
I agree "Irradiance" itself doesn't necessarily include extrasolar planets, ... "If a point source radiates light uniformly in all directions and there is no absorption, then the irradiance drops off in proportion to the distance from the object squared, since the total power is constant and it is spread over an area that increases with the square of the distance from the source." it doesn't necessarily mean stars either. So just because it is in the Astronomy section doesn't mean that "Irradiance" is enough. This is only an in-depth example. "Irradiance of Planets" perhaps.

Redbelly98 @ 10:23 AM Apr30-11
But "irradiance" is a more general term, and this article is specifically about the irradiance of planets. While a high autolink frequency is normally a benefit, it isn't the single most important criterion for selecting a title.

tiny-tim @ 10:03 AM Apr29-11
Shortened title from "Extrasolar Planet Irradiance" to " irradiance", since the former would get no autolinking (nor would "planetary irradiance").