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General commet questions!!

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18 years 1 month ago #27031 by albertw
Replied by albertw on topic Re: General commet questions!!

COMETS: The absolute magnitude of a comet is the brightness it would exhibit if placed 1 AU from both the Earth and sun.


Nitpicking... I thought it was defined as the brightness of the object at 1AU as seen from the sun; so be definition it would be fully illuminated. This amounts to the same thing if you include the phase angle in the above definition.

Seanie, in mathematical terms its
[1]
Where a is the albedo of the body, r the radius, d0 is 1AU.

So rather than relying on the black body emission of the object, which is where the luminosity comes from in a star, the brightness is a function of its size (radius/distance) and the albedo.

Dave, afaik anything that reflects in the solar system uses this absolute magnitude definition. Planets, asterids, the Moon etc. all use the reflection definition.

Cheers,
~Al

[1] insert rant about hosting365 not having latex on their servers...

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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18 years 1 month ago #27035 by dmcdona
Replied by dmcdona on topic Re: General commet questions!!
Al - just got this off Wilkpedia. I was not aware of the distinction amongst the various bodies in terms of absolute magnitude. It seems the 1AU from both the sun and the earth is the definition though...

You learn something new every day eh...

Cheers

Dave



Absolute Magnitude for planets
For planets, comets and asteroids a different definition of absolute magnitude is used which is more meaningful for nonstellar objects.

In this case, the absolute magnitude is defined as the apparent magnitude that the object would have if it were one astronomical unit (au) from both the Sun and the Earth and at a phase angle of zero degrees. This is a physical impossibility, but it is convenient for purposes of calculation.

To convert a stellar or galactic absolute magnitude into a planetary one, subtract 31.57. This factor also corresponds to the difference between the Sun's visual magnitude of −26.8 and its (stellar) absolute magnitude of +4.8. Thus, the Milky Way (galactic absolute magnitude −20.5) would have a planetary absolute magnitude of −52.

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18 years 1 month ago #27036 by albertw
Replied by albertw on topic Re: General commet questions!!

In this case, the absolute magnitude is defined as the apparent magnitude that the object would have if it were one astronomical unit (au) from both the Sun and the Earth and at a phase angle of zero degrees.


And the only place the phase angle can be 0 and both the Earth and the Sun are at 1AU is when the Earth is where the Sun is :-)

So you can simplify the definition down to the magnitude of the body at 1AU from the Sun as seen from the Sun. I'll have to check this in a textbook when I get home, there must be some reason for the phase angle and the Earth needing to be stated.

Speaking of wikipedia, I checked phase angle. Wikipedia says this:

The phase angle varies from 0° to 180°. The value of 0 corresponds to the position when the illuminator, the object and the observer are collinear, with the illuminator and the observer on the same side with respect to the object. The value of 180 is the position when the object is between the illuminator and the observer, known as the astronomical opposition.


The last bit is wrong surley? A phase angle of 180 is inferior conjunction right? opposition would be 0, as would superior conjunction and conjunction. So we have a full moon when its at a 0 phase angle. I'll add a note to the wikipedia discussion page.

Cheers

Albert White MSc FRAS
Chairperson, International Dark Sky Association - Irish Section
www.darksky.ie/

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18 years 1 month ago #27038 by dmcdona
Replied by dmcdona on topic Re: General commet questions!!

In this case, the absolute magnitude is defined as the apparent magnitude that the object would have if it were one astronomical unit (au) from both the Sun and the Earth and at a phase angle of zero degrees. This is a physical impossibility, but it is convenient for purposes of calculation.


Is the bold text above their get out clause? Let us know how you get on anyhow - I'd be interested to know as this is the first time I've come across these different definitions. Just like Seanie, I thought there was only the 10 parsec rule...

Cheers

Dave

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