View Full Version : Help with Apparent Magnitude in an alien sky

2014-May-23, 02:30 PM
So I'm trying to figure the apparent magnitude of a solar system body around a different star as observed from that star locally. I have the Absolute Magnitude (H) for the body - but this measures the total visual magnitude of the body as if it were at a zero phase angle and 1 AU from OUR sun. I do have the equations to work out apparent magnitude from H using the phase integral - and I assume this would work if I could convert the H to an H for a different solar body as the light source - or do I have that wrong?

m = H + 2.5 log [ ( dist(body-sun)^2 * dist(body-obs)^2 ) / (Phase Integral P(x) * 1 AU ^ 4) ]

I know it should be possible to convert this H into the H for a different star if I know that star's Luminosity and/or Absolute Magnitude (M), but I cannot quite work out the algebra / luminosity physics in my head.

Do I need to find an H relative to another star (and how) and use the phase integral calculation or do I need to use the solar H (that I already have) and somehow change the phase integral calculation?

Any help would be greatly appreciated. Thanks!

2014-May-24, 04:31 AM
If you already know how to calculate the apparent brightness of Solar System bodies using H, phase angle, and distance, then 90% of the work is done for you. I'd say keep it simple; adjust the magnitudes up or down in direct relation to the star's magnitude difference from the Sun. For example, Sirius is about 3.5 magnitudes brighter than the Sun; therefore, the Earth's H around Sirius would (?) be -7.36 instead of -3.86.