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Does a circumstance exist for subtracting one apparent magnitude from another to reach a solution to a problem? Is it ever suitable to do so?

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    $\begingroup$ If you assume two asteroids have the same albedo, then if you know the distances to each from the Earth and from the Sun you can get the ratio of their diameters from the difference in their absolute magnitude. It's a stretch though. $\endgroup$
    – uhoh
    Nov 9 '21 at 11:43
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Two stars in a binary system are at the same distance from Earth.

If they have similar spectral types then the difference in their magnitudes tells us the ratio of their luminosities.

$$\Delta m = m_1 - m_2 = -2.5\log_{10}\left(\frac{L_1}{L_2}\right) \ . $$

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