I know the apparent magnitude of sun is -26.74, but I wonder how to get this value if we don't know the absolute magnitude?
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The apparent magnitude is found by direct observation. You can find the apparent magnitude using a light-sensitive device.
This is true for all observed bodies. The apparent magnitude is how bright it "appears" to be from Earth. The only difficulty with finding the apparent magnitude of the sun is that it is so bright that the highly sensitive light detectors that would be used to measure the apparent magnitude of a star would be damaged by the brightness of the sun. But that is a practical difficulty.
Having found the apparent magnitude, one can calculate the absolute magnitude if (and only if) you know the distance to the star. The absolute magnitude can be calculated from the apparent magnitude and the distance of the sun. It is known to be 4.83.
If you know neither the absolute nor apparent magnitude, you can find absolute magnitude with the following formula:
M = (log10(L/L0)) * -2.5
Once you have the absolute magnitude, apparent magnitude is calculated with this formula:
m = (M-5) + (log10(d*5))
Credit goes to https://www.omnicalculator.com/physics/luminosity#absolute-and-apparent-magnitude. I just altered the formula to make sense for those with less experience in mathematics, such as myself.