Comparison of brightness
The brightness comparison is a method of distance measurement .
If the real brightness of light sources is known (or assumed / assumed), the distance can be measured by evaluating the apparent brightness , i.e. the brightness of the light arriving at the observer. If, for example, the brightness is a quarter of another, same light source, the distance of which is assumed to be known, this means twice the distance, because the decrease is square .
Since the lower brightness can also have other causes ( dark clouds ; different type of light source), the procedure has great uncertainties for the individual case; However, if, as in astronomy, billions of cases can be compared for which models verified by other methods are available, useful statements can be made.
One tries to determine the type of light source and thus the actual brightness at the location of the light source by examining the spectral lines .