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## [gnuastro-commits] master 24bb37e 1/2: Book: clarification in the descri

 From: Mohammad Akhlaghi Subject: [gnuastro-commits] master 24bb37e 1/2: Book: clarification in the description of surface brightness Date: Tue, 20 Oct 2020 18:58:28 -0400 (EDT)

```branch: master
commit 24bb37e9be9fa71a5a7b31b62d091ea0c3ce3f3a
Author: Raul Infante-Sainz <infantesainz@gmail.com>

Book: clarification in the description of surface brightness

With this commit, I am just adding a phrase in order to note that area
of astronomical objects are unit less.
---
doc/gnuastro.texi | 1 +
1 file changed, 1 insertion(+)

diff --git a/doc/gnuastro.texi b/doc/gnuastro.texi
index c3a2ece..8b61a45 100644
--- a/doc/gnuastro.texi
+++ b/doc/gnuastro.texi
@@ -17216,6 +17216,7 @@ Using the zero point magnitude (@mymath{Z}), we can
write the magnitude relation
@cindex Surface brightness
Another important concept is the distribution of brightness over the object's
detected area.
For this, we define the @emph{surface brightness} which is defined as the
magnitude of an object's brightness divided by its angular-coverage (or
``area'' in the sky, usually in units of arcsec@mymath{^2}).
+Note that since this ``area'' is an angular measurement of the size of the
astronomical object (projected on the celestial sphere), it is also unit less.
A common mistake is to divide the magnitude by the area, but this is wrong
because magnitudes don't have units.
It is the brightness that should be divided by area, and the magnitude of that
ratio is defined to be the surface brightness.
For example when the brightness is measured over an area of A
arcsec@mymath{^2}, then the surface brightness becomes:

```

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