Q. If the time between olas hashachar and Netz is equal to the time between sh’kiya and tzeis hakochavim, why do people end Shabbos at 72 minutes yet start a ta’anis at 16.1 degrees as 72 minutes which is generally longer that 72 minutes?

A. Although the Talmud (Pesachim 94a) does mention that the time interval from alois hashachar until neitz is four mil, the same amount as from sunset to tzeis, Horav Shlomo Miller Shlit”a pointed out, that in reality these two time periods are not the same. Before alois hashachar all stars can be seen. After alois most stars can still be seen, while for tzeis hakochavim you need three stars only.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a