פירוש על ברכות 4:19

In Shab. 34 b R. Judah defines "twilight" thus: From the time the sun sets and so long as the sky in the East is coloured red. When the lower horizon is pale but not the upper horizon, it is still twilight; but when the upper horizon is pale like the lower, it is night. Cf. Lane, p. 74 n. 2.

Should, therefore, the priest bathe about the time of twilight (according to B. Jose's view), although it is still day, it is legitimate to read the Shema' then, since it is practically night.