Regarding the halachic date line which I understand is set at 90 degrees east of Jerusalem; I wondered why it is set at the first sighting of the sun on the fourth day and not in the evening when a jewish day starts. I hope you do not consider this a frivolous question as I happen to find the subject and all its ramifacations very interesting.

There is more than one opinion regarding the halachic date line. The differences in opinion led to a famous question in 5701 (1940) as to when the Jews in Kobe Japan should observe Yom Kippur. The two most famous opinions- of HaRav Yehuda HaLevi in the Kuzari and of the Yesod Olam differ over how to see Jerusalem as the center of the world. HaRav Yehuda HaLevi sees Jerusalem as the center of the civilized world which he understood to be only one half the globe- in other words, the date line is 90 degrees to the East of Jerusalem. When the sun is at it height at midday in Jerusalem, it is setting 90 degrees to the east and a new day begins from there. The Yesod Olam sees Jerusalem as the center of the entire world so the date line is 180 degrees from Jerusalem (see HaRav Shlomo Zevin L’Or Hahalacha p283).