ויהי ערב ויהי בקר יום שלישי "It was evening, it was morning, the third day" (Bereshith 1:13). How do we understand what the Torah is saying? Light had not yet been introduced as of the third day. It was only on the fourth day that G-d created luminaries to separate between day and night.
Rabbenu Bahyei comments that it is the setting of the sun that causes it to become evening, and the rising of the sun in the morning which causes it to become day. He adds that if the Torah, nevertheless, does use the terms 'evening' and 'morning', then they must exist in a different sense.
In fact, it is the position of the planets that determined morning and evening. It is the position of the planets every 24 hours that determines what time of day it is. This is what the Torah appears to be referring to on the third day. From the next day however, the terms morning and evening are used in the manner we understand them to be nowadays.
(See Rabbenu Bahya on the Torah, Bereshith 1:13)
Parasha, Beireishis, Bereshit, Bereshith