Reference for Shevi'it 4:1

ר' ישמעאל כדעיתה דרבי ישמעאל אמר אין העומר בא מסוריא יצא קציר העומר שהיא מצוה:

If it becomes shadowy, the intermediate period (twilight) has arrived. If darkness has set in, so that the upper atmosphere has become indistinguishable from the lower, night has arrived. Rabbi says: When, at the period of the new moon, the sun commences to go down and the moon to appear, it is twilight.

Jastrow