"For the Jews celebrate their Passover on the 14th day according to the moon, and the death of our Saviour falls on the day before the Passover. But an eclipse of the sun can only take place when the moon comes under the sun, how then could an eclipse have occurred when the moon is directly opposite the sun?"
It was an act of GOD, clearly. (Then again, so are solar eclipses, though this was a special case.)
A little basic positional astronomy and calendrics. Passover is on the 14th of Nisan, in the Hebrew lunar-solar calendar. All months in the Hebrew calendar begin on a new moon, and full moon (both in Christian liturgical calendars and in the Hebrew calendar) falls on the 14th day.
A new moon occurs when the moon overtakes the sun. In Islam, the month begins when the first crescent of the “new moon” is spotted visually. A full moon occurs about 14 days later.
A solar eclipse always occurs on a new moon, a lunar eclipse on a full moon.