Moshe said, "So said Hashem: At about midnight I shall go out in the midst of Egypt. And every firstborn in the land of Egypt shall die...".
The Gemara in Brachos (ברכות ד ע"א) discusses why Moshe said that Hashem will slay the firstborns 'at about midnight', and not 'at midnight', and says as follows. Because Moshe knew the exact moment of midnight, why did he say 'at about midnight'? Moshe thought that Pharaoh's astrologers might err in their calculation in the precise moment of midnight, and would then claim that Moshe is a liar. Moshe's caution was based on the lesson that was taught, 'Teach your tongue to say "I do not know", lest you be caught in a falsehood.'
We need to understand why the astrologers would be so meticulous regarding the exact moment of the plague's occurrence, that were Moshe to have said "at midnight" and it would seem to them that it had occurred a bit before or after the exact moment of midnight, they would immediately have called him a liar. Furthermore, why would the astrologers seek out to dispute Moshe's prophecy, after having clearly seen that his previous prophecies were all perfectly accurate?
We first need to understand why indeed Hashem said, and why it was actually so significant, that the plague should occur precisely at midnight.
There are seven moving cosmic bodies who govern and exert their respective astrological influence on the earthly affairs according to their hourly rotations. These seven bodies are; Mercury, Moon, Saturn, Jupiter, Mars, Sun and Venus. Each of these seven bodies govern and exert their influence in a specific sequence and continuous cycle. The cycle begins with Mercury governing during the first hour after Shabbos, followed by the moon in the second hour, Saturn in the third etc. Following this pattern, we can determine which specific planet governs during any given hour.
The Gemara in Shabbos (פז ע"ב) tells us that the Jewish People left Egypt on Thursday, and, hence, the slaying of the firstborns was at midnight of Wednesday evening. According to the cycled pattern, we can determine that on Wednesday evening - which was the night of the firstborn's plague - Jupiter and its favorable influence govern in the hour preceding midnight, and Mars and its detrimental influence govern in the hour following midnight.
In view of this, we can understand why Hashem slayed the firstborns at the exact moment of midnight, not a moment before nor a moment after. The reason is because had they been struck during the hour preceding midnight, in which the influence of Jupiter governs, the clear recognition of the miracle would have been greatly diminished, because the redemption of the Jews from the plague would not have been attributed to the merit of the Jews, but rather to the favorable influence of Jupiter governing at that hour. Similarly, had the plague struck during the hour following midnight, the Egyptians would have easily dismissed the plague, attributing it, not as a punishment to their wicked behavior but rather to the negative influence exerted by Mars governing during that hour.
Thus, Hashem brought about the plague at the exact moment of midnight, which is the transition period of the planets, where neither Jupiter nor Mars govern or exert any influence, thereby establishing the twofold miracle; the salvation of the Jews being due to their merits, and the plague upon the Egyptians being as punishment for their wicked behavior.
In light of this, we can understand that had Moshe said that the plague would occur "at the very moment of midnight", the astrologers would have been extremely particular regarding the plague occurring at that very moment. Because the plague occurring at "the moment of midnight" would be a clear allusion to the double miracle - the redemption of the Jewish firstborns, proving the Jewish People's righteousness, as well as the punishment of the Egyptians, proving their wickedness - and the Egyptians would be more than happy to accuse Moshe of lying and thereby disputing at least one of the two miracles.
זרע שמשון פרשת בא אות ג
