Or HaChaim on Numbers 9:17/אור החיים על במדבר ט׳:י״ז

1 ולפי העלות הענן, and whenever the cloud would lift, etc. The reason this paragraph begins with the conjunctive letter ו is to remind us that the cloud served two different purposes. 1) Its function was to serve as a cover for the Holy Tabernacle and all its furnishings. As such its function was decorative, a compliment for the Jewish people who had become carriers of the שכינה. This is the meaning of the words in verse 15 that כסה הענן את המשכן לאהל העדות, "it covered the Tabernacle the Tent of Testimony." The Torah adds a second function of the cloud in our verse when it describes the motion of the cloud indicating that the Israelites were to break camp and to start marching on their way to the Holy Land. The cloud signalled both when it was time to move and when it was time to make camp.

2ואחרי כן יסעו, following this the Israelites journeyed. The expression ואחרי כן seems somewhat misplaced. We would have expected something parallel to ובמקום אשר ישכון שם יחנו without the words ואחרי כן. Why did the Torah write the words כן ואחרי? Had the Torah simply written ולפי העלות הענן יסעו בני ישראל, I would have concluded that the signal for the Israelites to move was the motion or lack of motion of the cloud. In order to make certain we understand that the Israelites journeyed at the direct command of G'd the Torah inserts the words ואחרי כן to direct our attention to verse 18 where the Torah spells out that the journeys were על פי השם. Our verse therefore deserves to be understood thus: "And in accordance with the cloud lifting from above the Tabernacle, shortly thereafter G'd would give the command for the Israelites to start journeying" so that they were actually journeying at the command of G'd. [The letter ו before the words אחרי כן means that the expression is not one describing a time frame, though it does describe a certain sequence of events. Ed.]

3Another meaning of these words could be that the Torah wanted to inform us that the Israelites did not ever journey before the cloud lifted but only after it had already removed itself from above the Tabernacle. This did not imply however, that they automatically journeyed as soon as the cloud lifted, rather they awaited specific instructions from G'd to do so. Had the Torah used the alternative wording we mentioned earlier the impression would have been created that the lifting of the cloud was equivalent to a command for the Israelites to break camp and to start journeying.