אין כא-ל ישורון, "There is none like God, Yeshurun;" after Moses had concluded blessing the tribes individually, he now addresses the whole people reminding them of how fortunate they are in receiving their share on earth by the Creator of the universe directly. He tells them that they will inherit the land of Canaan, something that has been denied him; he also explains aspects of the hereafter to them, as I will explain in greater detail. He seals his words with esoteric matters, and this is why he begins this paragraph reminding the people that there is No One comparable to God. Whereas all the other nations have been placed under the guidance of intermediaries, angels, etc., not so the Jewish people. The first time Moses had requested from God that this be so was in Exodus 33,16 with the words ונפלינו אני ועמך, "and let me and Your people be distinct." The reason he calls the Jewish people ישורון at this stage is not because of their character but because they have a "direct line," (ישר) to God, unlike the other nations. This is also why, in extolling the advantages of the Jewish people compared to other nations, he says אשריך ישראל מי כמוך, "Hail to you Israel; who is comparable to you?" (verse 29). They are unlike the other 70 nations. This is why he tells the people that their share of the worldly goods and their status on earth is different from those nations. He commences by describing the God of Israel as "riding the heavens," always available to assist His people ובגאותו רוכב שחקים, "in His majesty (He rides) through the upper regions." In the last part of this verse the word רוכב is to be added before the word שחקים, otherwise there is a verb missing.
The concept of God "riding" the heavens requires some analysis. According to the plain meaning of the text the verb "riding" depicts simply someone who is in control, much as the mounted police are in control of the mob on foot. It is similar to 32,13: "He makes it (Israel) ride on the heights of the Land," where "riding" also was a simile for being in a dominant position. Just as the horse performs at the command of its rider, Moses describes the function of the heaven as being to perform at the will of God Who rides it. We find this described in Job 39,18: תשחק לסוס ולרכבו, "making fun of both horse and its rider." (God describing that were it not for being deprived of its natural instincts, the ostrich would fly and not leave its eggs exposed to all kinds of danger.) The orbits of the celestial bodies must not be ascribed to their own ability and power, but only to the power of God Who is the source of their motion.
In this verse the word שמים refers to the heavenly regions known as ערבות. Our sages in Chagigah 12 deduce this from the verb רוכב used by Moses here in describing God's movements over the heaven and the same verb used in Psalms 68,5 סולו, לרוכב בערבות, "extol the One Who rides in the ערבות."
Maimonides writes in Moreh Nevuchim 1,70 "consider the wording of Chagigah 12 where the sages describe the Aravot as high and God 'dwelling' upon them." They quote in support the word רוכב from Psalms 68,5. The reason that David used the word רוכב in that psalm rather than the word שוכן is that the latter word would have created the impression that a fixed site had been assigned by him to God, something inappropriate seeing He is the site of the universe and not vice versa. At best, using the word שוכן would have made God appear no better than some of the other celestial phenomena none of which possess independent powers.
A Kabbalistic approach: when Moses described God as רוכב שמים, the word שמים has to be understood as a reference to the various attributes of God, whereas the One "riding" them is the Essence, Hashem, the אין סוף. The message Moses wishes to convey is that as long as the Israelites carry out God's commandments, the distance between God the Essence, and God in the form of His attributes is minimal, no more so than that of a horse and a rider. If the Israelites do not observe the Torah, however, He will gradually withdraw His Essence to more distant regions, thus distancing Himself from His attributes. This has been expressed in a similar vein by our sages who interpreted Psalms 60,14 where David says באלו-הים נעשה חיל, "with God we shall triumph;" i.e. at a time when Israel performs the commandments, whereas צור ילדך תשי, Deut. 32,18: "You weakened the Rock Who brought you forth," i.e. you weakened God's powers by not observing His commandments.