Or, as the Soncino (commentary) says: "You who rehearse Your majesty in the heavens." That is kind of interesting. We will get to that in just a minute.
"How majestic," David says, "is Your name!" The majesty of God is revealed in His creation, and one of His names is Creator.
What is also implied, in this same context, is that the same glory of that same God is equally shown on the earth, as well. So we have, day and night, an awesome display of the power and the majesty of our God. What else do we see there, besides power and majesty? We see order. Awesome beauty that is constantly changing and shifting—so that no two sunsets are ever exactly alike.
We see loving providence, as He provides for His creation every day. Awesome wisdom as everything is balanced and in harmony—except where man has gotten things disharmonious. We see reason and logic, and we see vastness of thinking—of this great God that we worship.
This psalm is intended to direct yours and my thinking toward the vastness of this God and His majesty as compared to the puny insignificance of you and me. And yet that great, awesome God has it in mind—it is His purpose—to glorify Himself in us. Yes, "out of the mouths of babes and infants." This is where that principle in I Corinthians 1:26 comes from. God has called the weak of the world—to glorify Himself in. He has chosen that which is weak and foolish (by this world's standards) to come to appreciate and to respect the glory of God—the glory that is in His name.