3 A great and honourable man was Abraham, but only great in comparison with men; and when he came before God, then speaking the truth candidly he says, I am earth and ashes. He did not say 'earth,' and then cease, lest he should call himself by the name of that great element; but he added 'and ashes,' that he might represent his perishable and frail nature. Is there anything, he says, smaller or lighter than ashes? For take, says he, the comparison of ashes to a house, of a house to a city, a city to a province, a province to the Roman Empire, and the Roman Empire to the whole earth and all its bounds, and the whole earth to the heaven in which it is embosomed;— the earth, which bears the same proportion to the heaven as the centre to the whole circumference of a wheel, for the earth is no more than this in comparison with the heaven: consider then that this first heaven which is seen is less than the second, and the second than the third, for so far Scripture has named them, not that they are only so many, but because it was expedient for us to know so many only.
And when in thought you have surveyed all the heavens, not yet will even the heavens be able to praise God as He is, nay, not if they should resound with a voice louder than thunder. But if these great vaults of the heavens cannot worthily sing God's praise, when shall 'earth and ashes,' the smallest and least of things existing, be able to send up a worthy hymn of praise to God, or worthily to speak of God, that sits upon the circle of the earth, and holds the inhabitants thereof as grasshoppers.
Source: Catechetical Lectures (New Advent)