10 This also is a part of the teaching of the Church, that there are certain angels of God, and certain good influences, which are His servants in accomplishing the salvation of men. When these, however, were created, or of what nature they are, or how they exist, is not clearly stated. Regarding the sun, moon, and stars, whether they are living beings or without life, there is no distinct deliverance.
Every one, therefore, must make use of elements and foundations of this sort, according to the precept, “Enlighten yourselves with the light of knowledge,” if he would desire to form a connected series and body of truths agreeably to the reason of all these things, that by clear and necessary statements he may ascertain the truth regarding each individual topic, and form, as we have said, one body of doctrine, by means of illustrations and arguments—either those which he has discovered in holy Scripture, or which he has deduced by closely tracing out the consequences and following a correct method.
Source: De Principiis (New Advent)