ויעש אלהים את־הרקיע ויבדל בין המים אשר מתחת לרקיע ובין המים אשר מעל לרקיע ויהי־כן׃
And God made the firmament, and divided between the waters which were under the firmament and between the waters which were above the firmament. And it was so.
The word translated in this verse as "created" is ויעש, a form of the verb עשה. The verbs עשה and ברא appear often in the Genesis 1 creation account, and they seem almost interchangable. So, why did the Author use the 3MS vav-consecutive imperfect of עשה instead of the 3MS vav-consecutive of ברא?
Day 1: Neither עשה nor ברא is used in describing creation of the light, but instead we have yehi owr wa-yehi owr ('Light shalt exist.' And light existed.)
Day 2: עשה is used to describe creation of the firmament
Day 3: Neither verb is used for God's creation of plants, but instead the earth "sprouts" (דשא) the vegetation. The verb עשה, however, is used to describe the tree "bearing" fruit (perhaps, in its own way, "creating" fruit?)
Day 4: עשה is used to describe creation of the luminaries
Day 5: ברא is used to describe creation of the sea-animals and sky-animals
Day 6: עשה is used to describe creation of the land-animals. עשה is also used when God says, "Let us create man in our image." But then, we have ברא used 3 times when God says "So God created man in his own image, in the image of God created he him; male and female created he them."
In these observations, however, I fail to see a pattern which might explain the use of עשה on Day 2.