The 2nd Day God Creates The Firmament

God created the firmament on the second day, which he called heaven. There were upper waters and lower waters, above the firmament and below the firmament. Gen 1:6-8 Then God said, "Let there be a firmament in the midst of the waters, and let it divide the waters from the waters." Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. And God called the firmament Heaven. So the evening and the morning were the second day. The "firmament" here refers to the atmospheric heaven where the clouds are as well as the celestial heaven where the stars are.

Link: https://bible-history.com/old-testament/creation