And God said, “Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons, and for days and years, and let them be lights in the expanse of the heavens to give light upon the earth.” And it was so. And God made the two great lights—the greater light to rule the day and the lesser light to rule the night—and the stars. And God set them in the expanse of the heavens to give light on the earth, to rule over the day and over the night, and to separate the light from the darkness. And God saw that it was good. And there was evening and there was morning, the fourth day.

Genesis 1:14-19 (ESV)

Then God said, “Let bright lights appear in the sky to give light to the earth and to identify the day and the night; they shall bring about the seasons on the earth, and mark the days and years.” And so it was. For God had made two huge lights, the sun and moon, to shine down upon the earth — the larger one, the sun, to preside over the day and the smaller one, the moon, to preside through the night; he had also made the stars. And God set them in the sky to light the earth, and to preside over the day and night, and to divide the light from the darkness. And God was pleased. This all happened on the fourth day.

Genesis 1:14-19 (TLB)
This chronicles a further change of the sky from a translucent atmosphere, where the light from the sun, moon, and stars are weak and diffuse, to a transparent atmosphere, where the light from the sun, moon, and stars can be seen clearly and bring about the seasons and mark the days and years.

Probable Time Frame ~580 mya to 575 mya

The scientific evidence for the clearing of the atmosphere comes from records for an increase in oxygen in the atmosphere and a decrease in methane. This article shows how modeling these changes in oxygen and methane levels leads to a clarity in the atmosphere that would allow the sun, moon, and stars to be visible.

Formation of the ozone layer to protect life from harmful UV radiation ( ~580 mya )

Videos about Day 4 of Creation

YouTube TitleDescription
Early Earth AtmosphereFirst earth atmosphere. Collision, moon, Hadean. Cynobacteria. Banded iron formations.
Evolution of Earth's AtmosphereEvolution of the atmosphere of the earth with chemical composition and the influence of temperature dependent reactions, volcanic activity, and living organisms.
Evolution of the Earth's AtmosphereAnother description of the chemical composition of the atmosphere of the earth and how that changes during earth's evolution.