We think in very scientific terms in the modern world. The sun is a ball of gas and the moon is a ball of rock. However, we see in Genesis that the reason the sun and moon were created was to mark time. Ancient people were not as concerned with what the heavenly bodies were made of so much as their purpose. The Psalmist invokes their purpose, showing how the sun and moon mark the days and seasons for both man and animals.
Thou hast made the moon to mark the seasons; the sun knows its time for setting. Thou makest darkness, and it is night, when all the beasts of the forest creep forth. The young lions roar for their prey, seeking their food from God. When the sun rises, they get them away and lie down in their dens. Man goes forth to his work and to his labor until the evening.
Christ is risen from the dead,
By death trampling down on death,
And to those in the tomb,
He has granted life!