[__ Science __ ] How likely was the first day a 24-hour period?

[TonyChanYT]

Genesis 1:

2 Now the earth was formless and empty, darkness was over the surface of the deep, and the Spirit of God was hovering over the waters.

Darkness was first.

3 And God said, “Let there be light,” and there was light.

Light was next.

4 God saw that the light was good, and he separated the light from the darkness.

Darkness was followed by light.

5God called the light “day,” and the darkness he called “night.” And there was evening, and there was morning—the first day.

The usage of "night" here was related to the darkness in verse 2. The usage of "day" here was related to the creation of the light and not the sun. Here I define the first day as the time elapsed from Genesis 1:2 to 1:5.
Could the first day be defined as a 24-hour earthly solar day?
I doubt it. The flow of time is not absolute but a relativistic experience. It depends on the local gravity or acceleration exerted on the time observer. To my reading, the first day seems short. God was the observer, 2 Peter 3:

8 But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day.

This is not an equation but a simile. God is timeless and above time. There were no human observers there to experience the first day. The first day marked the creation of light and delineated it from the creation of the following day. These days were markers of milestones of achievements placed by God.

See also How old is the earth?.

 

[Who Me]

Yom hebrew for day with as in english a multiple of meanings, so is dependant upon the context for its use.