This question comes up quite frequently in my conversations with Christians who subscribe to an old earth (i.e. having an age on the order of billions of years). I've got a 1/2-hour presenation on this topic that you can watch on my YouTube channel that goes into detail, but I thought I'd write this quick blog entry to give a quick-and-dirty summary.

What Are the Basic Ingredients of a Day?

A day, also known as a solar day, is simply the amount of time it takes for our earth to make one complete, 360° rotation about its axis. We mark the passage of these days by the appearance of the sun in sky–we know a day has passed when the sun appears in the same place in the sky. In order for this to happen, we need, quite simply, two things:

1. A rotating planet
2. A point source of light

Do we have these ingredients before Day 4 of creation (Genesis 1:14-19)? Let's take a look...

A Rotating Planet

Genesis 1:1-2 says:

In the beginning God created the heavens and the earth. The earth was formless and void, and darkness was over the surface of the deep, and the Spirit of God was moving over the surface of the waters.

So we clearly have the earth--one that is completely covered by water (cf. 2 Peter 3:5).

The rotational behavior of the earth is not explicitly stated here, but it does not seem unreasonable to infer this based upon what follows.

A Point Source of Light

Genesis 1:3 says:

Then God said, “Let there be light”; and there was light.

But apparently, this light was everywhere all at once and illuminated the entire surface of the earth uniformly, because in the next verse, we see:

God saw that the light was good; and God separated the light from the darkness. (Genesis 1:4)

If the light were already coming from an isolated (point) source, then there would be no need to separate it from darkness.

"There was evening and there was morning..."

In Genesis 1:5, we see these two "ingredients" come together:

God called the light day, and the darkness He called night. And there was evening and there was morning, one day.

This is where the rotation of the earth is inferred--the rotation of the earth about its axis, coupled with the point source of light (i.e., "light separated from darkness") gives us periods of alternating darkness and light on the surface of the earth, which God Himself defines as "one day."

Where'd the Light Come From?

A natural follow-up question is "Where did this light come from? The sun doesn't appear for another three days!"

Good question!

The best commentary on the Bible is the Bible, so let's see what it says about days and nights in the New Jerusalem, when God renews creation and restores it to perfection:

And the city has no need of the sun or of the moon to shine on it, for the glory of God has illumined it, and its lamp is the Lamb. The nations will walk by its light, and the kings of the earth will bring their glory into it. In the daytime (for there will be no night there) its gates will never be closed; (Revelation 21:23-26)

And there will no longer be any night; and they will not have need of the light of a lamp nor the light of the sun, because the Lord God will illumine them; and they will reign forever and ever. (Revelation 22:5)

Because we see on the lines in Revelation that God's glory will give light to the new Jerusalem so that there will be no need of a flaming ball of hydrogen 93 million miles away to illumine it, it seems quite reasonable to read between the lines in Genesis 1 that His glory was also the light that illumined the earth from Day 1 through Day 3.