Not part of the general conversation but I can answer this one. Basically, God created a light source on Day One, which was replaced with the sun on Day Four.
Full reasoning:
We may learn several things about the light God created on Day One (
Genesis 1:3–5). First, it was a created light, that is, the light did not exist one moment, but it existed the next moment. That light was not eternal, like God, even though “God is light” (
1 John 1:5), “the light dwells with him” (
Daniel 2:22), and he “dwells in unapproachable light” (
1 Timothy 6:16). This light of Day One seems to be separate from God himself.
Second, in order for there to be distinct daytime and nighttime, this light must have been localized and therefore directional (not a diffused or ambient light), and it must have been stationary relative to the earth. For “morning and evening” to have occurred successively, the earth must also have been rotating on its axis from Day One, allowing part of the earth to be exposed to the light while the opposite side was in the darkness.
Third, the light possibly also provided adequate heat to warm the earth, allowing water to exist in liquid form. God separated the “waters . . . from the waters” (
Genesis 1:7), and he gathered the surface water into seas (
verses 9–10). Heat from this light or another source would also be necessary for the plants, trees, and other vegetation prior to the creation of the sun on Day Four.
Fourth, this initial, temporary light was evidently replaced with the sun on Day Four. On several occasions we see this pattern of temporary physical realities being removed. The pillar of fire and cloud that led Israel to the Promised Land was a temporary provision from God (
Exodus 13:21–22,
40:34–38). So also was God’s gift of manna from heaven to feed the Israelites during their wilderness wanderings (
Exodus 16:4,
31,
35). And there was the star that led the magi to the child Jesus (
Matthew 2:1–10).
Plants certainly need light to survive, but there are several other necessary elements that God provided in his creation.
Light
God created plants to generate their energy from visible light by the process of photosynthesis. Chlorophyll in plant cells mainly absorb blue and red wavelengths, and largely reflect green wavelengths. Using light chlorophyll transforms carbon dioxide and water into sugars (made of hydrogen, carbon, and oxygen atoms). I already posited that God had already created a light source separate from the sun, so the plants created on Day Three would have had the light they needed.
Moderate Temperature
Most plants require a moderate surface and air temperature. Given a relatively constant distance from the sun, this temperature is primarily affected by the earth’s rotation and tilt. Because of the earth’s timely rotation, the heat generated during the day regulates and is regulated by the coolness of the night, thus providing a moderate temperature. Also, the tilt of the earth’s axis of rotation produces the tempering effects of seasons, allowing growing seasons and dormant seasons.
Atmosphere
God created the atmosphere on Day Two, thus protecting and providing for the plant life he subsequently created. The atmosphere protects plants from ultraviolet light and other cosmic rays which damage living cells. The air also provides some of the elements needed for plant life, including nitrogen and carbon dioxide.
Water
Plants need water to survive (
Genesis 2:5–6). Plants use water as a component of photosynthesis and as a medium to transport nutrients throughout the different parts of the plant. They also use water to cool the plant from the sun’s heat, and water fills the structure of plants to give them shape and support. In that first week of the world, the water vapor in the air as well as surface water contributed to form a suitable habitat for these organisms.
Land
God separated water and land on Day Two, creating a suitable habitat for both aquatic and land plants. The ground provides a stable location for a plant’s root system, and plants return the favor by helping soil against erosive factors like water and wind. The land also holds many nutrients that a plant needs, such as water and nitrogen, which are collected by the plant’s roots.
In the Genesis account, God recorded that he created the heavens and the earth, but he didn’t tell us why he followed this order. We may conjecture two possible reasons why the sun was not created on Day One. First, God may have wanted to underscore the supernatural origin of life, clearly showing that life did not come from the sun but from him. To be sure, in God’s design the sun is critical for the continuation of life on earth, but life on earth did not come from the sun.
Second, God may have wanted to undermine humanity’s inclination to worship the sun as the originator of life, by which they would have regarded the sun as a deity. God specifically forbade his people from worshiping “the sun or the moon or any of the host of heaven” (
Deuteronomy 17:3; cf.
4:19;
Psalm 121:5–6).