The greater light here is obviously the sun. Some point out, rightly, that the moon does not generate its own light, it just reflects it from the sun, and that this verse is, therefore, inaccurate. In response I would point out that, whatever the ultimate source, the moon is the major source of light in the night. The idea that this verse is intending to argue that the moon generates light via chemical reactions is very obviously off base.
Targum Onkelos: And the Lord made the two great luminaries: the greater luminary to rule in the day;* and the smaller luminary to rule in the night, and the stars. *Samaritan Version, “the plentitude of the greater light.”
Rashi: the two great luminaries: They were created equal, but the moon was made smaller because it brought charges and said, “It is impossible for two kings to use the same crown.” – [from Chullin 60b] Rashi (ad loc.) explains that this derash is based on the discrepancy of the two expressions, “the two great luminaries,” which intimates that the moon was a great luminary, and“the lesser luminary,” which intimates that the moon was smaller than the sun. To reconcile this difference, the Rabbis asserted that the moon was originally created equal to the sun, but, because of its complaint that the sun wielded the same power that it wielded, it was forced to relinquish that power.
and the stars: Because He diminished the moon, He increased its hosts, to appease it. – [from Gen. Rabbah 46:4 and Chullin 60b] i.e., The stars serve as the entourage of the moon. When it comes out, they accompany it, and when it sets, they too set. [Gen. Rabbah ad loc.]
Rambam: God in-formed the realm of the spheres with the forms of the two large crystal bodies, the larger crystal body to rule the day and the smaller crystal body to rule the night; he also in-formed the realm of the spheres with the forms of the stars and planets.
Ibn Ezra: God made the two great lights — the sun and the moon — which are great by contrast with the stars. The moon, though smaller than the stars, is closer and hence its light is greater and it too is, therefore, called “great.” The sun rules during the day when it is seen and the moon rules during the night when it is seen. God also created the stars.