The short comparison
Search traffic treats "Greek sun god" as one job with two applicants. Ancient Greeks did not always agree. In Homer and Hesiod, Helios (Homeric Greek often Ἠέλιος) is the daylight sun that rises, sees everything, and sets. Apollo (Ἀπόλλων) is Leto's son, the Delphic archer and musician, invoked for plague and purification long before poets casually call him the sun.
MythCairn already covers Helios's double label as god and Titan and Apollo's Delphic career on the Apollo page. This article is the side-by-side: domains, parents, transport, cult centers, and the date range of conflation. If you only need one sentence for a quiz: Helios is the sun; Apollo became a sun god by identification, not by birth story.
What Helios does that Apollo does not own in Homer
In the Odyssey, Odysseus's crew slaughter Helios's sacred cattle on Thrinacia after Circe's warning. Helios demands punishment; Zeus wrecks the ship. That plot needs a sun god who keeps herds and watches from above. It does not need Delphi's archer.
Helios is the witness who tells Hephaestus about Ares and Aphrodite in Odyssey book 8, and who tells Demeter what happened to Persephone in the Homeric Hymn to Demeter. Oath scenes in the Iliad call on Helios because the sun sees. Those are solar-witness functions, not Apolline oracles. None of that portfolio migrates to Delphi simply because both figures can be called bright.
Genealogy fixes the cast: Hesiod's Theogony makes Helios brother to Selene (Moon) and Eos (Dawn), child of Hyperion and Theia. Apollo's standard tree is Olympian. Calling both "sun gods" without that split erases why Greeks could worship a Titan sun and an Olympian healer in the same century.
What Apollo owns that Helios never monopolized
Apollo's brand is the sanctuary and the arts of order: the Pythia at Delphi, the bow that brings plague in Iliad 1, the lyre, and later medical associations shared with Paean/Asclepius language. Helios can be called Paean in some Orphic and magical texts, but that is syncretic layering, not Homer's default.
Apollo also absorbs other light metaphors. Epithets like Phoebus ("bright") made Latin poets comfortable using Phoebus for the sun chariot. Renaissance and later art often paint "Apollo" driving solar horses. That reception history is real; it is still a reception, not a Homeric identity card.
When Greeks started saying they were the same
Walter Burkert's line is useful: different names may refer to the same being, or they may be consciously equated. Apollo's link to the physical sun is attested by the fifth century BCE, but widespread, consistent conflation is later and uneven.
A famous early literary fuse appears in surviving fragments of Euripides's tragedy Phaethon, where Clymene addresses Helios and says he is "rightly called Apollo among men" who know the unspoken meaning of gods' names. James Diggle reads the joke through Apollo's folk etymology as "Destroyer": Helios destroyed her son, so the name fits. That is witty identification, not a claim that Bronze Age Greeks already ran one cult under two doorplates.
Hellenistic and Roman writers lean harder on the merge. Orphic and philosophical texts play with Helios-Apollo (and sometimes Dionysus or Asclepius). Emperor Julian later centers a philosophical Helios that can absorb Mithras and other solar figures. Meanwhile Rhodes still treats Helios as its own patron; the Colossus was Helios, not a Delphic Apollo statue with a day job.
How to read modern "sun god" lists
If a textbook says "Apollo was the Greek sun god," ask which century and which genre. For reading Homer, Hesiod, or the Odyssey cattle episode, answer Helios. For a Roman poem that says Phoebus mounts the chariot, the poet may mean the sun under Apollo's nickname.
If the question is cult practice, keep the centers straight: Delos and Delphi for Apollo; Rhodes and various solar altars for Helios. Syncretism happened, but Greek religion was comfortable with overlapping light gods the way it was comfortable with multiple storm or sea figures. A reader who only meets Apollo-as-sun in a modern textbook is seeing the end of that process, not the Homeric starting line. Keep Phaethon with Helios, keep the Pythia with Apollo, and treat equations as labeled historical moves rather than silent substitutions.
Related MythCairn reads: Helios's Titan-or-god status, Apollo at Delphi, and the Ares and Aphrodite affair Helios reports in Ares and Aphrodite as lovers.