This coin is found in the Cavern of the Gods in a corner of one of the corner rooms, found at A-4SW. This room is at the bottom-right corner of the map, and the coin's "blip" also contains two Striped White Sea Slugs. Nearby, a Tasselled Wobbegong should be swimming about.
It is part of two quests in Voice of the Night Sky: Solar System coins and Stars: Guidance of Apollo.
Stars: Guidance of Apollo
This is the only coin required to trigger Guidance of Apollo. When looking at it in the collection book, a voice comes over the radio mentioning "a cave with the same name." This cave is the Cavern of the Sun, a small, hard-to-find cave hidden in the cliffside of the Super Drop-Off, a little farther south than the Echoing Terrace, at co-ordinates F-4.
Within the Cavern of the Sun, one of the more notable features is a turtle skeleton near the far edge of the room. Once you have triggered the Guidance of Apollo quest, you should find a special tank called the Turtle Shell sitting within a glimmer atop this skeleton.
This coin is part of the Solar System quest, which must be completed in its entirety to unlock the hidden Cameroceras.