"supermoon, which is the full or new moon that occurs when the moon is at or near its closest approach to the Earth, and which can make the moon appear" contains a sentence construction error: the subject "supermoon" never gets a verb, so we are left with a fragment. "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and can make the moon appear" suggests either that "a supermoon" can make the moon appear larger or that "the moon" can make the moon appear larger, but neither of these construals is acceptable, since a supermoon is the moon and one would never say that "the moon makes the moon appear larger." "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and appears" correctly suggests either that "a supermoon" appears much larger than the moon typically does or that "the moon" appears much larger than the moon typically does. In "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth, a position which causes itself to appear", the phrase "causes itself" does not work logically, since the position does not make itself appear larger. "supermoon, which is the full or new moon that occurs when the moon is at or near its closest approach to the Earth, causes the moon to appear" suggests that the "supermoon" causes the moon to appear larger, but that claim is illogical, since the "supermoon" is the moon. Note: "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and can make the moon appear" and "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and appears are both characterized by the same flexible/fuzzy subject-verb relationships. In "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and can make the moon appear", either "a supermoon" or "the moon" (first instance) could be legitimately posited as the subject of "can make"; in "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and appears", either "a supermoon" or "the moon" (first instance) could be legitimately posited as the subject of "appears." What's important is that whichever construct one favors, the reasoning above holds: "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and can make the moon appear" doesn't work and "supermoon is the full or new moon that occurs when the moon is at or near its closest approach to the Earth and appears" does.