STC mentions at line 210 of his famous adventure,
Till clomb above the eastern bar
The hornÚd Moon, with one bright star
Within the nether tip.
It is easy to scoff at such a description, since every star and all planets are farther away from the earth than the moon is. Hence, it is impossible for a bright star to shine within the horns of the crescent moon. Still, lots of people seem to have witnessed such an event, judging by the flags of many Islamic countries, for example.
I don't remember ever seeing the horns point upwards or downwards, but apparently such a phenomenon is possible from certain points on the globe, with specific atmospheric conditions.
Still, looking at the site below, it would appear possible to almost be accurate, were the moon to travel just a little farther (not further) to the west (click for larger image):
Did Coleridge view such an event personally, or merely report an image from his rather extensive imagination? No way to tell at present, no.
This adds some insight re latitudes
A listing of transient lunar phenomena:
Either that or he was hopped up on Nopium