Near the west equator, celestial objects move down in the sky.
The equatorial line itself is marked by a pole extending a short distance above and below ground (down to bedrock). The "Sunset" achievement will be granted the first time a blockhead reaches this pole. The pole itself functions as a back wall, allowing the placement of ladders, light sources, and blocks. There is also a chance that it can be found in a ocean.
The Sunset Skirt Of Happiness can be found at the top of the pole.