Major circles of latitude and meridians

The style should depict the major circles of latitude and longitude: the Equator, Arctic and Antarctic circles, Tropics of Cancer and Capricorn, Prime Meridian, and perhaps the antimeridian (though the International Date Line is more conventionally seen in print atlases and globes).

A common treatment for these lines is a line in light blue, the same color as coastlines, with a matching label. I think we should display these lines with or without a proper graticule, but if we do show a graticule, then the tropics would need to be dashed rather than solid lines, since they don’t occur at the regular interval of any graticule.

These lines should probably fade away at higher zoom levels because of year-to-year fluctuations in the circles and tropics. If we end up showing the International Date Line, the de facto IDL would be far more recognizable to most Americans than the nautical line and consistent with the time zone boundaries proposed in #595.

Most of these lines aren’t mapped systematically in OSM, but we can get an approximation into a relatively small GeoJSON file and load it as a GeoJSON source.