Terms: hour circles, diurnal circles

On a Mercator projection of the globe the lines running up and down (confusingly called meridians) connect the poles and all points that have the same longitude. In the same way hour circles connect the poles on the celestial sphere. On the SC001 map hour circles run up and down the map.

On a Mercator projection of the globe the lines running side to side (called parallels of latitude) connect all the points that have the same latitude. In the same way diurnal circles are parallel to the celestial equator and run side to side on the SC001.

Both of these circles look like straight lines on the SC001, but that's the sort of distortion projection can produce. Hour circles really are great circles and diurnal circles (except the celestial equator) are small circles.

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