## 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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