The equator and the lines of longitude.
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On the earth (and planetary/stellar bodies) they are called latitudes and longitudes.
Latitude lines are imaginary lines that are parallel to the equator to fix a position on earth (along with longitude lines which are imaginary lines perpendicular to the equator)
normal lines or normals
A circle has an unlimited number of lines of symmetry. These lines are called the diameter. Why does a circle have an unlimited number of lines of symmetry? It's because a circle has a constant diameter regardless of where the diameter is measured from.
Radius And Diameter.