An observer located on the terrestrial equator is said to be at sphaera recta, for the stars rise perpendicularly to the eastern horizon. The celestial equator bisects the visible sky into equal northern and southern portions, passing east to west through the zenith.
For an observer on the equator, looking east, the celestial
equator would appear to rise perpendicularly (as shown by the dotted
line, labelled in hours of right
ascension): 
What a sight it would be to see Mintaka rising at a 90-degree angle to the horizon and then crossing directly overhead!
Set the planetarium to sphaera recta and observe the diurnal path of Orion.