See explanation.
At any instant -facing hemisphere of the Earth
has a zenith and a nadir.
The latitude difference for these points is nearly ##180^o##.
If there is no tilt the line of centers of the Earth and the Sun will
pass through the equator of ##0^o## latitude. So Sun’s rays will
strike normal to the equator casting no shadow at all at every noon.
Elsewhere the angle of in incidence of these ( parallel ) rays will
be the latitude for ever.. Noon-shadow length will be
(vertical height) X tan (latitude ). Latitude is ##90^o## at either
pole wherein the Sun will be at the horizon for ever.
The tilt of ##23.4^o##causes the oscillation of the solar rays between
the angles of incidence
##latitude – 23.4^o and latitude + 23.4^o##
during the orbital period of one year. This causes local seasons.. .