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.. .