The only explanation in RE theory for different day lengths is an elliptical orbit of the Earth around the Sun. However, is this just an example of circle logic (no pun intended) or retrofitting an explanation to fit an observation?
The elliptical orbit is not the explanation for different day lengths. Who has ever given that explanation? Can you link to someone who actually gives that as an explanation?
The explanation is a sphere which is tilted at 23.5°. That tilt explains the seasons, the planetary ecliptic, the solar ecliptic, the change in length of days, the changes in the azimuth change of sunrise and sunset. The elliptical orbit explains the subtle changes in angular diameter of the sun throughout the year, the reason why summer and winter are not equal in length, and why the solar analemma isn't symetric.