What is declination and how does it impact day length

Definition: declination is the angular distance of a point north or south of the celestial equator.

The declination angle, denoted by δ, varies seasonally due to the tilt of the Earth on its axis of rotation and the rotation of the Earth around the sun. If the Earth were not tilted on its axis of rotation, the declination would always be 0°. However, the Earth is tilted by 23.45° and the declination angle varies plus or minus this amount. Only at the spring and fall equinoxes is the declination angle equal to 0°. The rotation of the Earth around the sun and the change in the declination angle is shown in the animation below. It more or less affects the actual time of sun rise/set, but not the length of day

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explanation:

since the earth is flattened at the poles and bulges at the equator, geodesy represents the figure of the earth as an oblate spheroid. the oblate spheroid, or oblate ellipsoid, is an ellipsoid of revolution obtained by rotating an ellipse about its shorter axis.