If #vec U# represents the velocity of the person P, relative to the center E of the Earth, and #vec V# represents the velocity of E relative to the center S of the Sun, both vary with respect to time, in both direction and magnitude.

The rotating-about-Earth's-axis with period 1 day, # vec U# is perpendicular to the Earth's tilt axis.

The revolving-about-Sun with period 1 year, #vec V# is in the Earth's orbital plane .

It is possible to find #vec U#, in terms of time and latitude of P. #vec V# is already known .So, at any time t of the calendar day, it is possible to find approximation to #vec U + vec V#. Of course, the day's average is #vec V#, nearly. .