Synodic period of planets Many books explain the difference between synodic and sideral periods. The synodic period of planets are tabulated for centuries and it is defined as the time interval between two consecutive repetition of phase, phase in this case meaning the terrestrial measure of its angular position and speed relative to the Sun.

My question is, how do astronomers measure the angular position of a planet relative to the Sun if the major observations are made during the night?
I'm pretty far from an expert at ancient astronomy, but as I understand it, the ancient astronomers looked for the date when the planet was highest in the sky.   This is called being in opposition or opposite the sun.



Source of image

See here for further explanation.

I've also read that the synodic period can be calculated between periods of retrograde.   In neither case, the position of the sun is necessary.   

The synodic period, because it was initially based entirely on observation, works for both Geocentric and Heliocentric models.   

"Position of the sun" infers our current understanding of the heliocentric system.  The triangle, between the 3 points, Earth, Sun and observed planet and that only works for the heliocentric model.  Most of the ancient astronomers didn't use that model.  Their synodic period was based entirely on the motion of the planet relative to the fixed background stars.