# How are various orbital period measured?

How are the sidereal periods and synodic periods of celestial bodies measured from earth? The usual definition of the sidereal period of a celestial body is the time it takes to complete one revolution around the sun, relative to some other fixed point outside the solar system. I assume that relative to a fixed point here means the references frame of that point.

Is there a way to measure this sidereal period while still staying in the reference frame of the earth? For example, a sidereal day can be measured by noting the time between the first appearance of a distant star on two consecutive nights.

Similarly, what are the ways for measuring synodic periods?

• Take an accurate clock, a sextant and measure meridian passage times. Sep 8 '20 at 22:09
• @planetmaker that's not at all helpful. The question is about the sidereal period of the actual orbit not of the apparent position. "From Earth" refers to the observation point, not the reference system. The question clearly states "The usual definition of the sidereal period of a celestial body is the time it takes to complete one revolution around the sun, relative to some other fixed point outside the solar system."
– uhoh
Sep 8 '20 at 23:51
• @ThePixelGenome it's an interesting question. I'm pretty sure that steps include measuring apparent positions relative to stars over an extended period of time followed by a fitting procedure to calculated orbits in the solar system for Earth and the body of interest. One adjusts the initial conditions of the orbit of the body before each propagation, uses it to predict the apparent positions, checks against measured apparent positions, then adjusts again to improve the fit in an iterative way...
– uhoh
Sep 8 '20 at 23:56
• ... and positions for the Earth can be taken from an ephemeris like this.
– uhoh
Sep 8 '20 at 23:56