The Sun moves slowly under the effects of the gravitational perturbations of the planets (mainly Jupiter and Saturn). This movement is shown here from a distance of 10 pc.
Astrometry consists of measuring the variations of the apparent position of the star on the plane of the sky (compared with the other stars). This apparent movement is an ellipse given by (for a planet with a circular orbit) :
We can also measure the distance variation between the star and the observer. The best method to measure this variation is to measure the time it takes for light to travel from the star. At a given time, this variation is :
The simplest way to measure this variation is to measure the time variations in the arrival of a periodic signal. Then is the delay or the advance of the arrival time of the signal compared with a "nominal" time (without perturbation). The periodic signals are of two types :
- the "pulse" of a pulsar
- the minimum time of an eclipse, in the case of a binary star