The baseline paradox. Consider a thought experiment where an interferometer is equipped with two masks that rotate in the pupil of both telescopes. By doing so, the distance between the two apertures changes with time while the telescopes stay firm. In the left panel, the observation is done on a single target, acquired on the axis of the telescopes (perpendicular to the plane of the aperture). The OPD measurement is done between the white-light fringe and the zero reference of the interferometer. The OPD does not change with the rotation of the masks. In the right panel, the interferometer is recording two fringe packets obtained by observing two stars in the same field. The difference in OPD is measured between the two white-light fringes. δOPD does change with the rotation of the masks.