Illustration of the toy model. The left panel represents the WASP-33 system; the right panel illustrates a zoom in on the planet. The curved black arrows indicate the rotation direction and the gray arrow indicates the orbital motion direction. We assumed the presence of TiO near the terminator(indicated as red and blue regions). Due to the planetary rotation, the TiO signature is red-shifted before eclipse (red arrow) and blue-shifted after eclipse (blue arrow). The orbital phase is ϕ and the geographical longitude is denoted with α (west limb at –90 °; east limb at +90 °; substellar point at 0 °).