Geometry of the stellar sphere and spot feature. The origin of the coordinate system is the center of the star and spot parameters are defined using spherical coordinates. The gray circle on the stellar surface represents a spot at the distance rs (here: stellar radius in pixels) from the origin. The longitude θ (blue) can have values between − 90° and 90°, whereas 0° represents the center of the stellar disk. The co-latitude φ (purple) is defined between 0° to 180°, with the equator set at 90°, as seen from an observer lying far along the z-axis.
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