An ultraviolet risk indicator instrument comprising a dial and a shadow projection device on it, the dial having colored areas that indicate different UV risks, defined by lines of declination (solstices and equinoxes) and solar hour lines calculated based on the transformation of the Hour Coordinates of the Sun (Hour Angle and Declination) in a 3D Cartesian Coordinate System, whose origin is the tip of the shadow projection device (being a polar style parallel to the rotation axis of the Earth, or a straight style perpendicular to the dial plane) using matrix calculations to produce the necessary rotations of the system, taking into account the latitude of the place, the declination and inclination of the dial. The style may be a rod or a triangular shape. The tip of the style is the focus or point of view of a gnomonic projection, where the Sun is the “point” of space to be projected, the dial is the projection plane and the projected point is the shadow point, whose coordinates (x,y) in the dial plane are calculated. Hour lines for local solar time in the dial are calculated, converging to the intersection of a polar style with the dial, the lines of declination of solstices and equinoxes and the position on the dial plane of the shadow cast by the tip of the style. The shadow point moves according to the annual and diurnal variation of the altitude of the Sun crossing different sectors of the dial.