This is the teacher's edition of one of the eight units of the Intermediate Science Curriculum Study (ISCS) for level III students (grade 9). This unit focuses on the properties of sunlight, the use of spectrums and spectroscopes, the heat and energy of the sun, the measurement of astronomical distances, and the size of the sun. Optimal excursions, in addition to the activities, are described for students who wish to study a topic in greater depth. An introduction describes light, inverse squares, orbits, and relativity in the sky. Illustrations accompany the text. (SA)