The term 'PERTAINING TO THE SUN' can be summed up by the word 'SOLAR'. SOLAR refers to anything related to the sun, such as its energy, radiation, or light. SOLAR energy is used to generate electricity, while SOLAR radiation is used to study the sun's atmosphere. SOLAR light is used to measure the amount of sunlight that reaches the Earth's surface. SOLAR power is also used to heat water and provide hot water for homes and businesses.