The Total Irradiance Monitor (TIM) was launched in January 2003 aboard the NASA Solar Radiation and Climate Experiment (SORCE). TIM measures the total solar irradiance (TSI), which is the spatially and spectrally integrated solar radiation incident at the top of Earth's atmosphere. Relative changes in solar irradiance are measured to less than 10 ppm/year (0.001%/year), allowing determination of possible long-term variations in the Sun's output.