Is solar radiation a constant or does it vary periodically?

Solar radiation is not constant; it varies periodically due to several factors. These include the Earth's axial tilt and orbit around the Sun, which cause seasonal changes in solar radiation received at different latitudes. Additionally, solar output itself can vary due to solar cycles, such as the approximately 11-year solar cycle of sunspots and solar flares. Other factors, like atmospheric conditions and geographic location, also affect the amount of solar radiation reaching the Earth's surface at any given time.