ABSTRACT
On an annual basis, given that the Earth travels in an elliptical orbit rather than a perfect circle, the solar constant varies by around ±0.3% due to the varying distance to the sun. Interestingly, the Earth is closest to the sun in January rather than a summer month. On a bit longer timescale, there is also a variation of ±0.04% as a consequence of the 11 year sunspot cycle. Finally, the sun has evolved dramatically during the course of its life that began an estimated 4.5 billion years ago. Its brightness has changed drastically over that time, and will do so again as it nears the end of its life when it approaches the red giant stage in perhaps another 4.5 billion years. It is believed, however, that the solar output has been relatively constant throughout the last 2000 years, with variations of no more than 0.1%–0.2%.
