Late last Friday evening as I wasted a couple of hours trying in vain to buy 2 tickets from the London Olympics website, I filled my time looking further at the SORCE TIM solar irradiation data . The olympics website turned out to be broken but all was not in vain because instead, I had a brainwave about what causes the apparent regular small oscillations in the data. It is the Moon of course ! In reality the Moon and the Earth orbit each other around their common centre of mass. This is located about 4000 km outside the center of the Earth so as a result the Sun-Earth distance also changes in phase to the lunar month. The exact monthly variation in distance (D) will depend on orbital parameters but could be as much as 8000km. The data clearly show a monthly variation of about 0.2 watts/m2 (figure 1) during 2008-2009 which turns out to be in phase with the lunar cycle. There is a negligible probability that this occurs by pure chance – see below.
As far as I am aware this is the first time this effect has been seen as I cannot find any other reference to in the literature. Apologies if I am mistaken.
My original aim in studying the TIM data was to try to isolate a possible (annual) signal as the Earth passes through plane of the interplanetary dust cloud. Could ice ages be really caused by an oscillation in dust density lying between the Earth and the Sun in phase with the eccentricity of the earth’s orbit ? But that is another story…