The Earth's climate is determined by energy transfer from the sun to the Earth's land, oceans, and atmosphere. As the Earth moves around the sun, the fact that the Earth's axis is tilted means that the sun's overhead position moves from the Northern Hemisphere to the Southern Hemisphere and back from one summer to the next. This effect causes winters to be cold and summers warm in the Northern Hemisphere and the opposite in the Southern Hemisphere. This animation shows the incoming solar irradiance on the Earth at noon on the Greenwich meridian during an entire year, illustrating this movement. The magnitude of this irradiance comes from measurements by the TIM instrument on SORCE. Since the Earth's orbit is elliptical, the magnitude of the solar irradiance at the Earth is least when the Earth is farthest from the sun and greatest when the earth is closest. This 6 or 7 percent change can be seen in the animation by watching the dark bands move. When the bands expand from the bright spot, the Earth is getting closer to the sun, from July through December, and when they contract the Earth is moving away, from January through June. The sun's irradiance is also variable from day to day, but that effect is about ten times smaller than the effect of the earth's orbit.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 22.214.171.124.0