Seasons from Space
December 21, 2012
Summary Author: Maximilian Reuter
Moments from Space Web site
The image sequence above shows the Earth as seen from a geostationary satellite perspective approximately 22,370 mi (36,000 km) above the Equator at 0 degrees longitude (Greenwich or Prime Meridian). These images were captured at the spring and autumn equinoxes and the summer and winter solstices. All four images were acquired by EUMETSAT at 06:00 UTC, with the Sun illuminating the eastern half of the Earth. The Earth’s axis is tilted by about 23.4 degrees relative to the elliptical orbit of the planet as it revolves around the Sun. March 20 or 21 and September 22 or 23 are days of equinox and define the beginning of meteorological spring and autumn. June 20 or 21 is the day when the meteorological summer begins in the Northern Hemisphere. On this day and for the next three months, the Northern Hemisphere is tilted toward the Sun. The opposite is true on December 20 or 21 when the meteorological winter begins. However, the distance to the Sun is not the reason for the seasons. Actually, due to the elliptic orbit of the Earth around the Sun, the Earth is about 3.1 million mi (5 million km) closer to the solar disk in January than in July. As illustrated above, at 06:00 UTC, an imaginary line from the satellite to the equator would be perpendicular to a line connecting the Sun and Earth, and so regardless of the season one-half of the Earth is illuminated by the Sun. Thus, even though large portions of central Europe are still shadowed at 06:00 UTC in winter (lower left) while being sunlit in summer (upper right), in both cases half of the Earth is illuminated.
Image details: MSG satellites are operated by EUMETSAT and developed in close cooperation with the European Space Agency (ESA). The primary instrument on MSG is the Scanning Enhanced Visible and InfraRed Imager (SEVIRI), which measures light in 11 spectral channels from the visible to the thermal infrared spectral region. The resolution of most channels is about 1.8 mi (3 km) at the nadir point, directly below the satellite. Above images are composites of the detection of three specific spectral channels of SEVIRI combined with NASA’s "Blue Marble Next Generation” data. This combination enables the generation of true-color images from satellite instruments optimized for meteorological observations.