Keeping Up with the Sun
From its vantage 824 kilometers (512 miles) above Earth, the Visible Infrared Imager Radiometer Suite (VIIRS) on the NPOESS Preparatory Project (NPP) satellite gets a complete view of our planet every day. This image from Nov. 24, 2011, was the first complete global image from VIIRS.
Rising from the south and setting in the north on the daylight side of Earth, VIIRS images the surface in long wedges measuring 3,000 kilometers (1,900 miles) across. The swaths from each successive orbit overlap one another, so that at the end of the day, the sensor has a complete view of the globe. The Arctic is missing because it is too dark to view in visible light during the winter.
The NPP satellite was placed in a Sun-synchronous orbit, a unique path that takes the satellite over the equator at the same local (ground) time in every orbit. So, when NPP flies over Kenya, it is about 1:30 p.m. on the ground. When NPP reaches Gabon—about 3,000 kilometers to the west—on the next orbit, it is close to 1:30 p.m. on the ground. This orbit allows the satellite to maintain the same angle between the Earth and the Sun so that all images have similar lighting.
The consistent lighting is evident in the daily global image. Stripes of sunlight (sunglint) reflect off the ocean in the same place on the left side of every swath. The consistent angle is important because it allows scientists to compare images from year to year without worrying about extreme changes in shadows and lighting.
PHOTOS: Sunsets and Other Sky Wonders