Eastern Africa was cloaked in darkness when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite caught this image on October 3, 2005. The inky blackness that covers much of Ethiopia, Kenya, Uganda, and parts of Sudan and the Democratic Republic of the Congo in this image was caused when the Moon crossed in front of the Sun in an annular solar eclipse. During such an eclipse, the Sun is visible as a fiery circle around the black disk of the moon.
In the dimness beneath the Moon’s shadow, very little light remained for MODIS to capture this image. Under normal conditions, the land in the lower half of the image is a lush green, with patches of tan where the land is bare. Here, the tan areas have a red tint in the low light, while the green is completely black. Red dots show where fires were burning in vegetated areas. In the top half of the image, the orange of the Sahara Desert is shown in darker tones than normal. Only in the upper right corner of the image, where full daylight has returned, does the desert look normal.
Gradations of darkness within the shadowed area can be seen in the clouds. Bright white clouds reflect light well, so they are easily visible, even in the shadow. Since some light reached the Earth, the clouds remain bright along the outer edges of the shadowed region. As the eclipse progressed, the Moon’s shadow traveled southeast across the globe. When this image was taken, the deepest shadow lay over Kenya in the lower right corner of the image. Here the clouds are darker, an indication that there was less light to reflect back to the satellite.
If this had been a total solar eclipse, the shadow cast by the Moon would have been complete. The difference between a total solar eclipse and an annular eclipse is caused by the Moon’s orbit around the Earth. The Moon’s orbit is not circular; it is elliptical, like a squashed circle with the Earth at its center. This means that the Moon is not always the same distance from the Earth. When it is close to the Earth, the Moon appears larger than it does when it is far away. During a total solar eclipse, the Moon is closer to the Earth and so is able to block out the Sun entirely. An annular eclipse occurs when the Moon is farther from the Earth, and so the Moon’s disk is not large enough to cover the Sun.
The large image provided above has a resolution of 500 meters per pixel. It is available in additional resolutions from the MODIS Rapid Response Team. Tiny blue flecks in the large image are defects in the data.
References & Resources
NASA images courtesy the MODIS Rapid Response Team at NASA GSFC.
