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Referring back to the tutorial section about the electromagnetic spectrum, there is a certain band of energy that our eyes can detect, called visible light. Just beyond the red end of that range, at slightly lower energy (longer wavelength) than what our eyes can see, is a section called the infrared. Our bodies can detect some of the more moderate wavelengths of infrared. If you stand next to a wood fire, you feel "heat" radiating from the flames. This is actually the infrared radiation emitted by the burning wood. The NOAA polar-orbiting satellites carry carefully calibrated, very sensitive instruments that measure infrared radiation. They are basically "feeling" the heat that radiates from the ocean surface, even though they fly about 850 km (530 miles) above the ocean! The instrument used by Coral Reef Watch is called Advanced Very High Resolution Radiometer (AVHRR). If you want to learn more about this "radiometer" sensor and how it measures ocean temperature see these references: NOAA's description of the AVHRR instrument
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