The electromagnetic spectrum. The human eye can only see a limited range of the spectrum, whereas satellites can register both the visible, the infrared and the radar ranges. The human eye can only perceive radiation within a limited interval of the electromagnetic spectrum. So instruments for remote sensing outside the visible wavelengths actually represent an extension of our visual field and they give access to additional information about the physical reality surrounding us. Reflected sunlight is for obvious reasons only measurable in daylight, whereas emission can be measured at all times.
Surface temperature is decisive for emission.
The sun has a surface temperature of 6000 K and maximum emmission in the visible light range. A surface with a temperature of about 1000 K, e.g. a fire in the Amazonas, has its maximum emmission in the middle infrared spectrum. The surface temperature of the earth is about 290 K and a emmission maximum around 14 micrometres, also called the thermal infrared range, see illustration . There is a direct correlation between surface temperature and degree of emmission at a given wavelength. The surface temperature can be calculated on the basis of remote sensing of the thermal infrared emmission.