Why do you need sensors to see if the light or heat is coming out of the earths surface?

Question: Why do you need sensors to see if the light or heat is coming out of the earths surface?
- Keywords:
  earth,
  light,
  sensor
Asked by Achsu to Silvia, Betty, Isaac, Kieran, Will on 8 Nov 2016.
- Silvia Pardo answered on 8 Nov 2016:
  
  We need sensors because the human eye cannot see all the different colors that light can have. Another reason is that using sensors on satellites is more efficient: satellites are very quick and they can map the whole planet in just one day, it would take years for a person to take enough pictures around the world to do the same!

Question: Why do you need sensors to see if the light or heat is coming out of the earths surface?