With a baby you just stick a thermometer in its mouth and wait thirty seconds. But the Earth is big, doesn't have a mouth and cant say ahh. So how do you measure the average temperature?

Lots of thermometers spread evenly across the surface of the Earth might help. Are they all working? Are the ones in Bangladesh as accurate and reliable as the ones in Moscow? When I watch the local weather reports on TV, it seems there are always one or two stations reporting unusual temperatures and one or two more not reporting at all. And these are 21st century state-of-the-art thermometers.

How do you average out night and day and winter and summer. Are some measurements more important? What about elevation? How do you compare measurements in mile high Denver to sea level New Orleans? Do you need to measure the temperature at various altitudes or is the surface level temperature the only important measure? Are ocean temperatures important?

Recently satellites have been used. How does that work? How well do those results compare to traditional readings?

Has is been measured accurately over the past hundred years? Is the data in our possession being affected by other factors like city sprawl?