Why is it hot?

Answer 1

It isn't really all that hot. The parts that are especially warm get lots of fairly direct sunlight, and there are large areas of the Earth that only get sunlight at fairly low angles and which are pretty darn cold by comparison.

And just as we have warm periods in the summer and cold times in the winter, the Earth has much longer seasons as well. In Roman times and in the late middle ages (from about 900 AD to around 1300AD) we had fairly long periods of warmer-than-average temperatures, and between about 400 AD to 800AD, and from 1400 to about 1800, we had much colder than average temperatures. The Earth has been warming for the last century or so, and is still not as warm as the period called the "Medieval Warm Period" around 1200AD.

Of course, we've also had prolonged "ice ages" in which the world became VERY cold and stayed that way for 30,000 years at a time. At this point, we cannot know if the world is getting warmer or colder, and whether the present moderate weather will continue or if we will go into another ice age.

Answer 2

The Earth receives heat from the sun, which warms up the surface of the planet. Additionally, factors like greenhouse gases in the atmosphere trap heat, contributing to the overall temperature increase on Earth.

Answer 3

Because it's temperature is greater than surrounding temperatures.

A candle is hot compared to ice, but cold compared to say lava.