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I'm reading my first book on physics "Seven brief lessons on physics" by Carlo Rovelli, and in his chapter on "Probability, time and heat of black holes", he mentions that Stephen Hawking showed that black holes are hot, they emit heat. My question is, does the earth emit heat?

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    $\begingroup$ Yes, Google "blackbody radiation" $\endgroup$ – MaxW Sep 25 '18 at 18:54
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    $\begingroup$ From what I learnt, all objects with temperature above the absolute zero temperature(0K) emit energy in the form of electromagnetic radiation. $\endgroup$ – Mohan Sep 25 '18 at 18:56
  • $\begingroup$ Stellar mass and larger black holes aren't very hot. They're currently colder than the CMBR, so they absorb more heat than they radiate. Take a look at this Hawking radiation calculator, in particular the temperature and luminosity figures. $\endgroup$ – PM 2Ring Sep 25 '18 at 20:11
  • $\begingroup$ It also emits heat due to radioactive decay inside itself. But energy from the sun is the largest component by far. $\endgroup$ – zeta-band Sep 25 '18 at 21:05
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Yes; but not for the same reason as a black hole.

Every object emits electromagnetic radiation with an energy corresponding to it's surface temperature.

Earth emits heat; but it also absorbs heat from radiation. In the light of the sun for example, more radiation is absorbed by the planet than emitted, so it heats up; but the Earth is still emitting that thermal radiation.

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  • $\begingroup$ It may be worth clarifying that the part of the Earth that faces the Sun heats up when exposed to sunlight, but the Earth as a whole absorbs and emits approximately equal amounts of thermal radiation, and so is in rough thermal equilibrium and its average temperature is roughly constant. $\endgroup$ – tparker Sep 25 '18 at 19:44
  • $\begingroup$ Whether the Earth is currently heating up or cooling down depends on your time scale. In a scale of hours, it's cooling down, because it's currently 20:00 UTC and therefore night in most of the Eastern Hemisphere, which contains most of the Earth's landmasses and therefore dominates its temperature fluctuations. On a scale of months it's also cooling down, because it's fall in the Northern Hemisphere, which similarly contains most of the Earth's landmasses. On a scale of decades, it's warming up because of anthropogenic climate change. On a scale of hundreds of millenia, it's cooling ... $\endgroup$ – tparker Sep 25 '18 at 19:51
  • $\begingroup$ ... down because of the Milankovitch cycles, which will eventually cause the current Holocene interglacial epoch to end and return the climate to a colder, Pleistocene-like epoch. $\endgroup$ – tparker Sep 25 '18 at 19:54
  • $\begingroup$ @tparker On a scale of months, is landmass still the most important parameter? I'm thinking that on that timescale, the temperature of the water would be pretty significant as well, no? Or is it so much of a thermal sink that the timescale essentially washes out even with seasons? $\endgroup$ – JMac Sep 25 '18 at 20:15
  • $\begingroup$ Water has much lower thermal absorbance than land, so the Earth's temperature is relatively insensitive to how much sunlight is hitting its water. That's why the overall annual temperature variation of the Earth tracks the seasons in the Northern Hemisphere rather than the Southern, even though the Earth is actually slightly farther from the Sun than average during the Northern Hemisphere's summer, so that one might naively expect the Southern hemisphere's seasons to dominate. $\endgroup$ – tparker Sep 25 '18 at 20:30
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Yes, Earth emits heat; radioactive minerals in the crust and core are continually decaying (potassium-40 and uranium in particular)and the decay releases energy which percolates upward through the ground as heat. Volcanoes and hot springs are the more spectacular evidences of this, but there is lesser geothermal heatflow, from below, everywhere.

Earth also exchanges heat with (for instance) sunlight and the cold dark sky. The heat income from sunlight and geothermal heat balance the loss of heat to the dark sky, at the surface temperature we call 'temperate'.

The net emission (not including secondhand heat from the Sun) of heat is the geothermal contribution, plus some human-generated minor additions.

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  • $\begingroup$ This is really interesting. If the sun is emitting heat towards the earth, and the earth is emitting heat towards space, why isn't space warm? Or is it? :) $\endgroup$ – Paula Sep 26 '18 at 13:07
  • $\begingroup$ Radiation leaves Earth (or any planet or star) at lightspeed, and is quickly diluted in vast volumes of space. Space DOES have some temperature; Penzias and Wilson got the Nobel prize for measuring it, in 1978. $\endgroup$ – Whit3rd Sep 26 '18 at 20:28

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