* Earth radiates infrared light: The Earth absorbs energy from the sun, mostly in the form of visible and ultraviolet light. This energy heats the Earth. The warmed Earth then emits this energy back into space as infrared radiation. This is the kind of light we can't see with our eyes, but we can detect it with special instruments.
* Earth's radiation is weak: The amount of infrared radiation Earth emits is much weaker than the light from the sun. It's not strong enough to be visible to our eyes.
* We don't see it because of the atmosphere: The Earth's atmosphere absorbs a significant amount of the infrared radiation, which is why we don't see a glowing Earth.
In short, Earth does radiate light, but it's infrared light which we can't see. It's also not as bright as the light from the sun, and our atmosphere blocks a lot of it.
