The Earth absorbs radiation from the Sun and emits radiation of its own. The radiation the Earth emits has a longer wavelength than the radiation it absorbs because
the radiation slows down as it travels through the atmosphere.
the Earth reflects the shorter wavelength radiation into space.
the Earth is at a much lower temperature than the Sun.
the Earth's atmosphere stretches the radiation as it leaves.
40 exam-style questions on AQA GCSE Physics 6.3 Black body radiation, covering 6.3.1 Emission and absorption of infrared radiation and 6.3.2 Perfect black bodies and radiation. Each one has a worked solution and a mark scheme showing where the marks go.