For example, like if an asteroid was projected to pass Earth but not until 20 years from now, how do we calculate this? How is that even possible to know?
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The Doppler effect but for light, if you’re familiar with the sound that a race car makes when it is approaching you, it gets louder and louder. Once it passes it stars tapering off and getting softer and softer.
With light there is a similar principle except that light from an object moving towards us will look more “blue” as the light waves compress. The opposite is true for objects that move further away from us as the wavelength of their light increases and thus become more “redder”. This phenomena is called blueshift in the former case and redshift in the latter case.
As a result scientists can use the light of an object to determine how far away it is by extrapolating data based on how shifted the light is. The more it has shifted to the blue spectrum, the closer it is to us. The further it has shifted to the red spectrum, the further away it is from us. For example, the furthest galaxies from Earth billions of light years away and require extremely sensitive telescopes that can detect light deep into the infrared range.
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