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Why can’t we see galaxies moving away from us?

How are galaxies speeding away from earth at 1 million meters per second? Why dont they look like they are moving that fast?

Asked by: Tiana

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One Response

  1. – We CAN see galaxies moving away from us at high speed!

    We ‘see’ it from examining the light we see from them by passing it through a ‘spectroscope’. This is a sophisticated way of splitting light up into its component colours (a ‘spectrum’), just as rain drops split white sunlight into a rainbow. But when the source of the light is moving relative to us we see a shift in the spectrum and the amount of the shift tells us how fast the galaxy is moving. We find they are moving extremely fast. (This effect is called ‘Red Shift’, and it is rather like the Doppler Effect that we are all familiar with for sound – e.g. when an ambulance with a siren on is speeding away from us we hear its sound drop in pitch. This is the sound-equivalent of a ‘red shift’ for light. If you google ‘red shift’ you’ll find numerous references.)

    – Why galaxies APPARENTLY do not move is just due to their sheer distance from us. They need to move a huge distance before it is apparent to us and even at their great speeds this takes a very long time. The Universe is very very large indeed. The furthest galaxies detected have speeds which are 100-200 Million meters/second, appreciable fractions of the speed of light (300 Million m/s). The closest large galaxy to us, in Andromeda, is about a hundred million million million meters away (3 million light years). Obviously that’s a very great distance indeed.

    (Follow-up question: how many galaxies, if any, can one see by naked eye?!)

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