The nice thing about science is that it is accurate no matter how you apply it. Essentially, we use speed = distance/time to calculate the CME speed. It's a slightly fancier version of that, but that is the root of it. By looking at how far a CME travels in a given time, we can work out what its speed is. One of the big challenges is that we only have a flat, 2 dimensional image of the CME moving. So if it looks like it's moving out of the image, say directly to the east, it might actually be moving at a 45 degree angle towards the observer or away from them. This changes the speed that we can measure. If you've studied Pythagoras' Theorem then it's like needing to measure along the hypotenuse but only being able to measure *one* of the other two sides. We can try to correct for this using stereoscopic observations from the likes of the STEREO satellites, but that's quite difficult to do.
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