For example, if I were to stand on the Moon, I would move with the Moon at a speed X = the rotational speed of the Moon around the Earth. In addition, the Earth itself also revolves around the Sun at a speed Y, so if I were on the Moon I would have to move at a speed X+Y. The Earth & Sun in turn revolve around the center of our galaxy at a speed C, so then I move at a speed of X+Y+C. And the galaxy itself also moves in the cosmos at a speed K, so then my final speed is X+Y+C+K. Is this correct? And if so, isn’t this a way to reach enormous speeds that border on the speed of light?
Answer
If we stand still or move at a constant speed, it amounts to the same thing. You can quietly read your newspaper on the train.
What has an effect, and must therefore also be observable, is changing the movement, the acceleration. You can feel it when the train speeds up or slows down.
The point now is that the accelerations associated with the movements of the Earth around its axis and around the Sun, and due to the presence of the Moon, are much smaller than the acceleration due to the Earth’s mass. So we don’t feel those small effects.
Until now it was about ‘feeling’. But beware, we can measure the speeds. Through the doppler effect, we can accurately measure how fast the stars are moving in line of sight from us to that star; and from that we can neatly deduce that the Earth (with the Moon) makes a systematic movement around the Sun with an average speed of about 30 km/s. Looking further into our galaxy, we find that the Sun with all its planets revolves around the galactic center at a speed of about 250 km/s. Then we can also see how our galaxy moves between others in our environment.
It turns out that the greatest speeds caused by such local effects are of the order of 1000 km/s, so much less than the speed of light.
Answered by
Prof. dr. Christopher Waelkens
Astronomy
Old Market 13 3000 Leuven
https://www.kuleuven.be/
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