I'm guessing that, this would be someone in a rocket or something... When they hit their top speed, at what fraction of $c$ are they traveling?

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    $\begingroup$ lurscher's answer is absolutely correct... if you don't specify what the speed is relative to, the answer is any fraction you like. $\endgroup$ Oct 12, 2011 at 20:01
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    $\begingroup$ Speed of Earth around Sun is velocity=107,300 km/h ? Don't we all travel at this speed ? Does this count ? $\endgroup$
    – Andrei
    Oct 14, 2011 at 21:13
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    $\begingroup$ @Andrei, no. Doesn't count. $\endgroup$ Oct 14, 2011 at 21:20

2 Answers 2


When swinging my comfy hammock, I travel all day even up to 0.99 $c$, some days even more, depending on what particles are passing me by and measuring my exorbitant speeds with their atomic clocks and photons..!

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    $\begingroup$ Great job on thinking outside the box! $\endgroup$
    – xaav
    Oct 13, 2011 at 2:36

Maximum velocity attained by the Apollo spacecraft was 39,897 km/h which is $3.6\times 10^{-5}$ times the speed of light...

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    $\begingroup$ remember this speed was relative to earth....if you want to pump it up a bit further add up the speed of earth moving (around sun), sun moving(around the milky way) and the milky way moving(with respect of I don't know what... ) :P... $\endgroup$ Oct 13, 2011 at 8:45
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    $\begingroup$ @VineetMenon: I suppose that would be the velocity of the Milky Way with respect to the CBM, which is about 600 km/s I think. $\endgroup$
    – dotancohen
    Mar 2, 2012 at 13:17
  • $\begingroup$ By comparison Earth's rotational speed at the equator is 1674 km/h, so the above speed might need to be corrected by at most 1674 km/h if one wants the Apollo astronauts' speed with respect to a specific person standing on Earth's surface. $\endgroup$ Mar 1, 2015 at 15:05

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