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After I read that speed of earth orbit around the sun is 30 km/s, I looked at sky and I didn't notice any change when compared to the start location.

30 km/s can be converted to (30 * 60 ) km/minute = 1800 km/minute. This is a very large number in terms of speed, so I am confused.

How can this be explained?

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    $\begingroup$ A simple but very accessible example would be to stand at different distances from a highway/interstate (don't do this during rush hour as it will be pointless). When you are close, the cars will appear to move very fast but when you are far they appear to move very slow. It's similar to why commercial airliners look like they are moving slowly in the sky when at altitude, even though they are moving ~500 mph... $\endgroup$
    – honeste_vivere
    Apr 14, 2016 at 15:31

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We can detect it, but it takes very precise measurements. Stellar parallax, i.e. the relative displacement of close-by stars against the background of far away ones can be detected, but it's a very difficult measurement to make because the "motion" is very small (usually fractions of an arc second):

enter image description here

Recently we have learned to build satellites that can make these measurements really precisely and there are large databases with these parallaxes available. See https://en.wikipedia.org/wiki/Hipparcos and https://en.wikipedia.org/wiki/Gaia_(spacecraft).

Another way of detecting the motion is spectroscopically. The relative motion will change the wavelength of spectral lines in starlight. See this question: Doppler spectroscopy to verify Earth's speed around the sun.

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  • $\begingroup$ wow I got it now... we are very far away from stars and the moon moves at same speed of earth around the sun... :) $\endgroup$
    – Akam
    Apr 13, 2016 at 22:06
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But you can see the motion of the earth in it's orbit; it's called aberration of starlight, first measured by James Bradley in 1729.

Even earlier, parallax of stars had been detected, by 1680.

But you have to take detailed observations at widely separated times for the effects to be even the least bit obvious.

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