Usually the mean translation speed of the earth around the sun is calculated (approximating its elliptic orbit with a circular orbit) to be around 29.7 Km/s. This is fine if we consider the sun is still, but we know this is not the case (see for ilustration ), the sun is also moving thru space and the earth traces a helical path with very clear speed cycles according to the earth's distance to the sun. I was wondering if anybody knows about a more realistic calculation of the earth's speed in its helical trajectory, I would think it should be less than the 29.7 Km/s calculated with the circular orbit approximation taking into account that at certain points the sun surpasses the earth, and subsequently the earth has to accelerate to catch up with the sun and surpass it. Heuristically it would seem like the earth's accelerates and deccelerates a maximum around +/- 15 Km/s that adds up to the total of around 30 Km/s usually calculated. This would fit well with the pattern of the CMB dipole from COBE (see George Smoot astrophysics page where it says: "the COBE DMR observations clearly show the change in velocity at the 30 kilometers per second due to the motion of the Earth around the Sun. One can see a clear sinsusodial pattern in the amplitude and direction of the dipole with a one year period in the four years of COBE DMR data. Differencing maps taken six months apart produces the familar dipole pattern with the amplitude and direction of the Earth's motion." Any thoughts? corrections?