http://curious.astro.cornell.edu/about-us/41-our-solar-system/the-earth/orbit/91-at-what-speed-does-the-earth-move-around-the-sun-beginner
" I cannot give you a precise answer, because the speed of the Earth changes all the time as the Earth moves around the Sun. This is because Kepler's second law says that on its orbit, a planet will sweep equal areas in equal amounts of time."
See? It's either accelerating or decelerating.
But, if I put you into an underground lab with all the measurement devices available, you will never be able to measure this fact. That's why the Michelson-Morley experiment was bound to fail.
Unfortunately, we're not moving at a constant speed, as the Earth's orbit is elliptical. The Kepler's law still stands true.