In 1727, astronomer J. Bradley discovered the phenomenon of stellar aberration. All stars throughout the year on the celestial sphere pass ellipses with an semimajor axis observed from the Earth at an angle of 20.5. Aberration is caused by the movement of the Earth in its orbit around the sun at a speed of 29.8 km / sec. To watch a star from moving Earth you need to tilt а telescope tube forward motion because as long as the light passes a tube. eyepiece with the Earth will move forward.