The Moving Earth

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Department
Physics, Engineering Physics and Astronomy
Course Code
ASTR 101
Professor
Prof.

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Description
The Moving Earth • To understand how the Earth moves, we have to compensate for diurnal motion by observing the stars at the same time each day, month, or year  Would appear as if stars shift very slowly with the sun drifting in from the west, slowly moving east (left) across the stars in a counter-clockwise direction (in the northern hemisphere)  Sun moves independently of the background of stars • The Greeks understood this and believed Earth was completely at rest, with the sun (incorrect), planets (incorrect), and moon (correct) orbiting Earth • Four potential tests to see if Earth is moving through space:  1. Various changes in star brightness as we move around  Completely impractical as the distance between Earth and stars as Earth orbits the sun changes by such a small percentage that there is no difference in brightness (Earth's orbit is only 150,000,000km from the sun to produce significant changes)  Brightness over distance follows the inverse square law, where standing2 twice as far away from an object looks 1/4 as bright (since 2 --> 1/2 = 1/4)  The minute changes will be so insignificant that it is not feasible with modern technology to measure  2. Measure speed similar to how cops measure traffic speeds  Practical by measuring the Doppler effect in the light of the stars, which shows Earth clearly is orbiting in a way such that we move towards some stars while moving away from others  Seen in all stars within Earth's ecliptic orbital plane (stars above or below do not demonstrate changing velocities)  In June, stars appear to all rush towards Earth while in December, they appear to rush away, both at speeds of 30 km/s  Achieved by spreading starlight into a spectrum and observing black lines where light is missing (if more black lines are in the blue light, then the Earth is moving towards the star, and vice versa)
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