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2 years ago in Classical Mechanics , Physics By Geetesh Sharma
Why do directions in space remain constant even though Earth is rotating?
In celestial navigation, we use fixed right ascension and declination. This confounds me geometrically. Our planet spins, orbits the Sun, and moves with the galaxy. So what absolute or inertial "stage" are these directions anchored to? Is it the distant quasars, the cosmic microwave background, or a theoretical fixed frame? I need to understand the operational definition of a "constant direction" in cosmology.
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By Kumar Answered 1 year ago
This is an excellent question that gets to the heart of practical astronomy. The fixed stage is the International Celestial Reference Frame (ICRF). It’s an inertial frame defined by the precise positions of about 300 extragalactic radio sources, primarily quasars. Because they are billions of light-years away, their proper motion is undetectable, making them ideal fixed points. We anchor everything Polaris's position, satellite orbits, your telescope's coordinates to this quasi-inertial grid. So, "fixed" doesn't mean absolute in a philosophical sense, but operationally fixed relative to this consensus frame, which negates Earth's local motions.
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