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Ra/Dec modelling continued

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Old November 8th 17, 09:05 AM posted to sci.astro.amateur
Gerald Kelleher
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Default Ra/Dec modelling continued

The calendar based convenience where celestial objects move relative to each other regardless of what is moving around what provides a useful framework for predicting astronomical events or to aid magnification professionals and hobbyists find specific objects accurately. It does give the Earth's rotation precedence as all things are references off a celestial equator but this part and it deficiencies have been covered many times.

When accurate timekeeping emerged as a serious force in the 17th century, it had the same impact as computers have in our era. The rapid adoption of the Equatorial Coordinate System for predicting astronomical events had one major consequence, at least compared to others, so this is the focus of attention here.

As the RA/Dec framework does not recognize what is moving around what yet provides an accurate way to gauge positions for any observer, Newton got around this by manufacturing a notion of true and apparent motions by appealing to direct/retrograde motions of the planets seen from the Earth (apparent) and seen from the Sun (true).

"That the fixed stars being at rest, the periodic times of the five primary planets, and (whether of the sun about the earth, or) of the earth about the sun, are in the sesquiplicate proportion of their mean distances from the sun.... for the periodic times are the same, and the dimensions of the orbits are the same, whether the sun revolves about the earth, or the earth about the sun." Newton

Appreciate that the last thread was left alone.


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