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There was a section on my textbook on history of theories of sun's energy source.

It talks about how the Meteorite Theory was dismissed, as it would decrease the period of Earth's orbit by 2 seconds per year due to increased mass of the Sun.

This theory was dismissed due to disagreeing with observation. And the textbook says the change in period is "easily measurable"

My question is how is 2 seconds difference easily measurable in Nineteenth Century?

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  • $\begingroup$ FWIW, until the development of atomic clocks in the mid-20th century, our most precise clock was the rotation of the Earth relative to the stars. $\endgroup$
    – PM 2Ring
    Oct 25 '21 at 1:40
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Astronomer Norman Lockyer published The Meteoritic Hypothesis in 1890. By then the age of the Earth was known to be at least several hundred million years from geological evidence, and Darwin’s theory of evolution suggested life on Earth originated at least 100 million years ago. A change in the length of the year by 2 seconds a year over 100 million years would make the year over 6 times longer than it is now. It would also mean that 100 million years ago the Earth was over 3 times further away from the Sun than it is now. This would put the Earth between the current orbits of Mars and Jupiter, making it uninhabitable.

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  • $\begingroup$ This is a good answer to the underlying question (why reject the Meteorite Theory), but both the Title and the Body questions are about the measurement of relatively small variations in time in the nineteenth century. Someone searching the site for this question will be disappointed (but informed) by this answer. Perhaps a new question should be asked, and this answer moved to it. $\endgroup$ Oct 25 '21 at 14:27
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Yes, 2 seconds per year decrease of the Earth's revolution period would be easily detected in the 19th century. This is based on the general principle of astronomy, which was used since time immemorial. A 2-second decrease per year would result in 200 seconds decrease per century. And this is easily measured, since observations of the length of the year have been made for much more than a century.

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