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I couldn't understand the bold lines in the following excerpt (from the book "Astronomy - Principles and Practice 4th ed. - A. Roy, D. Clarke")

The month is the next period of any significance to our watcher. During this time, the ideas about the heavens and their movements change. It will be noted that after a few nights the first group of stars seen above the eastern horizon just after sunset is markedly higher at first sight, with other groups under it becoming the first stars to appear. Indeed, after a month, the first group is about thirty degrees above the eastern horizon when the first stars are seen after sunset. It is then apparent that the Sun must shift its position against the stellar background as time passes. The rate is slow (about one degree per day—or about two apparent solar diameters) compared with its daily, or diurnal, movement about the Earth.

I just got reading, so I don't know anything about Astronomy. I tried to install Stellarium to check it out myself, but I couldn't find any option though, which could tell the sidereal position of sun, so that I could see the phenonmenon myself. Can anyone please tell how to do so? (Probably giving me a neat formula or theory/text based on this) Also according to Stellarium, the apparant diameter of Sun is $0°32'$ so howcome twice of this is $1°$?

Also, could anyone also explain what does the author mean by "compared with its daily, or diurnal, movement about the Earth." Is the author putting a comparision that the diurnal movement of sun overshadows this $1°$ shift in sun's sidereal position over a day, hence "the rate is slow"?

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    $\begingroup$ Numbers do match : $2\times 0°32' = 0°64' = 1°4' \approx 1°$. (Note author words "about two"), i.e. approximately. $\endgroup$
    – Agnius Vasiliauskas
    Commented Mar 1 at 14:13
  • $\begingroup$ @AgniusVasiliauskas yes you are right, my bad $\endgroup$
    – some random person
    Commented Mar 2 at 5:40

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Imagine the Earth making a complete revolution around the Sun. As it does so, the stars "behind" the Sun from the point of view of the Earth appear to change. It takes a full year (365 days) for this process to complete. Not entirely by coincidence, there are 360 degrees in a circle, so the Sun's apparent position relative to the distant stars shifts by about 1 degree per day.

By contrast the rotation of the Earth causes both the Sun and stars to appear to move across the sky once per day.

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  • $\begingroup$ Ohkay so is it that if we just consider the star/constellation (the stellar background) while the earth revolves, it would be the same as if to keep that star/constellation steady and make the sun move but in the opposite direction (because relativity)?? Because sun is always at the centre/focus in our perspective... $\endgroup$
    – some random person
    Commented Mar 2 at 5:48

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