I just got reading this book- "Astronomy - Principles and Practice 4th ed. - A. Roy, D. Clarke" and I got stuck at the following bold lines (I am providing the full text for context to my question):-

Six months after the Sun has risen between north and east and setting between north and west, it is rising between south and east and setting between south and west. Another six months has to pass before the solar cycle is completed, with the Sun once more rising between north and east and setting between north and west. All this could be explained by supposing that the Sun not only revolved with the stars on the celestial sphere about the Earth in one day (its diurnal movement) but that it also moved much more slowly along the path among the stars on the celestial sphere, making one revolution in one year, returning to its original position with respect to the stars in that period of time. We have already seen that the observer who notes over a month what group of stars is first visible above the eastern horizon after sunset will have already come to the conclusion that the Sun moves relative to the stars. Now it is seen that there is a regular secular progression right round the stellar background and that when the Sun has returned to its original stellar position, the seasonal cycle is also completed.

And I couldn't understand what it meant. How could we come to the conclusion that the sun moves relative to stars only by the fact that "group of stars is first visible above the eastern horizon after sunset". Also could someone also explain what the author meant about the "seasonal cycle" here.

I am new to this world, could anyone please provide an easy-to-understand solution. I will be grateful!

  • 1
    $\begingroup$ The best way how to see motion of the Sun relative to stars: Go to SOHO Satellite Movie Theater (soho.nascom.nasa.gov/data/Theater), and select C3 for Image. I highly recommend dates at the end of May (the Sun moves between the Pleiades and the Hyades). $\endgroup$
    – Leos Ondra
    Mar 3 at 19:42

1 Answer 1


Everyone knows that you can't see stars during the day. Why is that? Prehistoric people might imagine that stars don't exist during the day. Children sometimes suppose that the stars are "turned off" during they day. But that is wrong.

The ancient astronomers supposed (correctly) that the stars are still there during the day, but you can't see them because the sun is so bright.

Now, they noticed that every day the sun seems to move across the sky. And every night the stars seem to move across the sky. And they move at about the same rate, going 180 degrees in 12 hours. But not exactly the same. The stars move very slightly faster across the sky than the sun.

They observed this because different stars are visible at different times of the year. In winter, for example, The Twins, Gemini, rise at sunset. In Spring they are already risen, at sunrise, and set during the night. In summer they are permanently hidden by the sun. They concluded that in summer, the sun was in the same part of the sky as the Twins. As summer proceeds to autumn, the sun moves relative to the stars and each year it moves from the Twins to The Crab (Cancer) and then the Lion (Leo) etc.

They deduced that the sun moves relative to the stars from the fact that different groups of stars are visible at different times of the year.

And this repeats every year. The Twins are always visible in winter and never in summer. This cycle of which constellations are visible at which times is the seasonal cycle.

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    $\begingroup$ thank you for adding historical evidence as well! it made things connect well $\endgroup$ Mar 3 at 8:19
  • $\begingroup$ You can watch that happen yourself with casual observation of the night sky. I tend to let my dog out around 9pm most of the year, and even though I'm in an area with awful light pollution, I can see Orion's very recognizable shape just fine and observe how it shifts from month to month. $\endgroup$ Mar 3 at 10:07
  • $\begingroup$ @PM2Ring yes, so the stars actually move faster then the sun... will fix that. $\endgroup$
    – James K
    Mar 3 at 12:38
  • $\begingroup$ @DarthPseudonym I did try that over stellarium, and yes it did change its position $\endgroup$ Mar 3 at 16:46
  • $\begingroup$ Oh, no doubt, I just always feel like there's something different about seeing it with your own two eyes as the seasons change, the way our ancestors did it. $\endgroup$ Mar 4 at 14:16

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