sun's movememt through the zodiac

[piff]

I am sure you have all read in books about astronomy something along the lines of this example taken from Patrick Moore's "Guide to the Night Sky" (in the "Philip's" series) on page 17 :
 
"The earth takes one year to go round the sun, so the sun seems to move right round the sky in a period of one year, during which it passes through the twelve constellation of the zodiac."

Similarly on the excellent website "Astronomy Notes" for example the author states: "the sun appears to drift among the zodiac constellations" and then refers to "the apparent yearly path of the sun among the stars".

I could give several other examples from books by other writers all saying pretty much the same thing. Now it is obvious that these eminent and knowledgeable writers know what they are talking about, but due to some misunderstanding on my part I am completely baffled as to what they mean! So I hope somebody can help me understand this.

My problem is that, so far as I can see, the sun does not appear to move round the sky once a year, but every single day, starting in the east and crossing to the west. Nor does it "appear" to drift among the constellations of the zodiac - or any other constallations for that matter - because the stars are invisible when the sun is in the sky and vice versa, so the sun does not "appear" among them at all.

Clearly I am missing something here, and I am failing to grasp what these writers were trying to convey, but I would be very grateful if someone who understands what they meant to say could explain these apparent paradoxes.

[JamesT]

Hi Piff

Like you I find this a difficult concept to envisage. I found the website below describes it quite well.............. Rgds James

http://www.astronomynotes.com/nakedeye/s5.htm

[David]

Hi Peter, there are three effects here.

The first is the illusion that the Sun and stars are moving.  It turns out that we are all moving, but the stars are so far away that to the naked eye you need hundreds of years to see the movement.  The Sun is moving but we are moving with it so you don't notice and finally we are moving in an orbit around the Sun.  This is what we see but with no other measurement you cannot tell if we are moving around the Sun or the Sun is moving around us.

Second the Earth spins on its axis.  Let us assume that the Earth does NOT go around the Sun on an orbit ie it is fixed in space.  We now see the stars rise in the east and set in the west.  How long they take to travel from east to west depends on our latitude and their position relative to our equator which we call the celestial equator.  If the Earth did not go on an orbit around the Sun then we would see the same stars every night at the same time.  You would see half the possible sky from our latitude because the other half would be daytime and the stars would be lost in bright sky (but they still would be there).

Third, the Earth moves around the Sun.  This time let us assume that the Earth does not rotate on its axis while it travels on its orbit so that the same point on the Earth always faces the Sun.  Let us assume that Bexhill is opposite side of the Sun to this point ie it is always in darkness of the night sky.  At this point it is probably best if you draw a picture of the Sun in the centre and the Earth in a number of positions on its orbit around the Sun with Bexhill always on the opposite side to the Sun.  You will now see as each day passes Bexhill faces a different part of the distant sky until a year is up and the Earth is back where it started 365 days ago and the whole thing stars again.  Now superimpose the constellations on the distant night sky (ie as if on the inside surface of a ball with the Earth/Sun at its centre) you will see that as each day passes Bexhill faces slightly different stars.

If we now combine 2 and 3 above as the Earth spins on its axis the stars in the night sky drift from East to west but at the same time the Earth moves slowly around its orbit (about one degree a day ~ it takes the Earth 365 days to complete 360 degrees) so gradually the a particular star rises about 4 minutes earlier each day and over many weeks and months we see different constellations.

As the Earth rotates on its axis at midnight Bexhill has stars high in the sky and the Sun on the opposite side of the world,  In 12 hours time Bexhill is facing the Sun in bright daylight but looking beyond the Sun we have the stars on the opposite side of the celestial sphere which we can't see because of the intense daylight.  But have no worries in 6 months time when the Earth is on the opposite side of its orbit at midnight it will see these very stars but at midday when looking in the direction of 6 months ago and the stars we saw then are obliterated by the brilliant sunshine.

Finally we go back to the first point.  Without detailed measurement it is difficult to tell whether we go around the Sun or the Sun goes around thew Earth.  We get easily drawn into the situation of the Sun going around the Earth.

[piff]

Thanks for your help on this and your careful explanations, which i do appreciate. However these don't actually address the problem I raised - perhaps I did not make my meaning clear.

I have no difficulty with the reason why the stars appear to change position as the earth orbits the sun, and different constellations appear visible at different times of year. That is all clear enough. Where I have problems is with the kind of statements which appear e.g. on that web page that James kindly referred me to in "Astronomy Notes" where the author writes:

"The Sun appears to drift eastward with respect to the stars (or lag behind the stars) over a year's time" and then again a little further down, "As the earth moves around the sun, the sun appears to drift among the zodiac constellations".

But it doesn't! That is my difficulty. The sun does not "appear" to do anything at all "among the constellations" because they are never visible in the sky at the same time! So what does this mean?

[David]

Hi Peter

Am I right in thinking that you issue you have is with the fact that you cannot see the stars during hours of daylight when the Sun is above the horizon?  Can you clarify that point (agree or disagree).

Thanks
David