What cause the apparent motion of the stars?
The real motion of stars is hardly noticeable even over a period of many years. It's the Earth's daily rotation the is the main cause of their apparent motion.
(Stars are so far away that the Earth's revolution around the Sun hardly cases any apparent motion.)
Why did the fact that stars not show any Parallax motion convince the Greeks that the Earth was not moving?
I am not sure whether the ancient Greeks were involved. In any case, any movement of the Earth should be reflected in the apparent motion of distant objects, such as stars.However, in practice, this apparent motion is very small for the stars - the yearly parallax for even the closest stars is less than one arc-second (1/3600 of a degree).
The stars that do rise do so in the east set in the west, just like the sun. It is because of the rotation of the earth. As is the case with the sun, the rising and setting of stars is an apparent motion and not a true motion; the sun and stars do not orbit around the earth. Depending on your location, some stars never go below the horizon.
We say that it is 'setting,' but this is apparent motion, and not true motion. The same is true for the rising and setting of the moon and fixed stars. There is another spectacular example of apparent motion that is different from true motion. Because the moon 'rises' in the east and 'sets' in the west, it is easy to think that it is "moving" from east to west. In fact, the moon's orbit around…
Actually they do. But the stars are so far away that it takes a long, long time to notice any movement. For example, Barnard's Star - one of the stars that is closest to us - has an apparent movement of 10.3 arc-seconds per year - that is, one degree in 350 years. The apparent motion of other stars is less than that.
Yes, every star is moving. We call the actual movement of a star "proper motion", to distinguish it from the "apparent motion" of the stars rising and setting. This is because the "apparent motion" is caused by the Earth spinning and us going around with it. However, the "proper motion" of most stars is so small, and the stars themselves are so far away, that with only a few exceptions no person could notice any…
the doesn't move at all actually. all the planets are in constant motion around the sun --------------------------------------------------------------------------------------------------------------- The apparent motion of the sun is the way the sun appears to be moving in relation to the motion of the planets around it.
Retrograde motion is in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. The term is often used when discussing the APARENT motion of the outer planets against the background of the fixed stars. The planets outside the orbit of the Earth appear to move backwards along their orbital paths as our planet (Earth) overtakes them each year. They do not actually start orbiting backwards, it…
Why do modern astronomers continue to use the celestial sphere when they know that stars are not all at the same distance?
Yes. "Retrograde motion" occurs when the apparent path of a planet through the background of the stars appears to turn backward from its usual direction. The cause is that the Earth and other planets are in orbit around the Sun. The inner planets orbit more quickly than the outer ones. At times in its orbit, the Earth seems to "leap ahead" as it passes the outer planet.
Every star has its own "proper motion", and some of them are moving pretty quickly - but the stars are so far away that the apparent motion even over the course of decades is unnoticeable. The planets move more slowly, but are relatively close; close enough that we can see the change in position from one night to the next.