Why certain stars seem to move very little in the sky

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Main Question or Discussion Point

Why is it that some celestial bodies when observed from the Earth appear to move very little in the the sky throughout the seasons, even though the Earth in the meantime has traveled millions of miles in its orbit around the sun?

From my location in Montreal, Canada, there are only very few stars or celestial bodies visible with the naked eye. One, in particular, is very bright and is one of the very few stars visible with the naked eye. It puzzles me that its apparent displacement throughout the sky, relative to my location on the Earth, seems to be minimal across the seasons. .

I am no astronomer and I cannot give a description of its position and the magnitude of its displacement in astronomical terms (declination and ascension). All I can tell is that this displacement seems to follow a circular path, with the radius contained within the span of only two, may be three hands throughout the seasons, in spite of the fact that in the meantime the Earth has moved hundreds of thousands, if not millions of miles along its orbit.

Since the North Star, from what I know, is not 90° vertical overhead, but probably less ( 70°, 80° ?), I reckon this body could be at 45°, 50° altitude East of the North Star.

I’d expect to see this body at almost diametrically opposed points of the compass at the winter and summer solstices. This is not the case. Why not?



Thanks for your insight



Ittiandro
 
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  • #3
vela
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In Montreal, the North Star (Polaris) will be about 45 degrees above the horizon, and over the course of a year, its position in the night sky won't change. The star close to Polaris in the sky will be circumpolar. You can see it all year long, and over the course of a year, its position night to night will slowly circle Polaris. It sounds like you're observing a circumpolar star.

If you still have Flash, check out this simulation:

https://astro.unl.edu/classaction/animations/ancientastro/dipperclock.html
 
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Only the planets move dignificantly throughout the year. The name planet comes from the Greek for "wanderer".

Try this to begin with:

https://www.universetoday.com/85730/do-stars-move/
Of course! That I know.
Thanks. I read the link . It somewhat clarifies the issue, but not completely.

The parallax point. I knew about it but somehow I overlooked it.

In this regard, I infer, then, that the closer the object is to the Earth, the greater its apparent displacement across the sky will be in terms of parallax. Conversely, the greater its e distance , the smaller its apparent displacement across the sky. This is probably the case with “ my “ object, which must be very very far indeed.

It is true that one contributing factor in the small apparent displacement of an object across the sky is its relative proximity (alignment) to the terrestrial rotational axis. This is however irrelevant in my case, because the object I observe is not at all aligned with( or close to) the terrestrial axis . The object appears to be quite East of it.

In the end, though, I fail to understand why, even admitting the greatest distance of a star from the Earth, its apparent displacement in terms of parallax remains so small, in spite of hundreds of millions of miles of distance covered by the Earth in its orbit.

Will there ever be ( or are there ) observed (or observable) stars whose distance from the Earth, if it could be observed with the most powerful optical telescopes, is so great that the parallax angle is 0°, in other words it remains truly fixed in the sky relative to different orbital positions of the Earth , like the winter and summer solstices ?



Thanks

Ittiandro
 
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DaveC426913
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I've split this off into its own post to distinguish your specific question of a given star from the more general discussion of how parallax works.

About parallax:

In this regard, I infer, then, that the closer the object is to the Earth, the greater its apparent displacement across the sky will be in terms of parallax. Conversely, the greater its e distance , the smaller its apparent displacement across the sky. This is probably the case with “ my “ object, which must be very very far indeed.
Sorry, that's not right.

There are no stars** in the night sky whose apparent motion due to parallax is visible to the naked eye .
As far as you are concerned all stars** are at infinity.

You'd need a good telescope to see even the slightest movement of any star** against the background.

**except the sun of course


Will there ever be ( or are there ) observed (or observable) stars whose distance from the Earth, if it could be observed with the most powerful optical telescopes, is so great that the parallax angle is 0°, in other words it remains truly fixed in the sky relative to different orbital positions of the Earth , like the winter and summer solstices ?
The question is a bit malformed. There is no such thing as "truly fixed" when looking out into the night sky, because the yardstick by which we measure positions of stars is ... the stars themselves.

Virtually all stars are collectively considered to be the unchanging fixed back ground of stars, against which we are able to measure the parallax of only the closer ones.
 
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  • #6
DaveC426913
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About your observation:

The most likely possibilities of your observations are:
  1. You are seeing a planet - Jupiter or Saturn or Jupiter, whose orbits are 12 and 20 Earth years long respectively, so they will appear hang in the sky for quite some time.
  2. You are mistaking different stars for the same star throughout the year. Do you go out and observe this star at the same time each night? If not, you won't be looking at the same star each time.
  3. You are looking at a star near Polaris. This is an unlikely possibility since there are very few bright stars near Polaris.
#1 is by far the most likely, considering you mention you can see only a few celestial bodies in the city glow. Saturn and Jupiter tend to outshine pretty much all stars. And they hang around in the sky from year to year.

Is your star visible now? Can you go out tonight and observe it? Tell us what time, what direction and what elevation you see it, and we will try to identify it.

I live in Toronto, so I see pretty much the same sky as you.

Since the North Star, from what I know, is not 90° vertical overhead, but probably less ( 70°, 80° ?), I reckon this body could be at 45°, 50° altitude East of the North Star.
As vela points out, Montreal is at 45 degrees North, so the Pole Star is at 45 degrees in your sky (half way between horizon and zenith), same as for me here in Toronto.

What time of night are you observing? And what day(s) of the year?
 
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  • #7
DaveC426913
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Here is the night sky for Montreal, tonight, around midnight. Facing East.

You can see Polaris, there on the left.
Other bright bodies are shown in the area you mention, along with their height above horizon and their distance from Polaris around the compass.
montreal.png


Jupiter is in the sky, at about 30 degrees up (you'd have a tough time seeing above the buildings), and about 100 degrees East of Polaris (in the South East sky).

The other three stars are by far the brightest in that area - together they are known as the Summer Triangle - generally the most prominent "ersatz" constellation in the summer sky.
 
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I've split this off into its own post to distinguish your specific question of a given star from the more general discussion of how parallax works.

About parallax:


Sorry, that's not right.

There are no stars** in the night sky whose apparent motion due to parallax is visible to the naked eye .
As far as you are concerned all stars** are at infinity.

You'd need a good telescope to see even the slightest movement of any star** against the background.

**except the sun of course



The question is a bit malformed. There is no such thing as "truly fixed" when looking out into the night sky, because the yardstick by which we measure positions of stars is ... the stars themselves.

Virtually all stars are collectively considered to be the unchanging fixed back ground of stars, against which we are able to measure the parallax of only the closer ones.

Thanks for your reply

You say

Quote

There are no stars** in the night sky whose apparent motion due to parallax is visible to the naked eye …. You'd need a good telescope to see even the slightest movement of any star** against the background.

unquote.


Of course! But this is not what I said. By “ visible apparent motion due to parallax’ , I meant, rather the parallax as observed and measured by the usual means employed in astronomical observation, typically telescopes and other means, which exclude the naked eye, as was the case in the days of the Babylonians or.Aristarchus of Samos.

This being said, in what sense, then, is my statement not true? It may be so, but not because I brought in the” naked eye”, which I never did and never meant.



Regarding my question

Quote

Will there ever be ( or are there ) observed (or observable) stars whose distance from the Earth, if it could be observed with the most powerful optical telescopes, is so great that the parallax angle is 0°, in other words it remains truly fixed in the sky relative to different orbital positions of the Earth , like the winter and summer solstices ? UNQUOTE

You say that my statement and the question behind it is a bit” malformed” on account of my use of the word “ truly fixed”.

Scientists and even more so science students sometimes tend to be more royalist than the King in dissecting statements. Precision is of course paramount in science, but in so doing there is always a risk of losing sight of the forest for the trees when we do it indiscriminately, without regard to the context or the drift of the question.

You are absolutely right. I do understand your point that in the vast expanses of the cosmos we cannot speak of “ truly fixed” positions of the stars in the absolute sense, because these positions are always relative to other stars or celestial bodies as a point of reference, just as motion is always relative to another body, even though we commonly use this word without thinking of this all important scientific fact..

Would then perhaps my statement and the question behind it be more correct without the word “ truly fixed” or if I specified that I agree with you on its relative meaning?

Then my question becomes:

1. Are there stars whose position relative to other stars appears to be unchanged when observed by telescopes from different positions of the Earth along its orbit?

2. I also grant you that we can only speak of parallax of the CLOSER stars.

3. For those stars that are closer to us ( but that could nevertheless be tens or hundreds of thousands l.y. away), is it envisageable:

  • A.
  • That the parallax angle , not based on the naked eye’s observation , of course, but on the telescopes’ observations, be 0° or close to 0° in spite of the millions of miles traveled by the Earth in its orbit?
  • B
  • Can we then speak of “ fixed” stars in the relative sense you specify, i/e. of stars whose positions ( relative to to the background of other stars or constellations as a point of reference) appear unchanged when viewed from the Earth along different points of its orbit?
Thank you



Ittiandro
 
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About your observation:

The most likely possibilities of your observations are:
  1. You are seeing a planet - Jupiter or Saturn or Jupiter, whose orbits are 12 and 20 Earth years long respectively, so they will appear hang in the sky for quite some time.
  2. You are mistaking different stars for the same star throughout the year. Do you go out and observe this star at the same time each night? If not, you won't be looking at the same star each time.
  3. You are looking at a star near Polaris. This is an unlikely possibility since there are very few bright stars near Polaris.
#1 is by far the most likely, considering you mention you can see only a few celestial bodies in the city glow. Saturn and Jupiter tend to outshine pretty much all stars. And they hang around in the sky from year to year.

Is your star visible now? Can you go out tonight and observe it? Tell us what time, what direction and what elevation you see it, and we will try to identify it.

I live in Toronto, so I see pretty much the same sky as you.


As vela points out, Montreal is at 45 degrees North, so the Pole Star is at 45 degrees in your sky (half way between horizon and zenith), same as for me here in Toronto.

What time of night are you observing? And what day(s) of the year?

You have a point that I may be observing a planet whose orbit is 12 or 20 Earth years. Never thought about it.
Unfortunately, I am not equipped well enough to give its position. I am using an App called Sky Safari.
Based on it, that star ( or planet?) is in the East sky. I have no clue about its elevation. It is well above the horizon, possibly 45 ° . It is not at the Zenith. From the App, it could be Muphrid or Arcturus. It is usually there almost the whole night , around 3 am.
I am pretty sure that I am looking at the same star, because it is the only bright star always in the same sky quadrant. If I strain my eyes, I can see other faint stars around, but this is pretty much the only easily visible star. I even looked at it my 20 x binoculars. It is a huge, bright blob, definitely not Mars, because it would be reddish. Never thought the North Star would be at only 45°, though.

I'll look at it tonight. If I can, I'll give you some more details.

Thamks

Ittiandro
 
  • #10
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Here is the night sky for Montreal, tonight, around midnight. Facing East.

You can see Polaris, there on the left.
Other bright bodies are shown in the area you mention, along with their height above horizon and their distance from Polaris around the compass.
View attachment 265384

Jupiter is in the sky, at about 30 degrees up (you'd have a tough time seeing above the buildings), and about 100 degrees East of Polaris (in the South East sky).

The other three stars are by far the brightest in that area - together they are known as the Summer Triangle - generally the most prominent "ersatz" constellation in the summer sky.
Dave
I overlooked the chart. My star could well be Deneb or Altair, which straddle almost exactly the East sky position. Vega is in the vicinity too ( so to speak!) but it is too close to the Z to be the one I see.
Regardless, it is always there or almost. It was last winter and it is now. Hence my original question raising the parallax issues, which I am still discussing with Perok, becaue there are a few contentious points I wanted to make.

Take care

Ittiandro
 
  • #11
DaveC426913
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Parallax:

Of course! But this is not what I said. By “ visible apparent motion due to parallax’ , I meant, rather the parallax as observed and measured by the usual means employed in astronomical observation, typically telescopes and other means, which exclude the naked eye, as was the case in the days of the Babylonians or.Aristarchus of Samos.
How do you propose to observe or measure this apparent motion unless it is with respect to a background of fixed stars?


This being said, in what sense, then, is my statement not true?
Sorry, I am not sure exactly which statement you are referring to. Using the quote feature would help immensely.

It may be so, but not because I brought in the” naked eye”, which I never did and never meant.
If you were using a telescope or binoculars, it would have helped to mention that earlier. :wink:

Even with binocs, you won't be able to spot any parallax. Good amateur scopes might get close to arc-second resolution, (which is still insufficient resolve 287 mas).


1. Are there stars whose position relative to other stars appears to be unchanged when observed by telescopes from different positions of the Earth along its orbit?
Yes.

61 Cygni exhibits a parallax of about 287 milliarcseconds (or a quarter arc second). Unfortunately, this is smaller than amateur telescopes can resolve.

2. I also grant you that we can only speak of parallax of the CLOSER stars.
Correct.


  • That the parallax angle , not based on the naked eye’s observation , of course, but on the telescopes’ observations, be 0° or close to 0° in spite of the millions of miles traveled by the Earth in its orbit?
Virtually all stars exhibit a parallax of zero. That's why we can treat them as part of the fixed and unchanging background against which we observe parallax.

Can we then speak of “ fixed” stars in the relative sense you specify, i/e. of stars whose positions ( relative to to the background of other stars or constellations as a point of reference) appear unchanged when viewed from the Earth along different points of its orbit?
Yes. Again, all but a select few that are close-by are fixed.

In the interest of readability, would you mind using the quote feature to highlight sections of text you specifically want to address? Also, as a matter of etiquette, typing in all bold is considered yelling, and generally frowned upon.
 
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  • #12
vela
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I have to admit, I'm a bit confused by this thread. You started off talking about the circular path of a star over the course of a year with a (angular) radius on the order of tens of degrees. Now you seem obsessed with parallax, which is measured in arcseconds. These are obviously completely different motions in the sky. I'm no longer sure what your original question was.
 
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  • #13
DaveC426913
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Your star:
I overlooked the chart. My star could well be Deneb or Altair, which straddle almost exactly the East sky position. Vega is in the vicinity too ( so to speak!) but it is too close to the Z to be the one I see.
Regardless, it is always there or almost. It was last winter and it is now.
Those stars are not visible in the winter - at least not in the same place at the same time as in summer. They're near or below the horizon at night. And they're in the North West, not South East.

Essentially, the entire sky is rotated 180 degrees around the pole star (which puts them below the horizon).

December sky, at same observing time:
1593297461964.png


You must be seeing a different star and assuming it's the same one as in the summer.

BTW, are you viewing them at the same time of night in both summer and winter? What time?
 
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  • #14
sophiecentaur
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You'd need a good telescope to see even the slightest movement of any star** against the background.
Yes. A good telescope and photographs, taken six months apart in order to make the parallax at all visible. Some while ago I was looking for a link that would actually show the results of parallax measurements in astronomy. It seems to be quite hard work because the actual movement, relative to very distant {so called 'fixed' stars) is very small . I found this graph which shows the parallax of one of the nearer stars to us. The parallax due to the Earth's orbital motion adds to the star's own motion (Proper motion) and produces the plot shown. Credit to Mark Munkacsy who was responsible for the actual work. There's another image that I found which shows what two actual photographic exposures (on a glass plate) recorded. Pretty well incomprehensible . I've no idea what the star was or what the parallax was - it just illustrates how danged clever and dedicated the old astronomers were without the help of electronics.

parallax slide.jpg

paralax stars 1.jpg
 
  • #15
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Parallax:


How do you propose to observe or measure this apparent motion unless it is with respect to a background of fixed stars?



Sorry, I am not sure exactly which statement you are referring to. Using the quote feature would help immensely.


If you were using a telescope or binoculars, it would have helped to mention that earlier. :wink:

Even with binocs, you won't be able to spot any parallax. Good amateur scopes might get close to arc-second resolution, (which is still insufficient resolve 287 mas).



Yes.

61 Cygni exhibits a parallax of about 287 milliarcseconds (or a quarter arc second). Unfortunately, this is smaller than amateur telescopes can resolve.


Correct.



Virtually all stars exhibit a parallax of zero. That's why we can treat them as part of the fixed and unchanging background against which we observe parallax.


Yes. Again, all but a select few that are close-by are fixed.

In the interest of readability, would you mind using the quote feature to highlight sections of text you specifically want to address? Also, as a matter of etiquette, typing in all bold is considered yelling, and generally frowned upon.
Parallax:


How do you propose to observe or measure this apparent motion unless it is with respect to a background of fixed stars?



Sorry, I am not sure exactly which statement you are referring to. Using the quote feature would help immensely.


If you were using a telescope or binoculars, it would have helped to mention that earlier. :wink:

Even with binocs, you won't be able to spot any parallax. Good amateur scopes might get close to arc-second resolution, (which is still insufficient resolve 287 mas).



Yes.

61 Cygni exhibits a parallax of about 287 milliarcseconds (or a quarter arc second). Unfortunately, this is smaller than amateur telescopes can resolve.


Correct.



Virtually all stars exhibit a parallax of zero. That's why we can treat them as part of the fixed and unchanging background against which we observe parallax.


Yes. Again, all but a select few that are close-by are fixed.

In the interest of readability, would you mind using the quote feature to highlight sections of text you specifically want to address? Also, as a matter of etiquette, typing in all bold is considered yelling, and generally frowned upon.


Dave
Sorry for the bold characters. No intention of yelling at all! It is only that sometimes in the Forums my replies are wiped off due to some glitch in the system. To avoid having to redraft them by memory,especially when they are long, I usually type them in Word first and then I copy and paste into the Forum. Sometimes the font in Word is not set correctly and the replies get pasted from Word in bold characters.

My statement that you qualified as not correct was
Quote
Me :
In this regard, I infer, then, that the closer the object is to the Earth, the greater its apparent displacement across the sky will be in terms of parallax. Conversely, the greater its e distance , the smaller its apparent displacement across the sky. This is probably the case with “ my “ object, which must be very very far indeed.
Unquote

You refuted this statement by saying

Quote
There are no stars** in the night sky whose apparent motion due to parallax is visible to the naked eye .
As far as you are concerned all stars** are at infinity.

You'd need a good telescope to see even the slightest movement of any star** against the background.
Unquote

In turn, I objected that I never said that the apparent motion due to parallax is visible to the naked eye.
You are right, it cannot be. What I meant is "visible" with telescopes and I agreed that the parallax can be measured only on closer stars.
I also agreed with you on the relative sense of " truly fixed " stars, because the positions of stars are always relative to the background of other stars or constellations.
From your other comments, our positions seem to converge and I thank you for your dialogue with me.
I'll be more keen on using the" quote" feature in my replies.

Take care

Ittiandro
 
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  • #16
DaveC426913
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My statement that you qualified as not correct was

Quote
Me :
In this regard, I infer, then, that the closer the object is to the Earth, the greater its apparent displacement across the sky will be in terms of parallax. Conversely, the greater its e distance , the smaller its apparent displacement across the sky. This is probably the case with “ my “ object, which must be very very far indeed.
Hoisted on my own pitard I am! :-p I should have quoted only the part I was directly addressing.

All the unbolded is correct. I was referring to your conclusion, which I have bolded. You are not seeing parallax with your object.


That very passage, by the way, is the wellspring of the confusion in this thread - where your observation of an object in the sky got intermingled with a discussion about parallax. They're not related.
 
  • #17
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I have to admit, I'm a bit confused by this thread. You started off talking about the circular path of a star over the course of a year with a (angular) radius on the order of tens of degrees. Now you seem obsessed with parallax, which is measured in arcseconds. These are obviously completely different motions in the sky. I'm no longer sure what your original question was.
This happens when there are several interlocutors ( some say too many cooks in the kitchen..) .
I don't know whom you are addressing your remarks to: Ittiandro ( myself) started the thread by asking why certain stars always appear almost in the same position in the sky, regardless of the movement of the Earth in its orbit and the millions of miles that the observer's location has in the meantime changed along the orbit.

You seem confused that two altogether different issues are conflated. You need not be.
You decry the conflation, on one hand, of the parallax, which is measured only in tiny arcseconds and the vastly different notion of the circular path of a star over the course of a year, which has an (angular) radius of the order of tens of degrees." Huge difference!
I believe that in my case both explanations could apply, depending on the celestial body I am looking at and which I have been so far unable to identify.

The apparently small displacement of this body during six months of Earth's orbiting could be due either to the fact that this star is aligned with the rotational axis of the Earth and therefore it appears to move along a relatively short overhead circular path, or that this star is so distant from the Earth that the baseline ( the distance traveled by the Earth along its orbit in six months, is so small relative to the distance of the star that the angular radius, instead of being of the order of tens of degrees could well be of the order of arcseconds, just as an ordinary parallax.. Or perhaps your remark is adressed to somebody else?

All I can say that the comments of people in this Forum are helping me to better understand. Even if these comments may not always address my question, there is much knowledge that, as a layman. I can gain from them by way of " obiter dicta", i.e., in legal Latin jargon, what is said indirectly .
 
  • #18
vela
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I don't know whom you are addressing your remarks to: Ittiandro ( myself) started the thread by asking why certain stars always appear almost in the same position in the sky, regardless of the movement of the Earth in its orbit and the millions of miles that the observer's location has in the meantime changed along the orbit.
What do you mean by "same position in the sky"? Relative to the fixed stars? That's why they're called fixed. They don't move. Of course, as someone mentioned earlier, this applies to all of the stars except the Sun, not just certain stars.

Or do you mean you always see it in, say, the northeastern sky no matter what time of year and time of night? If that's the case, I'd posit you're seeing a bright light at a top of a tower, not a star, or you're seeing different objects at different times and assuming incorrectly they're the same object. Or you're looking at Polaris.
 
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  • #19
DaveC426913
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I don't know whom you are addressing your remarks to:
Vela's comment was definitely directed to you:

You started off talking about the circular path of a star over the course of a year with a (angular) radius on the order of tens of degrees. Now you seem obsessed with parallax, which is measured in arcseconds. These are obviously completely different motions in the sky. I'm no longer sure what your original question was.

You seem confused that two altogether different issues are conflated. You need not be.
Vela had the same problem I did. You started with an observation, and digressed into a hypothetical about the principle of parallax. Those are two distinct discussions.

Anyway, let's put all that behind us.


The apparently small displacement of this body during six months of Earth's orbiting could be due either to the fact that this star is aligned with the rotational axis of the Earth and therefore it appears to move along a relatively short overhead circular path,
This is possible, yes, although I thought we'd ruled it out because you would have known if the star was near Polaris (and there aren't any bright enough anywhere near).
The brightest stars are so far from Polaris that they are below the horizon in the winter during optimum star gazing times.

or that this star is so distant from the Earth that the baseline ( the distance traveled by the Earth along its orbit in six months, is so small relative to the distance of the star that the angular radius, instead of being of the order of tens of degrees could well be of the order of arcseconds, just as an ordinary parallax..
I thought we had this sorted out. You are still misunderstanding parallax.

With few notable exceptions, all stars (except Sol) have a parallax of effectively zero. In other words - stars are effectively at a distance of infinity as far as parallax goes. Your object can't be further away than infinity.

Parallax has nothing to do with your object.
 
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  • #20
Drakkith
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@Ittiandro Perhaps it would be helpful to restate your question based on what you've learned so far, so that it's clear what you're asking. I admit that I don't know if your original question has been answered satisfactorily or not based on what I've read in this thread.

For reference, the positions of the stars in the sky change because of a few reasons:

1. The rotation of the Earth, seen as the daily/nightly circular path of the stars across the sky.
2. The orbit of the Earth around the Sun. This is seen in a few ways:
  • a.) In the day to day progressive shift of stars towards the west. In other words, if you measure the position of each star at the same time every night, you'll find that their positions shift about 1 degree per day (365 days in a year, 360 degrees in a circle).
  • b.) In the alternating parallax measurements of nearby stars over the course of a year.
3. Relative motion between the stars and the Sun. This is visible as a non-alternating change in parallax measurements over the course of months to years.

Because of the extreme distances between stars, only item 1 and 2a are visible to the naked eye, even over the course of a lifetime. Item 3 would be noticeable if you lived for thousands of years and had impeccable memory as to the previous positions of the stars.

I want to stress for anyone reading this that when I say 'extreme distances between stars' I really mean the 'extreme' part. The Earth's orbit is roughly 150 million km (93 million miles) in radius. That's a lot. But it's nothing compared to the distances between most stars. The nearest star system is about 4 light years away, or 3.78x1013 km. A 378 with 11 zeroes after it. That's more than 250 thousand times more than the radius of the Earth's orbit. That's a 5 minute walk to the corner store compared to a 2 year journey across the Earth.

And that's just the nearest star. The average star that's visible in the night sky is dozens or hundreds of light years away. These extreme distances mean that even though the Earth moves millions of miles/km through space, it doesn't look like it when you look up at the stars.
 
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  • #21
DaveC426913
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I admit that I don't know if your original question has been answered satisfactorily or not based on what I've read in this thread.
IMO, unless OP can give us more details about his observations - starting with times/dates of observations, number of samples, and a better idea of locations in the sky - we've answered it as best as can be expected.
 
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  • #22
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Vela's comment was definitely directed to you:





Vela had the same problem I did. You started with an observation, and digressed into a hypothetical about the principle of parallax. Those are two distinct discussions.

Anyway, let's put all that behind us.
Dave, I am glad to put all this behind us because you have been of invaluable help in giving me a very plausible explanation, namely that I may be seeing a planet like Jupiter or Saturn whose orbits are 12 and 20 Earth years long respectively, so they may well appear to hang in the sky for quite some time. I overlooked this ...

I appreciate that you remained focused on my original question rather than engaging in a display of scientific savvy, which however valuable, does not address my question.

Before closing this thread, I wish however make a few remarks.

I believe that what I did say in my post is not what you and Vela understood.

I willingly take the responsibility for this misunderstanding, though, first because English is not my native language and also because scientific discussions require a cutting edge precision which is not always within a layman’s horizon..

You seem to agree with the following statement of Vela and I will therefore address my comments to you

He said:

Quote

You started off talking about the circular path of a star over the course of a year with a (angular) radius on the order of tens of degrees. Now you seem obsessed with parallax, which is measured in arcseconds. These are obviously completely different motions in the sky.

I'm no longer sure what your original question was. Unquote


Well, my original question was why certain objects in the sky appear to move very little when viewed from different points of the Earth’s orbit. Period. I may be wrong in this perception, but I said no more than this.

While speaking of an apparently circular path of the body I observe, my original post neither mentioned the parallax, nor other precise scientific terms like angular radius in arcseconds or tens of seconds, simply because these precise scientific terms are not in my horizon.

In fact, what I did say was only, rather crudely and approximately, that the radius of this circular motion can be measured within the span of two, may be three hands.

To say that I conflated parallax ( in arcseconds) and the angular radius ( in tens of seconds) of the apparent stars’ circular motion is an extrapolation that is not warranted by the original words of my post. .

There is nothing I conflated, as Vela said. . My post contained a simple, factual observation ( however right or wrong it may be) and I hoped that somebody could elucidate the issue.

I wish I could identify the body I am referring to, so that our exchange may be more constructive and less vague, but unfortunately I am not equipped with a telescope or other appropriate astronomical observation tools.

I also find somewhat condescending that the other member should imply that I took a…tower for a ..star. .
Most assuredly this is not the case because.... I am not on drugs and I am in full possession of my mental faculties...



Ittiandro
 
  • #23
Drakkith
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I wish I could identify the body I am referring to, so that our exchange may be more constructive and less vague, but unfortunately I am not equipped with a telescope or other appropriate astronomical observation tools.
No worries. You need nothing but your eyes and a star/sky map. There are many apps available for your mobile device, programs for your computer, and even static charts viewable online and available for purchase. It takes very little time to get proficient enough to identify the brighter objects you're viewing.

Dave, I am glad to put all this behind us because you have been of invaluable help in giving me a very plausible explanation, namely that I may be seeing a planet like Jupiter or Saturn whose orbits are 12 and 20 Earth years long respectively, so they may well appear to hang in the sky for quite some time. I overlooked this ...
Indeed, the outer planets move slowly through the sky over the course of the year (relative to the background stars). Jupiter's orbital period is almost 12 years, so it will only move about 30 degrees from one day in the year to that same day the next year. Saturn's orbital period is more than twice that, at nearly 30 years. So it will only move 12 degrees per year.
 
  • #24
DaveC426913
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...a very plausible explanation, namely that I may be seeing a planet like Jupiter or Saturn whose orbits are 12 and 20 Earth years long respectively, so they may well appear to hang in the sky for quite some time. I overlooked this ...
Unfortunately, now that we have a little more detail on your observing habits, it turns out this can't be the explanation.

While it is true that Jupiter and Saturn have very long orbits, that cannot explain your accounts. You say:

My star could well be Deneb or Altair, which straddle almost exactly the East sky position. Vega is in the vicinity too ( so to speak!) but it is too close to the Z to be the one I see.
Regardless, it is always there or almost. It was last winter and it is now.
What you are saying, it appears, is that you saw this object now (in June), and also in relatively the same place in winter.

That cannot be so. The planets, just like the stars, will be rotated 180 degrees from summer to winter.

If you saw the object at a given time of night in the winter, you cannot be seeing the same object at the same time now in the summer. It will be near or below the horizon, and on the opposite side (North West) side of Polaris.

Likewise, if you are seeing the object at a given time of night now, you cannot have seen the same object at the same time in the winter. It would have been near or below the horizon and on the opposite side (North West) of Polaris.


So, we're back to possibility #2: you are seeing a different star in the summer than you are in winter, and assuming they're the same.

I'm afraid, without further observing details, that's the best conclusion.
 
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  • #25
DaveC426913
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While speaking of an apparently circular path of the body I observe, my original post neither mentioned the parallax...

To say that I conflated parallax ( in arcseconds) and the angular radius ( in tens of seconds) of the apparent stars’ circular motion is an extrapolation that is not warranted by the original words of my post.
Your opening post did not bring up parallax, but in your subsequent post, post #4, you introduced - and expounded upon at-length about - parallax.

But again, we all finally agree that was a red herring - an unrelated discussion - and we can leave it behind.


In fact, what I did say was only, rather crudely and approximately, that the radius of this circular motion can be measured within the span of two, may be three hands.
This infographic may come in handy the next time you go out.

1593405012554.png
 
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