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Determining which regions of the sky are observable year-round?

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#1 zleonis

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Posted 21 May 2020 - 10:48 AM

How would you go about determining which regions of the sky are observable year-round, i.e. they're observable at some point every night of the year. This question, of all things, kept me up last night, but I was not able to quite work it out.

 

To simplify the question, I first tried to think about which regions of the sky would be above the horizon when the sun was not. Obviously an observer's circumpolar region (for those of us outside of the arctic) would fall into this category, but there are points at lower declination that are below the horizon for less time than the length of the shortest night, so they must also be visible at some point every night. This region, if my thinking is correct, would be everywhere north of the Tropic of Cancer, roughly speaking (for a northern observer, and ignoring for now the fact that stars aren't visible immediately after sunset). 

 

But it's only a small area around the Summer Solstice that would rise and set with the sun on the shortest night of the year. A point on the Tropic of Cancer at RA 18h would be visible before and after sunset even when the sun was at the same RA, and this would hold true for points south, all the way down to the Winter Solstice or just north of it?

 

Does this thinking hold up? How would you determine whether a point falls into this category, and how would it depend on latitude? How would the calculation of this region change if you considered only the region that was above the horizon when the sun was at least e.g. 12° below the horizon?

 

From an observing standpoint, an interesting challenge (if you were generous with the definition of interesting) would be to see what would be the most southerly (northerly) object you could observe year-round. Has anyone attempted this?



#2 gatsbyiv

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Posted 21 May 2020 - 11:14 AM

A couple of years ago, I went down the rabbit hole on this and similar questions about the total amount of time a given spot in the sky is visible at night.  I then did some math, wrote a bunch of code to explore it, created graphs, and eventually wrote a book using some of that code.  You can see some charts I created that explore this question here:  https://digitalstars...the-night-sky/  Agree that it's not interesting to everyone, but I definitely find this sort of thing very interesting for some reason.


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#3 sg6

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Posted 21 May 2020 - 11:48 AM

North.

Unless you are odd and in Australia then South.

 

Guess for us in the North it has to be a circle equal to your Latutude around the NCP. Where Latitude is where you are. Anything outside that would go under the horizon for part of the year.

 

Sit at the rotation point and the stars just go round and round. Right down to 90 degrees on the horizon.



#4 zleonis

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Posted 21 May 2020 - 02:01 PM

A couple of years ago, I went down the rabbit hole on this and similar questions about the total amount of time a given spot in the sky is visible at night.  I then did some math, wrote a bunch of code to explore it, created graphs, and eventually wrote a book using some of that code.  You can see some charts I created that explore this question here:  https://digitalstars...the-night-sky/  Agree that it's not interesting to everyone, but I definitely find this sort of thing very interesting for some reason.

I really enjoyed your blog post, and although I'm not an imager, the charts in your book were elegant and informative. It looks like these calculations are, uh, pretty computationally intensive but the results are interesting and visually striking. I sometimes forget how much of a miracle it is that I can wonder something about how and when an object will appear in the sky and just punch a few buttons on my phone to find out. There's lots of hard work behind those miracles!



#5 kathyastro

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Posted 21 May 2020 - 02:12 PM

You are talking about "circumpolar" stars.  Any stars that are closer to the pole than your latitude are circumpolar.  In other words, stars where declination >= (90 - latitude).


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#6 gatsbyiv

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Posted 21 May 2020 - 03:12 PM

I really enjoyed your blog post, and although I'm not an imager, the charts in your book were elegant and informative. It looks like these calculations are, uh, pretty computationally intensive but the results are interesting and visually striking. I sometimes forget how much of a miracle it is that I can wonder something about how and when an object will appear in the sky and just punch a few buttons on my phone to find out. There's lots of hard work behind those miracles!

To run the calculations for every day of the year on a grid with 1/4 degree resolution was definitely enough that I had to get up and go have a coffee while the computer cranked away!  



#7 zleonis

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Posted 21 May 2020 - 08:12 PM

You are talking about "circumpolar" stars.  Any stars that are closer to the pole than your latitude are circumpolar.  In other words, stars where declination >= (90 - latitude).

Thanks, Kathy. I think I wasn't quite clear in phrasing my question (probably a reflection of my own thinking on the question). I'm familiar with circumpolar stars, but I also got to wondering about stars that aren't necessarily circumpolar, but that would still be visible every night, even if only for a few minutes after dusk or before dawn. Are you familiar with a term for this region of the sky? 



#8 Love Cowboy

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Posted 21 May 2020 - 08:39 PM

I remember first noticing this phenomenon with Vega in December here in Texas where I live, but I'm not aware of any term for those objects, or the calculations needed to determine what falls in that zone.  Definitely would be an interesting project though.  


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#9 NinePlanets

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Posted 21 May 2020 - 09:16 PM

Get a planisphere. They cost from $3 to $12 depending on paper or plastic or quality and need no batteries. Questions solved.



#10 zleonis

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Posted 21 May 2020 - 09:51 PM

Get a planisphere. They cost from $3 to $12 depending on paper or plastic or quality and need no batteries. Questions solved.

That's a good thought. I did take a look at my planisphere (Firefly) and it surprisingly doesn't have a great way to track the sun. It has the ecliptic, but it assumes you know where the sun is at a given time of year (which maybe isn't entirely unreasonable). I wound up playing around with Sky Safari quite a bit to see if I could find a way to see such and such a star year round, and Sky Safari is terrific for that sort of thing. 



#11 PNW

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Posted 22 May 2020 - 12:29 AM

I keep a log in a spiral notebook. On the left hand side of the open book, I list the best objects, by constelation, that night. After a year or so, I have a quick reference. Call me old school.


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#12 kathyastro

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Posted 22 May 2020 - 08:01 AM

Thanks, Kathy. I think I wasn't quite clear in phrasing my question (probably a reflection of my own thinking on the question). I'm familiar with circumpolar stars, but I also got to wondering about stars that aren't necessarily circumpolar, but that would still be visible every night, even if only for a few minutes after dusk or before dawn. Are you familiar with a term for this region of the sky? 

Thanks for the clarification.  Yes, that would be a tricky calculation.  I don't know if there is a name for such regions or objects.

 

So you are looking for objects that, when they have the same RA as the Sun (i.e. the worst case for visibility), are still above the horizon long enough after sunset or before sunrise to be visible.  Definitely latitude-dependent: at the equator, there would be none.  It would also be magnitude-dependent: brighter objects will be visible in brighter twilight.  Also declination-dependent: the closer to the pole the object is, the longer it would be visible after sunset before it itself sets.

 

A truly messy calculation, which is why I can see that it would be fun to try. :D


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#13 LDW47

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Posted 22 May 2020 - 02:03 PM

How would you go about determining which regions of the sky are observable year-round, i.e. they're observable at some point every night of the year. This question, of all things, kept me up last night, but I was not able to quite work it out.

 

To simplify the question, I first tried to think about which regions of the sky would be above the horizon when the sun was not. Obviously an observer's circumpolar region (for those of us outside of the arctic) would fall into this category, but there are points at lower declination that are below the horizon for less time than the length of the shortest night, so they must also be visible at some point every night. This region, if my thinking is correct, would be everywhere north of the Tropic of Cancer, roughly speaking (for a northern observer, and ignoring for now the fact that stars aren't visible immediately after sunset). 

 

But it's only a small area around the Summer Solstice that would rise and set with the sun on the shortest night of the year. A point on the Tropic of Cancer at RA 18h would be visible before and after sunset even when the sun was at the same RA, and this would hold true for points south, all the way down to the Winter Solstice or just north of it?

 

Does this thinking hold up? How would you determine whether a point falls into this category, and how would it depend on latitude? How would the calculation of this region change if you considered only the region that was above the horizon when the sun was at least e.g. 12° below the horizon?

 

From an observing standpoint, an interesting challenge (if you were generous with the definition of interesting) would be to see what would be the most southerly (northerly) object you could observe year-round. Has anyone attempted this?

As time passes there is always something new showing up in every sector of the nite sky, every nite of every season, of each year ! Like the soap opera its ‘ As the World Turns ‘, lol !  Clear turning skize !




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