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Seasons and Right Ascension?

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21 replies to this topic

#1 Gschnettler

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Posted 23 February 2021 - 01:52 PM

Hello. Is there a correlation between the season and right ascension? I’m just wondering if some people say things like, “oh, that object is at 2 hours and 20 minutes R.A. That would be best viewed in February.”

Is there a table whereby you can map season or month to right ascension for viewing purposes?

The reason I ask is because there are a lot of spreadsheets of things to view. And to determine what’s visible for this month’s viewing it would be nice if you could sort on something like R.A. to get a sub-set of objects to create an observing list for the night.
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#2 rob1986

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Posted 23 February 2021 - 02:11 PM

Is there a table whereby you can map season or month to right ascension for viewing purposes?

you could, but most would use a planisphere to determine the RA range for the night. After a while you memorize it. 0 ra is at midnight on sept 21 approximately, so take your observing hours, add say +- 3h on either side, and then ad 2 hours for every month afyer sept 21. so this month, 10pm to 1 am, make it 7 to 4 and ad 5*2 or ten hours. do over those three hours objects from 5h to 14 will be within around 45 degrees of maridian durring that observing window.


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#3 havasman

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Posted 23 February 2021 - 02:13 PM

Yes, certainly. The big clock that is the sky overhead processes with the seasons. I recommend you get a planisphere like the 16" KenPress Guide to the Stars for an in-hand demonstration of the phenomenon. The procession of the sky and its constellations is integral to the hobby. Also, any good set of charts like the Sky and Telescope Pocket Sky Atlas will include RA info. But the planisphere most simply illustrates the seasonality.


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#4 clearwaterdave

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Posted 23 February 2021 - 02:15 PM

Hello.,Any simple planiphere will show you.Lets use 12h RA.,  At aprox.,9pm on May 7 ,.,12h RA will be due south.,One month later 14h RA will be due south at 9pm. 

  I have the Orion DeepMap 600 and it let's you see the whole sky ( minus the poles ) at once.,

 

  Rob is quicker then me.,lol.,

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#5 sg6

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Posted 23 February 2021 - 02:16 PM

Yes, but don't ask me what the relationship is.

Comes from the measurements being at the equinox, 6 months afterwards the object would be in daylight so not really visible. The sun is a really heavy source of light pollution.

 

There are a number of sky guides that are Spring, Summer, Autumn, Winter (I have one, 3 copies of a field guide). Actually useful as over the 3 month period the change is reasonable.

 

The guide is called: Guide to the Night Sky by FSC - Field Studies Council.

Not sure if available in the US, they seem to be a UK organisation.

Rather simple easy laminated fold out of 3 month sky intervals.

 

It would give an idea of RA and time of year.



#6 Starman1

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Posted 23 February 2021 - 02:18 PM

Hello. Is there a correlation between the season and right ascension?

<...snip...>.

So, technically, there is no seasonality to the sky except, perhaps, in the deep south where objects are only above the horizon for a short time.

 

What you mean, then, is is there an RA to month association when observing objects right after the end of astronomical twilight and for the 2 hours after that?

And the answer is yes.

 

You can easily determine what is visible any hour of any night all year if you own a planisphere, like the ones from David Chandler.

Or, if you download the monthly chart from SkyMaps.com

If you download the last 12 months, you have all the charts you need to discover what constellations can be seen at dark any month of the year.

This chart can tell you what RA hour is appropriate at any month--see the areas denoted as "Evening Sky"

http://www.ifa.hawai...05/DarkSky.html

I think a planisphere is easier.

 

There is also this:

https://twitter.com/...7352961/photo/1

Which you can get from S&T:

https://www.shopatsk...0-degrees-north



#7 jeffreym

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Posted 23 February 2021 - 02:19 PM

I find these very handy for planning.

 

(Created by Cromwell and Espenak)

 

Have Fun,

Jeff

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#8 jeffreym

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Posted 23 February 2021 - 02:20 PM

And this one.

Jeff

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#9 Gschnettler

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Posted 23 February 2021 - 02:42 PM

Thank you. I do have plenty of star charts and the Orion deep map 600 (which is awesome!). I got my answer in the replies above. Very helpful.

What I was really looking for was a quick way to sort through lists like the one at this website to see which of the objects on the list I can see between say 7pm-10pm tonight: https://skyandtelesc...s-for-everyone/

That website is just an example since as you know there are many lists like this for various types of objects. Rather than searching for each one in Sky Safari for the desired date and time to see where it is, it would be faster for me to simply determine a range of RA values and then circle the objects in that range as the ones to look for tonight.

Thank you all!

#10 clearwaterdave

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Posted 23 February 2021 - 02:54 PM

With Skysafari you can limit your search to RA.,Or look ahead with SS for the time you'll be out and see which RA is in the direction you want to look.,Then make your list.,Good luck.,



#11 Starman1

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Posted 23 February 2021 - 03:29 PM

Thank you. I do have plenty of star charts and the Orion deep map 600 (which is awesome!). I got my answer in the replies above. Very helpful.
<...snip...>

Get a planisphere.  Look at the constellations crossing the meridian at the time of night you plan to observe.

Look at objects in those constellations.  Simple.  Virtually every object is best viewed on or near the N-S meridian.


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#12 jeffreym

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Posted 23 February 2021 - 03:31 PM

Thank you. I do have plenty of star charts and the Orion deep map 600 (which is awesome!). I got my answer in the replies above. Very helpful.
<...snip...>

Aaron Clevenson has published an Excel file that might get you specifically to a date and time:

https://www.humbleisd.net/Page/81224

 

What's Up Dock? page.

Jeff



#13 Ulmer Spatz

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Posted 24 February 2021 - 10:02 AM

Since a star's coordinates are fixed, you could ask "Is there a correlation between the season and Sirius?" and perhaps have your answer intuitively. If you're at all observant, Sirius is sort of a winter star. So its fixed right ascension could be called a 'winter right ascension.'



#14 SeymoreStars

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Posted 24 February 2021 - 10:23 AM

Thanks for the thread and the responses, I just ordered a planetsphere. This Forum is going to make me into an astronomer yet.


Edited by SeymoreStars, 24 February 2021 - 10:26 AM.


#15 Dave Mitsky

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Posted 24 February 2021 - 11:23 AM

You can easily determine the right ascension of an object in relation to when it culminates using a planetarium program such as Stellarium.  For example, Sirius will be on the meridian at approximately 8:30 p.m. EST tonight at my longitude.

 

http://stellarium.org/

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#16 Dave Mitsky

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Posted 24 February 2021 - 11:54 AM

Perhaps the chart at https://in-the-sky.o...a/ra_season.php will be of some use.  (It's set for my location.)



#17 kathyastro

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Posted 24 February 2021 - 12:04 PM

If you want a quick relationship between RA and the calendar, try this.  Take 10:00 pm to be an average observing time.  In late October, 0h RA is due south at that time.  Since there are 12 months and 24 hours of RA, each month adds two hours to the RA.  So 0h at the end of October; 2h at the end of November, 4h at the end of December, etc.  Daylight saving time is going to mess with that by an hour twice a year, but it will give you a rough approximation.


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#18 belliott4488

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Posted 24 February 2021 - 01:01 PM

Maybe I'm just strange, but my answer to the OP's original question would be, "Yes - they're correlated because the Sun's RA is seasonal."

 

In other words, every constellation that's ever visible from your location will pass overhead ever day of the year. It's just that for certain times of year the Sun will also be overhead, which leads to difficult viewing. wink.gif

 

When I look at an object on a map of the sky and wonder when I'll be able to see it, I generally check for when the Sun will be at the object's RA + 180 degrees.

 

Maybe I should just use a planisphere ...



#19 Starman1

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Posted 24 February 2021 - 01:31 PM

Maybe I'm just strange, but my answer to the OP's original question would be, "Yes - they're correlated because the Sun's RA is seasonal."

<...snip...>

The sun is out of the way when it is about an hour and a half (18°) away from the object.

But who wants to view the object on the horizon, eh?

So add another 30° to get the object high enough, and you're at +/- 48° from the sun.

And if you prefer to view an object on the meridian, +/- 108° of RA from the sun will do it, and everything in between (about 144° of sky over the night).

 

If you only consider the meridian, then it will take longer than 3 months to see the whole sky.

But if you consider 30° or higher acceptable, 3 months can reveal the entire sky.


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#20 Tony Flanders

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Posted 24 February 2021 - 05:31 PM

I sort of know what's visible when by heart by now, but every now and then I retreat to first principles, especially when I want a little more accuracy.

 

By definition, the Sun is at R.A. 0 on the first day of spring, the vernal equinox. That happens in late March. Every month after that, the Sun's R.A. increases by two hours. Knowing that, it should be easy to estimate the Sun's R.A. on any given date.

 

For instance it is now late February. So the Sun is currently about two hours before R.A. 0 -- in other words, R.A. 22. (The whole R.A. system wraps around at 24.) That means that the objects that are least visible lie on R.A. 22. You can indeed see some circumpolar objects (ones close to Polaris) that lie on R.A. 22, but they will be placed about as badly as possible.

 

If you were to observe at true local midnight, the objects that would be best placed are the ones 12 hours away from the Sun -- at R.A. 10. As kathyastro says above, most people are more likely to be out at 10 p.m., roughly two hours before true local midnight -- at least when daylight-saving time isn't in effect. So the objects currently best placed are roughly at R.A. 8. I am actually duplicating kathyastro's post here, except that I'm explaining why objects at R.A. 0 are best placed around 10 p.m. in late October.

 

For best accuracy, obviously, use a planisphere or an electronic equivalent.


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#21 Gschnettler

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Posted 01 March 2021 - 01:33 PM

I think I have a formula. I’m looking at my Orion Deep Space 600 chart which has RA on it along with the date when that RA is at the meridian at 9pm.

Using that it looks like from Jan-Oct you can say that the 9pm RA at the meridian is approximately equal to the numeric month * 2 + 2 for the beginning of the month and + 3 for the second half of the month.

For Nov and Dec it would be equal to the (numeric month minus 12) * 2 + 2 for the beginning of the month or +3 for the end.

For example:

- early May: 5*2 + 2 = 12 hours RA

- late August: 8*2+3 = 19 h

- late Nov: (11-12)*2 + 3 = 1 h

Those all match up to the atlas. They don’t take into account daylight savings so need to factor that in, or just go with the map which says assume 9pm viewing for standard time, 10pm for daylight savings. Those are the times when I would be out most of the time anyways.

From there, I think can say that most objects should be easily visible with +/- 3 hours of the meridian RA.

So, for example, with the early May example above, I could take a spreadsheet of double stars, sort by RA, and then select those whose RA is between 9 -15 and feel confident that they would be high enough in the sky for good viewing.

Please let me know if you see any flaws with this approach.

#22 Tony Flanders

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Posted 03 March 2021 - 07:25 AM

I think I have a formula.

<...snip...>

That's all just fine. However, you can do better than just +-3 hours. It all depends on the object's declination. For far-southern objects that barely rise above the horizon, even being off 2 hours can be a serious problem. For circumpolar objects, you can easily go plus or minus 6 hours. For objects on the celestial equator, your 3 hours is probably about right.




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