7 replies to this topic

[jlg84]

This is a *very* basic question, but I hope you'll indulge my ignorance! I am trying to make better use of my observing and photography time by planning what will be located in prime viewing/photographing locations on any given night. I would have thought there'd be some easy or at least practical way to take a look at an object's RA/Dec and determine when it will rise, when it will set, and when it will be crossing the meridian, but I cannot figure out what the method would be. Would anyone care to illuminate it for me?

[rutherfordt]

An object crosses your meridian when its right ascension is equal to your sidereal time-- another way of thinking of that is that the sidereal time is the same as the right ascension of objects crossing your meridian.

 

I would think two hours on either side of that time should still put your targets in a pretty good position.

[jlg84]

An object crosses your meridian when its right ascension is equal to your sidereal time-- another way of thinking of that is that the sidereal time is the same as the right ascension of objects crossing your meridian.

 

I would think two hours on either side of that time should still put your targets in a pretty good position.

That is EXACTLY the sort of information I was after! That makes perfect sense to me, and is something I can now work with! Many thanks!

[DeepSky Di]

Astrospheric app now has a circular diagram where you can display objects of interest. Also check out Martin's Astrophotography series - here's September https://youtu.be/-lt...Uk6LlvNZY84eWGL and here's the instructions if needed: https://youtu.be/fvZJd3Q4Bq4?si=Zex6j0-kz5bhLhjE

[Tony Flanders]

Realistically, you should use a planetarium app that will allow you to input the object's name and will then tell you precisely the information that you want to know. There are many good, free planetarium apps, depending which electronic device you prefer to use. And also some good, cheap ones that are well worth their price.

It's possible to compute all of this from first principles, and it's worth understanding the calculation for your sake. But unless you really enjoy looking things up in trig tables, it's much more fun to let a computer do the work for you.

To expand on the excellent answer given above, sidereal time is equal to civil clock time on the September equinox (that's more or less now!) if you live on the central latitude of your time zone and if your location doesn't observe daylight-saving time. So right now at 1 a.m. -- which would be called midnight if not for daylight-saving time -- the eastern side of the Great Square of Pegasus, which lies almost spang atop the RA 0 line, is at its highest. Four hours earlier, around when the sky gets fully dark, an object at RA 0-4 = 20 (mod 24) is at its highest.

 

Every month the difference between sidereal time and civil time gets two hours longer, until after a full year it gets 12*2 = 24 hours later, is to say, back where it started.

 

So around October 20, an object at RA 2 will be at its highest around 1 a.m. And on November 20, when we will blessedly be off daylight-saving time, an object at RA 4 will be highest around midnight.

 

Since you live at mid-southern latitudes, you can figure on observing far-northern objects one hour plus or minus transit time. For objects on the celestial equator, you can reasonably stretch that to three hours plus or minus. And far-southerly objects are still high in the sky six hours before or after transit time. In the extreme case, Sigma Octantis doesn't change altitude at all.

Edited by Tony Flanders, 19 September 2023 - 05:37 AM.

[BlueMoon]

 

I would have thought there'd be some easy or at least practical way to take a look at an object's RA/Dec and determine when it will rise, when it will set, and when it will be crossing the meridian, but I cannot figure out what the method would be. Would anyone care to illuminate it for me?

Grab a copy of Stellarium: https://stellarium.org/ (It's free) install it and it should set itself to your location, or close to it, automatically. Click on the Astronomical Calculations icon (or press F10) then click on the WUT (What's Up Tonight) icon. It's a powerful tool to use. You can filter a huge number of different targets for observation using this feature. You can also manipulate the time scale by advancing time for minutes, hours, days by using the time controls in the control bar to see what targets will be up at certain dates and times. I use this function frequently to plan my photography targets for particular nights.

 

Clear skies.

Edited by BlueMoon, 19 September 2023 - 05:49 AM.

[MikiBee]

Advanced astronomy apps like Sky Safari or Stellarium do these things really well. They are an excellent tool for planning your night whether it is visual or AP.

 

However, just so we don't depend on technology 100% it would be good to know roughly what can be observed in each season. That comes slowly, through hours of observing at different seasons.

[rrpallechio]

Polar Scope Align Pro (PS Align Pro) has a neat little function that gives the current local sidereal time. AND, you can change the local time and it will calculate the new LST. So, for example, right now as I type it says the local time is 11:02 and the LST is 09:44. Then, if I manually set my local time to 22:02, it shows that LST will be 20:46.

 

THEN, I can go to my star charts and turn to the pages that show RA around 22 and see what is up.

 

It's probably a lot easier to use one of the apps mentioned above