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Old 26-11-2009, 02:55 PM
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reading star atlas' and finding the objects in the sky

I'm trying to learn more about finding objects without relying on computers/GOTO etc. I have printed out the Mag 7 star atlas from the sticky in this forum. I can read the charts in terms of RA and degrees, happy with that.

What I'm having trouble with is working out where the object is going to be in the sky in RA depending on the time of night and the time of year. Must be missing some trick or something.

I am familiar enough now with constellations and bright stars that I can get my bearings from other objects I suppose, but would still like to know the answer to the above question.

Also, do you have any tips on roughly visualising degrees etc? Seem to remember a guide about your fist at arm's length is so many degrees, thumb might be something else.
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Old 26-11-2009, 03:29 PM
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Once your eq. mount is aligned, you can align the setting circles on a known bright star, there are many listings of bright stars out there. From then on the circles will be set for the night to find whatever you want.

I had heard something of 'arm's length, fists, thumbs' and the such, but I'd leave this to someone else.
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Old 26-11-2009, 03:38 PM
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Aah, should have clarified. Didn't mean with the mount. More for viewing through binoculars or maybe a dob.
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Old 26-11-2009, 03:52 PM
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I still use a planisphere to plan sessions & find out what is up, been doing that since I was 12yrs old, nearly thirty years now. I use it in conjunction with charts.
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Old 26-11-2009, 03:52 PM
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You need to know what the Local Sidereal Time (LST) is. The RA of an object on the meridian is the LST at your location. eg when the Orion Nebula is overhead the LST is about 5hr 32min. LST runs about 4 minutes faster than clock time.

You can download an LST clock from various sources. Most astro software has LST time.

The LST at 3:54pm is 19hr 11min. Orion will be overhead in about 10 hrs.

Last edited by astro744; 26-11-2009 at 03:56 PM. Reason: added LST at 3:54pm
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Old 26-11-2009, 04:19 PM
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Thanks astro744. This is the sort of info I want. Questions. I get that Orion is RA 5h 35m, M45 is 3h 47m etc. Right now Astroplanner is telling me my LST is 19:43 (it's 3.14pm Qld time).

I don't get how you calculated that Orion will be overhead in about 10hrs. I need to understand that math.
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Old 26-11-2009, 04:37 PM
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Aah, think I got it. LST say 20h, add the object's RA 5h 35m, subtract current real time 15:30 say, gives 10 hours to transit?

Edit: hang on. that can't be right. checked on some other objects and it's not quite adding up. close but not accurate.
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Old 26-11-2009, 04:55 PM
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Quote:
Originally Posted by astro744 View Post
You can download an LST clock from various sources.
Yep. There's one right here:

http://www.iceinspace.com.au/files.html

AlsAstroClock down the bottom under miscellaneous downloads.

Al.
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Old 26-11-2009, 05:02 PM
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Quote:
Originally Posted by troypiggo View Post
Thanks astro744. This is the sort of info I want. Questions. I get that Orion is RA 5h 35m, M45 is 3h 47m etc. Right now Astroplanner is telling me my LST is 19:43 (it's 3.14pm Qld time).

I don't get how you calculated that Orion will be overhead in about 10hrs. I need to understand that math.
I just said 19 + 5 = 24hrs = 00 hrs, add 5 (approx) for Orion = 10hrs. Orion transits about 10 hours from 4pm (AEDT) which makes it about 2am AEDT. Sounds about right for this time of year but I haven't been out at 2am lately.

Older Planispheres used to have RA on them but most current version do not which is a pity.
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Old 26-11-2009, 05:09 PM
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But what is the first 5h you're adding to the 19h for LST? Seems like you're adding 5h twice, I understand one of them would be Orion's RA, but what's the other?
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Old 26-11-2009, 05:29 PM
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Quote:
Originally Posted by sheeny View Post
Yep. There's one right here:

http://www.iceinspace.com.au/files.html

AlsAstroClock down the bottom under miscellaneous downloads.

Al.
Al, can you explain this? The PC clock is showing 17:21 Sydney daylight saving.
If I run your program with "Use TZ" I get 22:43 sidereal.

If I switch to "Use Long" with 150deg 59" 40' E I get 20:47 sidereal, which matches the value from http://tycho.usno.navy.mil/sidereal.html

Daylight saving sign error somewhere?
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Old 26-11-2009, 05:57 PM
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Originally Posted by troypiggo View Post
But what is the first 5h you're adding to the 19h for LST? Seems like you're adding 5h twice, I understand one of them would be Orion's RA, but what's the other?
Just think of it in 24hr clock terms (24-19 = 5). The first 5hr brings you to 24/0hrs LST which = 00hrs RA on the meridian. Add another 5hr and Orion is up high.

LST is now just after 21hrs so add 3 to get to 24/00 and then add the RA of the object in question (approx 5 for Orion). In 3hrs time the LST is 24/00 so any objects with an RA of 00 will be on the meridian. Approx 5hrs later again will be Orion.
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Old 26-11-2009, 06:03 PM
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Originally Posted by mithrandir View Post
Al, can you explain this? The PC clock is showing 17:21 Sydney daylight saving.
If I run your program with "Use TZ" I get 22:43 sidereal.

If I switch to "Use Long" with 150deg 59" 40' E I get 20:47 sidereal, which matches the value from http://tycho.usno.navy.mil/sidereal.html

Daylight saving sign error somewhere?
I would just about have to agree. I haven't found that one before... I'll have a look at the code.

Al.
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Old 26-11-2009, 06:32 PM
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Quote:
Originally Posted by astro744 View Post
Just think of it in 24hr clock terms (24-19 = 5). The first 5hr brings you to 24/0hrs LST which = 00hrs RA on the meridian. Add another 5hr and Orion is up high.

LST is now just after 21hrs so add 3 to get to 24/00 and then add the RA of the object in question (approx 5 for Orion). In 3hrs time the LST is 24/00 so any objects with an RA of 00 will be on the meridian. Approx 5hrs later again will be Orion.
Got it. I think. If we say directly overhead/transit is 24h on some local RA grid, 18h would be rise, 6h would be set.

To find transit time: 24h - LST + RA(object) + local time
eg LST is currently 22:00 or 22 in decimal for simplicity; RA of NGC1499 is 4h (nice round number); local time is 5:30pm or 17.5 in decimal.

NGC1499 should transit at 24-22+4+17.5=23.5 which is 11:30pm. Got it.

Thanks for sticking with me on that.
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Old 26-11-2009, 06:50 PM
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And reading the star atlas, if I point whatever the LST is straight up, that shows me the stars in the current position. Got it. Thanks mate.
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Old 26-11-2009, 08:01 PM
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Originally Posted by mithrandir View Post
Al, can you explain this? The PC clock is showing 17:21 Sydney daylight saving.
If I run your program with "Use TZ" I get 22:43 sidereal.

If I switch to "Use Long" with 150deg 59" 40' E I get 20:47 sidereal, which matches the value from http://tycho.usno.navy.mil/sidereal.html

Daylight saving sign error somewhere?
I have fixed the bug. As suspected it was daylight saving error in the determine time zone routine. I added an hour to convert the time to daylight saving, when I should've subtracted to get standard time.

I've attached the updated version (v2.1) with the bug fixed, and I've sent a copy to Mike to update the link on the resources page.

Thanks for letting me know Mithrandir!

Al.
Attached Files
File Type: zip AlsAstroClock21.zip (229.5 KB, 17 views)

Last edited by sheeny; 26-11-2009 at 08:13 PM.
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Old 26-11-2009, 09:04 PM
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Quote:
Originally Posted by sheeny View Post
I have fixed the bug. As suspected it was daylight saving error in the determine time zone routine. I added an hour to convert the time to daylight saving, when I should've subtracted to get standard time.

I've attached the updated version (v2.1) with the bug fixed, and I've sent a copy to Mike to update the link on the resources page.

Thanks for letting me know Mithrandir!
Wasn't it a sign error in a program that lost a Mars mission?
It works now, and shows I'm almost 4 mins east of AEDT, even though I'm in northwest Sydney.
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Old 26-11-2009, 09:17 PM
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Its hard to beat having an old beat up planisphere on hand. I too still use the one cut out of a book and glued together 30 years ago from time to time - its like talking to a very good old friend.

I'm sure you could download one off the net. Gives you a great understanding of what you can expect to see and when in each hemisphere of the sky.
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Old 26-11-2009, 10:13 PM
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Yeah, I have a planisphere too. But there's so much more on that star atlas.
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Old 26-11-2009, 10:17 PM
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Its pretty much down to star-hopping then Troy once you've got your map and know roughly where you're looking. Comfy chair, binocs or alt/az scope, and away you go - just like the good old days....

(oops - forgot the red torch...)
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