Celestial Coordinates Questions and other stuff

vgeric · Jul 13, 2004

Ok.... I'm back, but thats besides the point. I have just recently wanted to try the setting circles and use celestial coordinaes to find objects when I thought of something (which will very likely sound very stupid... have mercy!). If an object's celestial coordinates are almost always the same, would things get messed up with the stars movements? If my telescope is pointed at one spot in the sky, most likey something different will be there if it is a different time at night. How does it all work? I've tried sites, books, instructions and to no avail. I was just wondering if you guys could help me out on this.... I REALLY want to try and find things this way. Also, whats the difference in magnitude limitation between an 8 and 10 inch telescope? Thanks!!! VGE

Saiph · Jul 13, 2004

it depends upon your system. If you are using azimuth and altitude, yes. As the sky rotates (small lie, I know) they constantly shift through these coordinates since they are based upon the horizon of the observer. RA and DEC are always constant. However, if your telescope's setting circles (or software) doesn't shift with the sky, it'll become innaccurate shortly. Why? Because you set you scope to look at a particular point in the sky, relative to the horizon, and said, that, right now has ___Ra and ___Dec. but that just means you're using the mount to convert between alt-az coord to ecliptic. That may be your problem, if your setting circles are arent' designed to track the stars too. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

vgeric · Jul 14, 2004

http://www.telescope.com/shopping/product/detailmain.jsp?itemID=381&itemType=PRODUCT&RS=1&keyword=4.5 Thats the scope I have. I have no clue about which system my scope uses- azimuth and altitude, or RA Dec. In the instructions, it talks about RA , and DEC, so I would assume thats what it uses. I just still don't see how it would remain a constant through RA and DEC. Is there a set time for all the coordinates? I am really confused...(sorry, I'm only 15 you know! <img src="/images/icons/laugh.gif" />) Thanks again, VGE

Saiph · Jul 14, 2004

I wish I had more experience with non-automated scopes (I basically work only on research grade scopes via my university). There is no set time for RA and DEC, those lines move with the stars. So you're right, your setup should get more innaccurate as time goes one. Now, here's a jury-rigged method for using RA and DEC to find objects. Find a bright star near your target. Center it, set the RA and DEC (making sure to track with the slow adjustment screws is youtake a while) then quickly shift over to the target area. You'll probably have to hunt around a bit despite this (Heck, even with my automated setups I have to hunt 50% of the time). Check to see if your slow adjustment screws (that you use to track) shift your RA settings. If you are properly polar aligned, your DEC won't be an issue, and it should shift as you move the scope. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

heyscottie · Jul 15, 2004

Eric: An object DOES remain constant in RA and DEC coordinates. Here's how: Declination is measured from Earth's polar axis. Since the north star is approximately along this axis, we can think of it as a measurement from the north star. The rotation and revolution of the Earth doesn't change what direction the axis is pointing in, so the "rotation of the sky" is a rotation that goes around the north star. If you've ever seen pictures of star circles, you'll know what I mean -- all stars "orbit" in circles around our axis. It should be obvious that at least Declination stays the same. Right Ascension is a little bit trickier, because there is an arbitrary circle set through the heavens that indicates R.A. of zero, like the Prime Meridean in Greenwich. All RA coordinates are referenced to this. Now as the earth rotates, this reference rotates as well, so while it is NOT accurate to say that a star will appear in the same spot in the sky at all times, it IS accurate to say that it will always be the same distance from that reference, and that therefore its RA will be the same. Incidentally, you have an Equilaterally mounted telescope, so you are capable of finding things in RA-DEC coordinates. Here are the steps you need to go though, keeping in mind that you (like me) do not have the most expensive telescope, so precision of the setting circles and mount machinery may be somewhat low. 1) Polar align your telescope. There are many sites talking about how to do this. It involves levelling your mount, setting declination to 90, setting your latitude adjuster correctly, and pointing your telescope directly along Earth's axis. 2) Using your RA and DEC controls only, find a bright object whose coordinates you know, and check its RA coordinate. Set your RA circle to zero. Then move your RA control so that your RA circle registers the difference between the RA coordinates of the bright object you found and th

Saiph · Jul 15, 2004

I probably wasn't to clear on that point. The RA and DEC coordinates are fixed in the stars. Any "change" or innaccuracy is on part of your scope not tracking as the RA and DEC rotate with the night sky. Okay scott. Who am I? And how'd I give it away? <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

heyscottie · Jul 15, 2004

I believe you to be the poster formerly known as ricimer. You mentioned telescopes at your university. That makes me think ricimer. There are a few other possibilities, such as pulsar4259, AlexBlackwell, or others, but I think you are ricimer. If you're not, you may take it as a compliment that I mistake you for him! Scott

heyscottie · Jul 15, 2004

Oh, and jeez -- I just noticed you have a black hole reference post. That's a dead giveaway too!

Saiph · Jul 15, 2004

Well, kudos to you, though I did admit my identity over in another thread a few days ago. But good deductioin nontheless! Probably time to change my sig. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

the_id · Jul 26, 2004

Would someone explain the difference between the celestial poles and the ecliptic poles? Which one would be more relevant for locating an object outside the solar system?

Saiph · Jul 26, 2004

There are 5 major coordinate systems one needs to know when dealing with astronomy. 1) Latitude and Longitude. It's a good basis, and works much the same as 2 of the next ones. 2) Altitude and azimuth. It's based on your local horizons and zenith. So what works for you, doesn't work for a guy in china. Heck, it won't work for you in a few hours (it doesn't account for earths rotation). it's basically : This far from north, and this high in the sky. 3) Equatorial (a.k.a. celestial): This is the classic Right Ascension and Declination. It's an extension of the latitude and longitude lines (#1) projected into the sky, but fixed relative to the stars. The stars all rotate about the celestial axis, which has two end points in the sky (where the earth's rotational axis points) the north celestial pole (near polaris) and the south celestial pole (near the southern cross). these points are at 90 degree declination, while right ascension is like longitude, going from pole to pole (the 0 hr line goes through the vernal equanox) The celestial equator is in line with the earth's equator (key point here) All the stars have a fixed latitude and longitude. So the stars in orion always have the same RA and DEC, regardless of your local position and time. For observing you often have to convert between #3 and #2 (at least in your head) since only certain RA and DEC areas are up at any given time. 4) Ecliptic: Same system as equatorial (though its not called RA and DEC anymore, its le and be. It's difference is the "equator" region is in line with the sun's equator, and planetary orbits which make up the plane of the solar system. It's offset by 23.5 degrees from the "celestial equator" (#3). Conversion between this and equatorial requires a bit of spherical trig (see link). 5) Galactic. Same sorta deal as equatorial, except the "equator" is in line with the plane of the mil <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

the_id · Jul 27, 2004

Thank you, Saiph. What I’m wanting to do is determine the position of a star as it would be seen from orbit around Jupiter. So it sounds like ecliptic or galactic coordinates would be the best ones to use. This is a bit tricky as I’ve only just begun learning the math in the last year. But I’m using Starry Night so that’s been a big help. I’ll take a look at that link, too. Thanks again. The_Id

petepan · Jul 27, 2004

Just a small nitpick Saiph, about the South celestial pole. The Southern Cross tends to 'point' to the actual pole. The closest star is in Octans, called Sigma Octans, about Mag 5.5 It is only 'close' to the pole, not virtually on top of it. which is a real bummer too because it makes it that much harder to 'polar align', not like Polaris in the North. Cheers

Saiph · Jul 27, 2004

Id: actually, equatorial will work just fine for that too. Since you can find the RA just as easily (it's based on the stars, and are basically arbitrarily placed in the first place) and Dec starts at 90 (at polaris) and decends from there (to -90). Because that small of a displacement won't do diddley to a stars location in the sky, the coordinates are basically unchanged. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

Saiph · Jul 27, 2004

the astronomy community has started charting "epochs" in their coordinate systems. There are currently 2, 1950 and 2000. And it's just what it sounds like, they've updated the coordinates, correcting for precession (as you mentioned) and scanned the sky for proper motion corrrections (to see if the stars really did move). <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

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