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RA and DE Help PLEASE!!


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I consider myself a smart guy. I can grasp certain concepts really well. However right ascension and declination are really making my head pound. Its time to gets the meds out and take something. Here is what I do understand, and I am going to start at the basics. The north and south celestial poles are the imaginary lines drawn out through the rotational axis of the earth. It is the axis in which the sky appears to rotate on. I got that. Right ascension is the equivalent to longitude here on earth. It tells how high something is from the celestial equator. Right ascension is divided into 24 hours because that is the rotational period of the earth. I got that barely. From here on out is where my brain apparently begins to shut down and the head ache starts. Declination is how high something is off the celestial equator..... what the hell is the celestial equator... or rather how do I know where the celestial equator is to figure out where an object is. Lets say I had no idea where M42 is in the night sky but I knew its RA/DE how, by looking at the north celestial pole at 11pm, do I find M42. There has to be a starting point for me to start at. I know that the vernal equinox (I even know what that is. The point in space where the ecliptic crosses the celestial equator) is 0h and 0 degrees. So if I am in an open field at night exactly where do I look to go "Hey there is where I start"? If I can figure out the "starting point" I know how to tick back 05 34.5 (RA) and down 05 27 (DE) to find M42. But whats killing me is that if I am outside at night with ten books, a compass, a bottle of headache meds, and whiskey I still couldn't figure out where something is based off of the RA and DC system. Am I making this way more difficult than it really is, or am I just as little stupid here and need to give up on learning how this works.

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For the most part, you got it right except for RA. RA is equivilant to our longitude (E-W) on Earth and DEC is the equivilant of out latitude (N-S) on the Earth. RA starts at 0H, 0M and 0S ( a star atlas will show where this point is on a line from the north celestial axis to the south celestial axis (90 degrees from each point to the "ecliptic" - "+" when above it and "-" below it) . The ecliptic is the averaged path the planets pass through the night sky and is almost always marked on good quality atlases. "0 degrees" is equivilant to our equator on Earth with positive (+) degrees above (moving north) and negative (-) below (moving south)it.

When we look at a globe representing the Earth, we locate points on it with latitude and longitude. When we look up at the sky, it's like being inside a sphere and looking at its surface or edge and we use RA for E-W locations and DEC for N-S locations on this imaginary sphere.

That's basically all you have to know to locate positions of objects in the night sky (using a star atlas) - any more info and you'll be reaching for the meds again :grin:

This link may let you see the whole thing better. Look in a star atlas and you can locate O degrees DEC and O hrs RA.

Edited by Mr Q
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Have a look at Stellarium, a free download, it shows you the night sky at any particular time and place and is a very handy tool.

The problem with using RA and DEC is that the zero point for RA, the first point of Aries) moves with the earths rotation unlike the "fixed" longitude reference Greenich Meridian.

Best way of finding things is through star hopping, locate known bright stars that are near your target and go from there.

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But whats killing me is that if I am outside at night with ten books, a compass, a bottle of headache meds, and whiskey I still couldn't figure out where something is based off of the RA and DC system

You are missing something - you need to know the sidereal time when you are outside. Basically this tells you what the RA is of something which is due South. This varies at almost, but not quite, the same speed as clock time (24 sidereal hours happen in 23h56m of real time). If you know this, and where the pole is (the celestial equator is the line on the sky which is always 90 deg from the pole), you can just about work out where an object should be (if you are good at imagining the inside of spheres!).

NigelM

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That's for the help. I think I'm finally grasping it. I think I'm making most of this a ok harder than it is really. I do tend to over complicate things lol. Yeah I know star hoping is by far easier and I'm really good at that. But the RA/DE has always intrigued me and I can't stand it if there is something I can't understand. Its like a personal challenge to me to figure it out. I think the sidereal time is the one thing I was missing for this to make sense.

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I find it really difficult as well. Most folks I know that understand it fully say that they had it explained to them first hand at astro clubs, which is what I reckon I need to do. To be honest, the setting circles on my EQ5 are next to useless, so I haven't bothered up to now. Stellarium, or a good sky atlas is what I use, and I also have a Telrad; this way I can find most anything if I look long enough :icon_confused:

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It's not the easiest thing in the world to get your head round but once you do (...trust me, it will happen) you will never forget it. Some people find it very easy to transfer an abstract written concept and apply it to model that sits inside their head which they can manipulate to further embed the process, I suppose it's a type of visualization. It's not an intelligence thing, its just simply having a mind that is wired that way. Nothing in astronomy is difficult, it's rather that there is a lot of it but you will be surprised how quickly it all starts to blend together e.g phases of the moon and how they work. Your question was a really good one to ask, because I am sure there are many people reading this that also haven't quite nailed it - yet :grin:.

Clear skies

James

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I am not sure why you are worrying as you have an alt-az dob? with e.g. setting circles you won't be able to use RA/Dec co-ordinates to find objects on an alt-az scale unless you use a widget to convert them at a specific time and you are accurately leveled and zeroised. I might have the wrong end of the stick though and maybe you are clouded out and thinking about stuff?

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I am not sure why you are worrying as you have an alt-az dob? with e.g. setting circles you won't be able to use RA/Dec co-ordinates to find objects on an alt-az scale unless you use a widget to convert them at a specific time and you are accurately leveled and zeroised. I might have the wrong end of the stick though and maybe you are clouded out and thinking about stuff?

I don't have a good reason really. I just was reading in and astro book and I came across that chapter and it interested me. Lately I have been reading and learning about how the earth rotates on its axis and the celestial sphere How precession works etc... Its not a big deal really, Its just something I kinda started reading on and then I kinda became obsessed with.

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I don't have a good reason really. I just was reading in and astro book and I came across that chapter and it interested me. Lately I have been reading and learning about how the earth rotates on its axis and the celestial sphere How precession works etc... Its not a big deal really, Its just something I kinda started reading on and then I kinda became obsessed with.

don't get me wrong, I am all for learning and diving right into hobbies myself. I just didn't want you to be disappointed if you thought that RA/DEC was the same as ALT/AZ and your new setting circles didn't work. :smiley:

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the tip i was told was to point your scope at a known star and use something like SkEye or stellarium to get the RA/Dec numbers for that moment in time and then adjust the setting circles on the mount accordingly.

Once you have it set against a known star at that moment in time, you can then use ra/dec to find anything else in the night sky.

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the tip i was told was to point your scope at a known star and use something like SkEye or stellarium to get the RA/Dec numbers for that moment in time and then adjust the setting circles on the mount accordingly.

Once you have it set against a known star at that moment in time, you can then use ra/dec to find anything else in the night sky.

And, of course, you can do much the same with the AZ reading if you have a dob or other AZ mount.

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