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Thread: Right Ascension Declination

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    Default Right Ascension Declination



    I can set up a 'scope but don't understand the theory. Couldn't find anything at my level of stupid. Any suggestions? I really don't want to waste anyone's time; I can keep looking.

    Thanks to all and thanks for having me.

    Marc

  2. #2
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    Default Re: Right Ascension Declination

    Welcome to the forum. This is the beginners' forum, so beginner questions are just fine.

    Declination is just like latitude, from +90 at the north celestial pole, to 0 at the celestial equator, to -90 at the south celestial pole.

    Right Ascension takes the place of longitude. Instead of being measured from -180 to - to +180 degrees, it is measured in hours, minutes and seconds, increasing eastwards from an semi-arbitrarily-chosen point in the sky. The numbers increase from 0 hours to 24 hours, with no negative numbers.

    What confuses people starting out is that the RA and Dec coordinates move with the sky, not with the Earth, so the point that is over head is constantly changing. It takes a clock and some math (or a computer doing the math for you) to figure out the coordinates of what is overhead.

    If that doesn't answer your question, feel free to ask something specific.
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    Default Re: Right Ascension Declination

    Download and install Stellarium from Stellarium on your computer
    Enable the equatorial grid. This will make it much clearer.

    Scott
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    Default Re: Right Ascension Declination

    To add a bit more to Keith's great explanation...

    The simpler system to understand is the "altitude / azimuth" system. But it has some drawbacks -- being aware of those drawbacks may make it easier to understand the need for the right-ascension & declination system. In the "altitude / azimuth" (alt/az) system, azimuth is simply the compass heading ... the direction in which the scope is pointing. Altitude is the angle up from the horizon.

    The "drawback" of alt/az is that the values only make sense to you and nobody else. You cannot, for example, say that you were viewing an object in 31º altitude and 170º azimuth and have anyone else know what you were looking at... this is because elsewhere in the world, a different observer would need a different alt/az to view the same object. Also... YOU would need a different alt/az to view that SAME object at a different day or time. This really limits the usefulness of the system.

    The RA/Dec system is universal...

    The star 'Sirius', for example, is located at Right Ascension 6h 45m 9s and Declination -6º 42' 58". That's not only where it is for me... that's where it for you (and I don't even know where you live). Also it doesn't matter what day or time it is... it's even at that location when it's below the horizon and cannot be viewed. This makes the system far more useful for cataloging, publishing, or sharing information with others because there's no dependency on other factors to find those coordinations and view the object.

    Also, it's easier to operate in the ra/dec system if you have an "equatorial" mount. There's no simple way to do this with an alt/az mount (unless your mount is computer driven and the computer handles the conversion for you.) An equatorial mount tilts the telescope's main axis of rotation over on an angle so that the axis of rotation is exactly parallel to the Earth's axis of rotation.

    If your scope is on an equatorial mount and the mount has been aligned to the celestial pole, then the declination axis is already going to be accurate (in the northern hemisphere the scope is pointing to declination +90º).

    To adjust the RA axis, move the scope to point to any star (or object) you can see with a known RA/Dec coordinate. Sticking with my earlier example of Sirius... after centering the scope on Sirius, you'd locate the "setting circle" (the indexed marks which run from 0 to 23) along your right ascension axis. That scale is adjustable... you can rotate the scale with your fingers WITHOUT actually moving the scope. So you'd adjust the setting circle so that it indicates 6h 45m (and most setting circles won't have enough accuracy to dial in the "seconds" value). Incidentally... you might notice your setting circle has two sets of labels... one set goes from 0 to 23... the other goes from 23 down to 0. This is simply a convenience so that the mount can be used in either the northern or southern hemispheres. The side of the scale you want to use is the one where the numbers get HIGHER as you point progressively farther EAST (well... until it gets to 23h 59mins.. then it wraps back to 0h 0m again).

    Assuming you've adjusted your RA scale, you can NOW hunt for another object by using that object's RA/Dec position. Suppose I want to find Betelgeuse (after the scope was centered on Sirius). Betelgeuse has an RA value of 5h 55m. You'd move the scope to the west (values go down as you go to the west) and you'll notice the RA setting circle moves as you move the scope... and just move it until the setting circle points to 5h 55m and snug the axis. Betelgeuse has a declination of approximately +7º 24'. You'd loosen the declination clutch and adjust the declination up to that new value and snug the dec axis. You'll find your scope is now pointing at Betelgeuse... and all without any electronics.
    Tim Campbell


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  6. #5
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    Default Re: Right Ascension Declination

    Thank you, all, for all of your help!

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    Default Re: Right Ascension Declination

    Marc,

    All good advice to you above. Please see attached graphic showing the Equatorial Grid projected onto the celestial sphere which is that imaginary glass ball surrounding the Earth in which all stars are embedded.

    For those of us involved with backyard astronomy it is probably sufficient to continue with that model and envision that the stars are stuck firmly in place in the glass and do not move relative to each other. It is the celestial sphere, or imaginary glass ball, that appears to rotate around the polar axis of the Earth.

    Again not quite true but sufficient in the first instance.

    If you do follow the advice above and download Stellarium, please try the following:

    Change your location to anywhere on the Equator, latitude = 00:00:00, longitude matters not.
    Change the view of the sky such that you're looking Northwards (N)
    Enable the Equatorial grid to be drawn.
    (You should see a series of concentric circles in DEC increments and radial lines in RA units, all centred on the horizon in the middle of the display.)
    Accelerate time to a comfortable rate and watch those RA radial lines turn slowly in an anti-clockwise direction.

    You, Sir, are now watching the lines of celestial longitude rising or ascending in the east, or more colloquially, to your right. That is the origin of the confounding term RA or Right Ascension.

    When you're on the equator, looking north, the lines of celestial longitude will be seen ascending to your right. Cool huh?

    Hoping this helps you as it did me when trying to figure out why these lines of celestial longitude were named as such.

    (They could very well have been called Left Descension LOL!)

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    Default Re: Right Ascension Declination

    One of the best descriptions of this topic is in "the stars, a new way to see them", by HA Rey. He goes into how the stars move through the night depending on the latitude of the viewer. Once you know that, you'll have a much better understanding of countless other topics, like weather, movement of the moon, and history. For example, ancient sailors found their way home with a tiny portable 1X Altaz solar telescope, the sextant. Even though those guys were living in an age when the world was flat, they were so brilliant they could see beyond the bias of the day. It's such a cool feeling, when learning one thing that may seem very dry spins off into much more fascinating topics.
    Dave

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    Default Re: Right Ascension Declination

    Video primers:
    https://www.youtube.com/watch?v=XUbG8jboh4M

    https://www.youtube.com/watch?v=s7LCK4goKBo

    If you don't have Stellarium, both videos use it, and it's FREE. Get it.

    Gary

 

 

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