RA & Dec question

[Llamanaut]

I'm bracing myself for a barrage of abuse over this however they do say the only silly question is the one you didn't ask... i have a question about RA & Dec... In starcharts and messier lists the ra and dec are often listed, as the earth is always moving surely these co-ordinates are only useful for a particular geographical area at a particular adte and time? How are they useful in pinpointing the object?

[brantuk]

Woops - I'm thinking of alt/az - I really shouldn't answer stuff when I just got up lol

[Vega]

Hi Llamafarmer

I see you're logic but I think you are thinking of what happens with Alt/Azm co-ordinates. Alt/Azm are the co-ordinates that require a specific date and time as they are relative to the horizon (and the sky is moving all the time). So any given objects alt/azm is constantly changing.

However RA and DEC are the very co-ordinates used to solve this. The RA and DEC co-ordinates are like an imaginary grid painted on the sky. This grid is moving all the time just as the sky is. So the co-ordinates for any given object never change. You do however need a telescope mount to follow these co-ordinates.

Hope im making some kind of sense, its difficult to describe without nice little diagrams lol

Matt

[Rui]

The celestial coordinates give you the position of every point in space (or in the celestial sphere, if you imagine it as a globe around us). Of course this celestial sphere seems to rotate around Earth, but it gives you the position relative to the other objects in space and gives you a position in a sky chart.

[Vega]

Woops - I'm thinking of alt/az - I really shouldn't answer stuff when I just got up lol

hehe.. oh well. Yeah I only find alt/azm useful when trying to calculate when an object is going appear from behind my neighbours tree or if an object will get high enough to peek above the fence.

Matt

[Talitha]

In starcharts and messier lists the ra and dec are often listed, as the earth is always moving surely these co-ordinates are only useful for a particular geographical area at a particular adte and time? How are they useful in pinpointing the object?

They're essential when i'm navigating the sky via the setting circles on my LX-10, but other than that i rarely look at them.

[themos]

How are they useful in pinpointing the object?

Their use helps when you'are already pointing at an object with a known RA/DEC and you want to find another one.

Say, you're at 6h/+30 and you want to find 8h/+40. That means you need to swing North (in declination) by 10 degrees and "wait 2 hours" or swing East (in right ascension) by 30 degrees (the sky turns 15 degrees every hour, 360 degrees in 24 hours). That last 30 degrees, note, does NOT mean an angular separation of 30 degrees but a 30 degree rotation around the RA axis of the mount. If you're pointing straight at the North Pole (declination +90), all RA values correspond to the same point.

The Sun is at 1hour 27minutes and 24seconds at the moment (according to Stellarium), Mercury at 2h32m36s and Venus even more to the east at 2h53m40s.

[themos]

Oh, it's you!

[Vega]

I find it much easier to pick up synscan controller and press ... 'Mars' or 'M51' or.... lol

Sorry couldn't resist.

Joking apart , I have very recently found a use for RA/DEC co-ordinates. When I want to image an object over a long period of time which is in the Eastern half of the sky and I want to avoid the mounts automatic meridion flip. I slew to the opposite side of the mount (eastern) to prematurely bypass the meridian flip. This means not using goto as it would normally slew the scope to the Western side of the mount. So instead I slew manually using the RA/DEC coordinates of my target with hand controller.

Matt

[themos]

sweet! I'm going to try this.

[FraserClarke]

I find it much easier to pick up synscan controller and press ... 'Mars' or 'M51' or.... lol

Good post, and nicely demonstrated point. GoTos and the such are great -- but you'll improve your observing (and understanding of the night sky) if you learn about RA/Dec/time, and understand how different parts of the sky relate to each other.

You would be surprised at the number of professional astronomers who try to observe objects in the wrong hemisphere, or at the wrong time of year, because they forget about inconvenient things like RA, dec and LST...

[acey]

I never bothered much with RA/Dec when I was starting out, but these days I find them very useful. Suppose I was reckoning on doing some deep-sky observing at 10pm tonight. A quick check with a planisphere would show me that the sector of the sky that will be due south at (roughly) that time is "12 hours RA". Objects in that part of the sky are at their highest then, and best for viewing. The NGC is listed by right ascension, and 12 hours corresponds to about NGC4000. So anything numbered, say, 3500 to 4500 is going to be a good target.

So I get myself set up and look on my target list of galaxies, and let's suppose I want to look at NGC4000. My atlas page is covered with little galaxies numbered around 4000, but I can see that the numbers are increasing as I scan across the page from right to left. So I can quickly see how far across the page I should need to go to hit 4000 - I just need the Dec number to tell me how far down the vertical scale I need to go to pinpoint my target. Then of course I have the fun of finding it with the telescope.

For a beginner, RA and Dec are still useful. Subtract your latitude from 90 and you will get (minus) your horizon limit: for example, my latitude is 55, so my horizon limit is -35 degrees. If I see a galaxy listed as Dec -30 degrees then I know I can see it (at the right time), but it will be only 5 degrees above my horizon - so in practice it's unfeasible. And knowing that 12 hours RA is due south at 10pm tonight tells me that Leo, Coma and Virgo are the places to look. If my target list has an object at 6 hours RA then I know can forget about it for the time being.

So for navigating around the sky, RA and Dec are actually more useful than constellations, once you get used to them - which is why astronomers have been using them for centuries.

[themos]

You could also do with a print out of a table of what RA crosses the meridian at astronomical twilight (sampled every fortnight, say).

[ollypenrice]

Llamafarmer, your question does have a sound basis. Lines of latitude have a natural meaning because the earth spins on an axis. Lines of latitude are, therefore, perpendicular to that axis. (If the earth did not spin then where we chose to put the 'equator' would be entirely up to us as long as it was on one of the great circles.)

However, lines of longitude are entirely arbitrary apart frpom having to connect the poles, which is why it was so hard to find out where they were if you were at sea. There is nothing in nature at all special about Greenwhich or its meridian but when you have the best navy and the biggest economy and a fine observatory you can tell the rest of the world where the definitive line of longitude is going to be. And we did!

The same arbitrariness is then projected out into space when an extension of the Greenwich meridian is imposed on the imaginary celestial sphere and made the first line of RA, 00.00hrs. Aha, but since the Greenwich meridan is moving (as you point out), when do you shout 'now' and stop the clock for a mo while someone climbs up a ladder and paints this line on the celestial sphere?? Anxious to render the moment repeatable it was decided to do so when the Greenwich Meridian crossed the ecliptic, the sun's path, on the vernal equinox. This is known as the first point of Aries. Of course the ruddy Earth has a wobbly axis so the first point of Aries is not in Aries any more but Hey-Ho...

And so to the way the professionals, and diehard amateurs, use RA. On the moment when the man with the white paint runs up the ladder from Greenwich into space each year to paint on the zero line of RA we also set our sidereal clocks to zero and this is sidereal midnight. Sidereal time paces ahead of regular time by about 4 mins a day because the earth moves around its orbit. However, the professionals, and anyone with a decent planitarium programme, can always check their sidereal time (corrected for their offset from Greenwich), wait till it is the same as the object's RA, open the dome and discover that... it isn't night!

Time of a cup of tea.

Olly

[Vega]

Good post, and nicely demonstrated point. GoTos and the such are great -- but you'll improve your observing (and understanding of the night sky) if you learn about RA/Dec/time, and understand how different parts of the sky relate to each other.

You would be surprised at the number of professional astronomers who try to observe objects in the wrong hemisphere, or at the wrong time of year, because they forget about inconvenient things like RA, dec and LST...

I agree with you fully on that one. For the first 2 years I used an EQ3 mount and relied heavily on charts, finderscope and starhopping. For me, this made me learn the sky more quickly than anything else.

I must admit however, my lazyness and love of gizmo's that make nice whizzy noises got the better of me. The GOTO has become a brilliant time saver for my already time consuming imaging sessions. It also has made it a much more pelasing experience for friends and family when I want to quickly give them a whistle stop tour of night sky gems. There's nothing worse than explaining how wonderful an object is that they are about to see... only to make them wait 15 minutes for me to lign everything up. lol Above all that, I didnt have too many choices when it came to mounting my 250px

Matt

[ollypenrice]

Ah, but when I wnated to image the soap bubble nebula, newly discovered and without a catalogue number, it was RA and Dec all the way...

Olly

[Astro_Baby]

Declination doesnt change so you can always set the Declination and then 'sweep' along the RA until you find the object. With that said I have yet to own a mount where the DEC/RA scales are worth a damn.

[ollypenrice]

Declination doesnt change so you can always set the Declination and then 'sweep' along the RA until you find the object. With that said I have yet to own a mount where the DEC/RA scales are worth a damn.[/QU

Me neither, A-B, but Santa is comng to France next week...

Olly

[FraserClarke]

Anxious to render the moment repeatable it was decided to do so when the Greenwich Meridian crossed the ecliptic, the sun's path, on the vernal equinox. This is known as the first point of Aries. Of course the ruddy Earth has a wobbly axis so the first point of Aries is not in Aries any more but Hey-Ho...

Isn't it a bit more fundamental than that?? I thought the first point of Aries is defined as where the Sun crosses the celestial equator going North (i.e the Vernal equinox). I don't think it has anything to do with the Greenwich meridian or any Earth based time/longitude systems.

As you say though, it's all a bit arbitrary, as that point moves in the sky due to the wobble!!

[Llamanaut]

Thanks for all your answers, i had to read the ra/dec section in my basic astronomy book at least three times before it started sinking in... think ive got the concept now i was confusing lat and long with ra and dec... thanks everyone