Right Ascension

[Chris Stacey]

I'm getting confused with this, and the book I'm reading which is explaining it is confusing me more. As I understand it the co-ordinates change given the time of day, but I'm not sure where my starting point is, do I start north and work round clockwise, i.e east, south, west etc...

Or have I got this completely wrong?

[GazOC]

The RA of an object never changes (unless its moving like a comet or a planet) they stay fixed on the inside of an imaginary sphere but as the Earth rotates we can see different parts of that sphere.

[Chris Stacey]

The RA of an object never changes (unless its moving like a comet or a planet) they stay fixed on the inside of an imaginary sphere but as the Earth rotates we can see different parts of that sphere.

I see, so if I'm trying to look at a specific thing, say M51 for example, i'd take the RA and declination co-ordinates and locate from there? if so, where's the starting point for measuring RA from?

[Dave]

The right ascension is assigned to a line across the sky from north to south pole. Much like the longitude lines on Earth. As the sky rotates, the lines of right ascension move with it. So all objects stay the same RA measurement. The RA of the sky passing through the meridian changes according to time of day or year. The RA passing through the meridian at the any moment gives us our sidereal time. Hope that helps?

[scotastro]

Right Ascension (RA) and Declination (Dec) are fixed to the sky in the same way that Latitude and Longitude are fixed on Earth. It is Alt Az co-ordinates that change with the time as these are reference to the compass points and from horizon to zenith.

There is a point near Pisces which is 0h 0m RA and 0 deg 0 Min Dec. RA is measure in time hours, minurtes, seconds and Declination is measured in degrees, minutes, seconds.

The only reason why the sky changes with time is the rotation of the Earth on its axis and around the Sun.

Mike

[Chris Stacey]

The right ascension is assigned to a line across the sky from north to south pole. Much like the longitude lines on Earth. As the sky rotates, the lines of right ascension move with it. So all objects stay the same RA measurement. The RA of the sky passing through the meridian changes according to time of day or year. The RA passing through the meridian at the any moment gives us our sidereal time. Hope that helps?

Bingo! I've got it. Those three lines made more sense the the 5 pages of the book I've been trying to get my head around. lol.

Thank you eagleseye.

[Chris Stacey]

Right Ascension (RA) and Declination (Dec) are fixed to the sky in the same way that Latitude and Longitude are fixed on Earth.

It is Alt Az co-ordinates that change with the time as these are also reference from North, South, East and West on Earth.

There is a point near Pisces which is 0h 0m RA and 0 deg 0 Min Dec. RA is measure in time hours, minurtes, seconds and Declination is measured in degrees, minutes, seconds.

The only reason why the sky changes with time is the rotation of the Earth on its axis and aound the Sun.

Mike

I understood the rotation issue, the diagram in the book I was reading gave the impression of RA runing round the earth North, East South and West, instead of the way eagleseye described it. It all makes sense now. Cheers.

[Chris Stacey]

I thought I understood it, but I don't. Take M40 = 12:22:24. I don't understand how I find the location of that in the sky. I understand Declination, but not how I convert the above RA co-ordinate from my current location. Think I need a lay down.

[Dave]

Zero Right ascension is measured Eastwards from where the ecliptic crosses the celestial equator going northwards. This is the so called first point of Aries. Unfortunately this point does gradually move, as the earth wobbles. At the moment it is in Pisces. It will very slowly move into Aquarius.

Hence the term and the song "The dawning of the Age of Aquarius"!

That's why we have to have new editions of star maps to show the differences over a period of time to show how these lines have moved.

Dave

[MikeP]

Hi Chris,

As you now know the RA and Dec of an object are fixed. Unfortunately the Earth moves, so when you are trying locate that object you need to take into account date, time and your location on the Earth's surface. That would let you know in which direction to look / point your telescope.

I find the planetarium programs very helpful (Cartes du Ceil, Stellarium etc) because they allow you to see what is where at any time. For example, at the time of writing, SkyMap shows Betelgeuse is roughly due south (not that I can see it for clouds). Betelgeuse has an RA / Dec of about 5h 55m / 7 degrees 24 minutes but SkyMap tells me the Alt / Az is 46 degrees / 173 degrees.

If I fast forward a couple of hours, SkyMap gives the same RA / Dec but now the Alt / Az is (roughly) 41.5 degrees / 214 degrees.

Since Alt is degrees up from the horizon and Az is degrees from North (going round via East), the Alt / Az makes it easy to know where to look.

Don't know if that helps.

Mike

[Astro_Baby]

I think I understand the question. YOu need to know where the zero point is for RA.

Normally you locate an object for which you know the RA - ie 22:20:15 from there you can work out where the object you want to find is.

Theres a goodish article here http://www.skyandtelescope.com/howto/visualobserving/3304206.html?page=2&c=y

[Chris Stacey]

Thanks Mike, Astro_baby and eagleseye.

Would I be right in thinking the following is right then, taking M40 as an example. 12h:22m:24s = 12+22.24/60 = 12.37. 12.37 x 15 = 185.5 Degrees. I then rotate 185.5 degrees from the vernal equinox. Or I can follow the instructions in Astro_baby's link? (i.e subtract or add declination from a known star from the star I'm trying to loacte to get my new degrees in which to move from the first known star).

[Whippy]

Hi Chris, would I be right in thinking you're asking about RA & DEC in relation to using setting circles in your search for DSO's? If that's the case, you may find you're on a hiding to nothing as the circles on most mounts are next to useless. You may find a more practical method of searching would be star hopping where you use a nearby bright star as a starting point and 'hop' from star to star to find your target. Books like turn left at Orion show you how.

Tony..

[Chris Stacey]

Hello Tony,

That's how I'm doing it anyway, hopping from star to star. I'm getting on fine with that too. It's more of a case of teaching myself how it's done - I was reading about it and couldn't understand it, I wanted to get the formula of how it's done clear in my head, more of a technical exercise to show myself I could do it rather then out of necessity. Ended up giving myself a headache though. lol.

[Whippy]

Ended up giving myself a headache though. lol.

that sounds about right! I've found star hopping to be much more practical.

FWIW, the best description I read about understanding RA and DEC is imagine you're sitting inside a giant beach ball. On the ball itself is all those lovely celestial objects marked out. Now, *we* rotate inside the ball and the objects stay where they are, not the other way around. Hence, the given RA & DEC of a given object stays the same.

Tony..

[Barry W]

I think the GOTO systems take all of this into account for us. Manual Equatorial mounts need this RA information more as you move around the celestial coordinate system. Conversion doesn't help much with an alt azimuth mount (ie a dob, like I use). I find visually identifying the celestial equator can give a good understanding of how the stars are moving around my location and can assist with "roughly" adding or subtracting for RA to point in the general direction needed.

Moving to any any astro expertise I may have, I've copied a couple of astro-navigation terms below. They're generally the same as or show the relationship to astronomy terminology. Everything is based on Aries and calculations go from there.

It's likely not relevant to the discussion questions, but the definitions may aid in understanding.

For Chris - You said you were trying to understand the calculations and how they apply. This link (http://www.irbs.com/bowditch/) will take you to an on line book called "The American Practical Navigator", commonly called "Bowditch". He has some very good chapters on astro navigation and how to apply the various calculations. It might help you gain a better understanding.

Some definitions:

Declination : This marks the height above or below the equatorial plane for celestial objects. Equivalent to the latitude on earth. it is followed by North or South same as latitude.

GHA : GHA (Greenwich Hour Angle) indicates the position past the plane of the Greenwich meridian measured in degrees. Equivalent to longitude on earth. It can be followed by East or West.

LHA : LHA (Local Hour Angle) is the angle between the meridian of the celestial object and the meridian of the observer, LHA = GHA(of celestial object) - Longitude (of observer).

SHA : Sidereal Hour Angle. Because the fixed stars (they are not really fixed, but they are so far away that they don't seem to move much), don't move relative to each other, rather than give the GHA of each star for each hour of the day, for each day of the year, the Nautical Almanac publishes only the list of GHA for a fictitious point called "first point of Aries" (also denoted by the Zodiac sign of the Ram). The SHA of the stars is simply a coordinate relative to this point. So to calculate the GHA of a star all you have to do is: GHA(star) = SHA(star) + GHA(aries). Ideally the GHA of aries should not vary relative to the stars, unfortunately, as Hipparchus noted a long time ago, the earth wobbles, its axis doing a full circle in about 26000 years (if my memory is correct) which means that the first point of Aries slowly drifts across the background of stars, making a full circle in 26000 years. It is called the first point of aries because it is the point where the sun crosses the equatorial plane on its way from the southern hemisphere to the northern one (around the 21st of March) and it happens to fall in the Zodiac sign of Aries the Ram. Will the point change name when it has drifted into another sign? Well the answer to this one is already known as the drift has already occured and the 1st point of Aries is actually in Pisces at the moment but we still call it 1st point of Aries. Old habits die hard.

Note that astronomers and land surveyors use RA (Right Ascension) instead of SHA, usually measured in time (hours and minutes) and increasing in an Easterly direction, to convert just remember that 1hour=15 degrees, 1 minute of time = 15 minutes of arc and don't forget to change the sign. Instead of GHA Aries they use GST (Greenwich Sidereal Time).

Hope you find this has some relevance.

Cheers,

Barry

[Astro_Baby]

Ohhh I am going out on a limb here as I never use setting circles for the reasons already stated that they are generally not (on most mounts) accurate enough.

With that said the EQ3-2 has remarkably good setting circles and I am told the EQ5 does as well. The HEQ5 and EQ6 setting circles are pants which is a bit odd but I suppose those mounts were designed mainly to be used with GoTo systems.

Back to your question. A stars RA is based on the vernal equinox - ie at a fixed point in time the star is for instance 18 hours ahead of the equinox.

When your observing of course the star doesnt appear at its 'true' RA because that only occurs at the equinox. BUT if you find a known star (Aldabaran for instance) you can look up its RA coordinates which are 04h 35m 55.2s

With Aldabaran centred you set the RA dial to 04h 35m 55.2s the

RA dial is now calibrated for the stars position and by happy cooincidence every other stars position. So if you were to move the RA axis to 00h 42m 41.8s you would ( assuming the DEC was set up as well - be pointing at M32. Those being RA coordinates for M32

Now what your looking at is not actually a point in the sky that accurately refelects those RA numbers but by getting one star aligned you have set the mount to a kind of zero position. ie a known object is correctly set on the dial as all other stars retain the same separation then all other stars will appear at the correct positions on the dial.

Loosely speaking if a star 'A' is 4 hours ahead of star 'B' and you can set the RA for 0 when centred on 'B 'then you know that when the mount is rotated for +4 hours you should be looking at star 'A'

Thats how I undertsand it anyway.

Theres another way of calculating it from hour angles and time offsets but whenever I listen to anyone explain that I either go blank, get confused, fall asleep or am overcome with an urge to do violence

[Chris Stacey]

Thank you for writing that up Barry, I've certainly learnt some more by reading that. Thank you for the link too - that will come in very handy, and i'll be going through that later.

Astro_baby - I didn't have a clue about the setting circles or anything about EQ mounts, I've got an 8 " dob on it's original base. I will be getting an EQ at some point though so your post has been very informative and handy to know.

I think I understand moving from star to star using an existing known stars RA now. I shall put it to the test later and see if I'm on the right track.

Theres another way of calculating it from hour angles and time offsets but whenever I listen to anyone explain that I either go blank, get confused, fall asleep or am overcome with an urge to do violence :):)

I know that feeling well, my laptop nearly took the brunt of my frustrations yesterday, narrowly avoiding a swift right hook!

Thanks again

[adamsp123]

I took "O" level Navigation about 100 years ago we dealt with the old system of taking star sights (and doing spherical trigonometry calculations using 6 figure log tables - my, those were the days) and as I remember it works like this.

RA is equivalent of longitude in the heavens.

On earth the zero longitude is Greenwich meridian so we can measure lines of longitude from that datum. We use degrees but we could use time as 15 degrees = one hour (angle) due to earths rotation of 24 hours.

In the sky the "Greenwich meridian" equivalent is the first point of aries and the stars angles (RA) are measured in time (1 hour = 15 degrees) from this point. ie a star with a hour angle of 12 hours is 180 degrees from the first point of Aries, time instead of degrees is used to eliminate confusion between earth and stellar "longitudes".

So now at any given time for an observer to locate a star/object she/he has to know the angle (in time) between the observer and the first point of Aries and add the angle (in time) between first point of Aries and the star/object concerned.

Does that help???

Pete

[Chris Stacey]

Thanks Pete,

I'm using this link:

http://www.stargazing.net/kepler/altaz.html

...and following the steps in it every time I want to check where I should be looking for any given object - well, I will be using it the skies clear. If I print it off and follow the methods enough times it should sink in so I remember it parrot-fashion.

It's not a necessity, I just hate it when I don't understand something, and like to keep going until I do understand it!

[Barry W]

You're welcome Chris,

I actually just went back and re-read chapter 15 of Bowditch. Honestly - that's the first time in over 20 years that I've looked through those pages (it was the biggest and scariest book in our reference library at the time!)

I'd recommend this for all you other beginners out there trying to get some reference material and gather a good basic understanding for astronomy. It's as good or better that half of the beginners' guides I've gone through in the past 6 weeks with outstanding explanations and some very nice sky diagrams for basic star hopping (i didn't even remember them being there). Like other books out there now - it's also written for a novice astronomer (or young sailor with no understanding of the skies!)

Again, I strongly suggest you check it out and hope it helps (It's free in PDF format broken down by chapters!)

Cheers,

Barry

[Chris Stacey]

It certainly will help Barry, I've saved each section onto CD rom so I can go through it at my leisure.

Thanks for all the advice!

[scapps42]

Hi after reading this thread and getting a headache just thinking about it , i would like to share with you how i locate objects with my 10" dob . i have simply fixed a plastic tape measue to my base( the bit that doesn't move) which reads in MM. i have also brought a wixey angle gauge,

Next i find a easy bright star that is very easy to locate, centre in in the scope then note the Alt Az co-ordinates from stellarium. From this i know how many degress i am out in altitude on the object i want to find, so with the wixey angle gauge i move it to the correct altitude .

Next i work out how many degrees i am out in AZ , then i convert this to MM for my plastic tape measure on the base, move it according to what my calculator tells me and i am usually spot on the desired object - simple

[Chris Stacey]

Hello Scapps,

How do you convert degrees into mm on your calculator, what's the formula for that?

[scapps42]

Hi Chris, the complete diameter of my dob base reads 164.4cm on my tape measure that i had taced on, therfore 1 degree = 0.45683 (164.4 divide by 360) so if i looked on stellarium and i found that saturn was 4.3 degrees out from my desired object then i would type into my calculator 4.3 x 0.456 which would tell me to move 1.96cm . of course all you have got to work out is which way to turn the scope left or right but that should be the easy bit