Right Ascension Differences

[SniffTheGlove]

Hello, now this might sound strange and many might hink I am daft but I am starting to make a list of Messier Objects and I am writing some software to keep track and also predict rising times etc.

Now, I have look at several sites on the net that show Messier objects but the right ascension and declination are different and would like to know if they are set for different long/lat on earth.

For example in the SEDS messier object list for M1 (Crab Nebula) they list the details as:-

RA 05:34.5

Dec +22:01

However on the Nasa for beginners site they list the details as

RA 05:31:30

Dec +21:59

Which is correct or are the adjusted for a certain location. Intriged to know so I can put/use the correct details in my software.

Thanks

[Breakintheclouds]

They won't be adjusted for a particular location - RA and dec co-ordinates refer to a specific part of the sky and are the same no matter where in the world you are. My guess is that either there is just a little error in one of the measurements, or one person has taken the co-ordinate of the object's edge and the other person has used the object's centre, or something like that. Bear in mind that some objects are really quite large, and it's a matter of choice which bit you refer to with a co-ordinate.

[Merlin66]

RA and Dec positions vary due to the "precession of the equinoxes" (google)

The positions therefore must be defined by the date of the Epoch. You will find some positions quoted as 1950 or 2000 or based on the current date.

[brianb]

RA and Dec positions vary due to the "precession of the equinoxes" (google)

The positions therefore must be defined by the date of the Epoch. You will find some positions quoted as 1950 or 2000 or based on the current date.

Yeah, I'm sure that's it.

[SniffTheGlove]

Thanks for the replies and the clarifications.

[ollypenrice]

One other thing; some sources engage in the absurd practice of decimalizing seconds of arc into tenths of a minute of arc. Utterly nuts but there you go. This is what has happened in your case, 30 seconds of arc being expressed as .5.

Olly

[themos]

If you look at

SIMBAD basic query result

you will see that these numbers are close to the "FK4" and "FK5" coordinates for the object. FK4 refers to 1950 "epoch" and FK5 to 2000 epoch.

Essentially, the axis of rotation of the Earth is pointing to a different place now than it did 50 years ago so what was declination 90 then is not 90 anymore!

This is an unwanted complication and software should be able to handle RA/DEC coordinates given in some "popular" coordinate systems.

[brianb]

some sources engage in the absurd practice of decimalizing seconds of arc into tenths of a minute of arc. Utterly nuts but there you go.

Bear in mind that very few scopes point to an accuracy approaching a second of arc ... In any case what is "sensible" depends on which scope you are using. The Skywatcher Synscan handset shows / accepts positions hh:mm.d, dd:mm whereas the Celestron Nexstar wants hh:mm:ss, dd:mm:ss ... and my charts usually go to 2 decimal places of seconds in RA, 1 decimal place of seconds in Dec, which is the sort of accuracy you can get from good astrometry but is totally pointless from the point of view of actually aiming a scope.

In the Bad Old Days (before computers) 1 min in RA and the nearest 1/4 degree in Dec was considered near enough for finding an object using manual setting circles. You have to have a very accurately aligned mount and an accurate sidereal time clock for it to be worth getting any closer, especially if using e.g. 1950.0 coordinates in 1970. Get the field into the low power eyepiece & fine slew to the exact position by comparing what's in the eyepiece to what's shown on the chart ... most beginners goto scopes need this technique anyway as the pointing accuracy is no better than 10 minutes of arc due to the mechanical engineering compromises in the mount.

[SniffTheGlove]

I have now gathered and created my own online database in MySQL using the NGC catalogue and kept to what was specified it this particular list for the RA and Dec

eg.

RA Hour x

RA Minute x.x

Dec Symbol +/-

Dec Deg x

Dec Minute x

I have also added the Messier 110 and also the Herschel 400 data and then removed the duplicate information.

These just appear on my website as the 3 separate catalogue lists which are pull from the single table.

I am now going to write a location based setting and rising times for each object and to show if the are viewable or not.

Then I will create a viewing list based on the data so I can just look at my site and see the top currently non-viewed objects and the times to make an easy night.

[ollypenrice]

Brian, I agree. I just think it a bit comical to mix units in this way! A headteacher in Australia hoaxed his school into trying to decimalize their watches when they decimalized their currency...

Olly