epoch 1950 to 2000, how?

[kitsuna (Adam)]

Hi all, just looking for a bit of advice.

I've recently bought all 3 volumes of Burnham's Celestial Handbook. I'm really enjoying it, but the books are quite old (1977 for my edition, AND YES THAT QUALIFIES AS OLD). As a result, all the RA and DEC coordinates are all recorded in epoch 1950. This makes it an interesting historical document, but not much good for actually finding celestial objects.

I'm vaguely aware that the difference between epochs occurs due to the movement of the earth and the difference between where the first point of Aries was in 1950 as apposed to 2000.

I'm wondering if anyone would be able to advise me how to convert the RA and DEC coords from epoch 1950 to 2000. Can this be done simply (as in, could a reasonable person do it in their head?) or does this require any complex calculations, or is there a calculator available somewhere.

Thanks guys.

[Rob_K]

Yep, it's because of precession. Well there's a hard way:
http://www.stargazing.net/kepler/b1950.html

Or an easy way:
online calculator

Trouble is, you don't know what assumptions are being made with the online calculator. Good luck!

Cheers -

[gary]

H Adam,

Hopefully the link that Rob kindly forwarded to the online calculator will help.

The transformations required are complex and beyond what the vast majority of
people could do in the head. Even with the benefit of a note pad, pen and non-programmable
pocket calculator, the steps involved would still be tedious by 21st Century standards, though it was
not that long ago that people had to perform them using much less.

So, to convert from one epoch to another, those online calculators are
all you need to know if the ends justifies the means.

But for the curious, here is some additional background.

Originally Posted by kitsuna

I'm vaguely aware that the difference between epochs occurs due to the movement of the earth and the difference between where the first point of Aries was in 1950 as apposed to 2000.

It is for the large part, but by way of background, it is a little bit more complicated
than that.

The two Epochs are B1950.0 and J2000.0.

The 'B' and 'J' prefixes are also important.

The 'B' stands for Besselian, named after Wilhelm Bessel, and the "Besselian epochs"
such as B1950.0 were based on the tropical year (i.e. a solar year) in 1900,
the reference being when the Sun's longitude was 280 degrees toward the
beginning of the year. The 'J' stands for Julian and such epochs are based on a year
being exactly 365.25 days long.

An additional subtlety is the fact that the J2000.0 system uses a revised
reference frame, known as FK5, which in 1974 replaced the previous system known
as FK4. For example, FK5 takes into account the orbital motion of stars around the
Galactic center - a very tiny difference - but nevertheless arose as knowledge
of the Galaxy grew.

As a footnote, the FK systems, which stood for Fundamental Katalog, grew in a
large part out of the efforts of two astronomers who's names also end in F and K,
namely Fricke and Kopff.

On a personal note, I am distantly related to August Kopff, who passed away in 1960.
When we developed Argo Navis, I found myself at one point in the UNSW library having
to read some of the introductory sections in FK4 and FK5. It is not light reading.
We only wish we could have had August Kopff available as a consultant!

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Outbackmanyep]

The discrepancies in the co-ords wouldn't make that much difference in finding the object would it? Not visually at low power, surely.

[Rob_K]

Originally Posted by Outbackmanyep

The discrepancies in the co-ords wouldn't make that much difference in finding the object would it? Not visually at low power, surely.

I think it does OBMY - just looking in Starry Night at a random star, the angular difference between JNow & J2000 was almost 7 arcminutes in RA direction (0.5m in co-ord RA). Run back fifty years further and you're well over a degree different.

Cheers -

[gary]

Originally Posted by Outbackmanyep

The discrepancies in the co-ords wouldn't make that much difference in finding the object would it? Not visually at low power, surely.

Hi Chris,

As Rob pointed out, it comes as a surprise to many enthusiasts how significant
the rate of precession is.

It shifts at about 50.3 arc seconds a year - about the diameter of Jupiter.

So going between the B1950.0 Epoch to J2000.0 is about 2515 arc seconds,
which is equivalent to 41.9 arc minutes. Depending upon how extended the
target object is, that can make quite a difference in reliably
locating and identifying it through 'blind pointing', if one relies
on the B1950.0 RA/Dec co-ordinates alone.

Best Regards

Gary

[Barrykgerdes]

Originally Posted by kitsuna

Hi all, just looking for a bit of advice.

I've recently bought all 3 volumes of Burnham's Celestial Handbook. I'm really enjoying it, but the books are quite old (1977 for my edition, AND YES THAT QUALIFIES AS OLD). As a result, all the RA and DEC coordinates are all recorded in epoch 1950. This makes it an interesting historical document, but not much good for actually finding celestial objects.

I'm vaguely aware that the difference between epochs occurs due to the movement of the earth and the difference between where the first point of Aries was in 1950 as apposed to 2000.

I'm wondering if anyone would be able to advise me how to convert the RA and DEC coords from epoch 1950 to 2000. Can this be done simply (as in, could a reasonable person do it in their head?) or does this require any complex calculations, or is there a calculator available somewhere.

Thanks guys.

Hi Adam
Yes as Gary says Precession makes a bigger change in Coords than most people realise.

Here is a little program in compiled basic from Peter Duffet Smith's book "Astronomy with your personal computer" that I wrote some time ago and just tidied it up a bit. You can enter any date and calculate precession for another date for the RA and DEC of an object.

[Blue Skies (Jacquie)]

Originally Posted by Outbackmanyep

The discrepancies in the co-ords wouldn't make that much difference in finding the object would it? Not visually at low power, surely.

I've always thought that if you're just starhopping for visual enjoyment that it wouldn't be a huge issue, but with all the goto gear that's around these days it certainly looks like it is.

[Outbackmanyep]

Thats incredible, definitely a pub fact to remember! Thanks guys n gals!

[kitsuna (Adam)]

I'd just like to thank everybody for their advice, input and links to calculators. It's been illuminating.

Honestly I reckon I'll just get the full Stellarium library and use that where possible.