"STELLAR TWINS" IN THE SLOAN DIGITAL SKY SURVEY

Chris L Peterson wrote:
Keep in mind, Martin is working with data extracted from professional
catalogs, many of which use degrees for RA. This includes the Sloan
Digital Sky Survey. It is perfectly reasonable for him to use the native
units, rather than converting them.

The SDSS uses degrees because of the involvement of physics
institutions. What they have done is a regrettable mistake. Repeating
that mistake, particularly when the audience is amateur astronomers, may
not be particularly unreasonable, but it isn't very friendly either.
After all, it would be a lot more efficient to divide the values by 15
once rather than force everyone who visits the page to do so on their
own. This is particularly annoying if one wishes to copy and paste the
values directly into a web site site as SIMBAD or NED. But whatever.

What I can't get over is that Martin couldn't be bothered to actually
state the units he is displaying. I was taught that a number without
units is meaningless. We get away with not stating the units for
astronomical coordinates only because we adhere to the standard of RA in
hours. If one is going to deviate from that standard one should make it
clear what units they are using.

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://comets.skyhound.com

To reply take out your eye

On Sat, 01 Dec 2007 20:42:20 -0700, Greg Crinklaw
wrote:

While it might be argued that hours of R.A. are not
necessary, there are very few (if any) reasons why degrees would be any
more useful. Very few, if any, astronomers do spherical trigonometry on
their calculators and it is trivial to do unit conversions on the more
sophisticated math applications.

Well, the example I gave was meteor science. We do spherical trig
calculations all the time. And while there was a time that using hours
for RA made good sense, that time has mostly passed. These days, the
choice of units in general isn't all that important, since we usually
have tools of one kind or another handling things for us. I do think it
is rather ugly to have a spatial coordinate system that uses different
units for each axis.

And of course computers use radians
internally anyhow.

Radians would be okay, but IMO degrees are better, since one degree
provides a very convenient level of integer granularity for so many
applications. Radians would require a couple of decimal places, which is
a little more awkward. But my main complaint with using hours for RA
isn't specifically that unit, but the fact that it is a different unit
than is used for declination.

Whether you will admit it or not, the standard unit for R.A. in
astronomy is hours and has been for a long time.

I believe that's pretty much what I said. But I think "convention" would
be a better word than "standard", and conventions change. There has been
an increased use in degrees for RA in recent years, and I predict that
trend will continue, both in papers and in catalogs. I don't expect the
convention of using hours to change on printed charts for a long time.

I will personally
continue to use hours because this unit relates to the sky in a more
useful way than degrees.

That's fine. As I pointed out recently to a SkyTools beta tester,
however, my decision whether to purchase the next version will hinge on
whether or not it provides me- the user- with a global option allowing
me to use the units of my choice, and the format of my choice. As you
said, computers don't care, but humans do. No modern astronomical
software should lock its users into the favorite units of the developer.

Not to restart the argument--I just feel that it should be pointed out
that your views do not necessarily reflect those of other astronomers.

Clearly. But they obviously do reflect the views of quite a few.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Greg - In any material you publish you are fully entitled to use
whatever units you wish. Please allow me the same courtesy in the
material I create.

As you correctly point out there are a number of views on the topic
but at the moment I am not fully convinced by your reasoning.

Martin Nicholson, Daventry, England.
http://www.martin-nicholson.info/1/1a.htm

Visit the Astronomical Hall of Shame at http://www.geocities.com/queen5658/


Not to restart the argument--I just feel that it should be pointed out
that your views do not necessarily reflect those of other astronomers.

Greg

--
Greg Crinklaw

The SDSS uses degrees because of the involvement of physics
institutions. What they have done is a regrettable mistake.

The people who made the decision for SDSS to use decimal degrees are
astronomers, not physicists.

After all, it would be a lot more efficient to divide the values by 15
once rather than force everyone who visits the page to do so on their
own.

Actally the reason to use decimal degrees is to make it less likely to
have computer bugs, all the programs take sines and cosines of the
angles, and it makes sense to have both the RA and dec in the same
type of units. And for some reason no one wanted to use radians.

Greg Crinklaw wrote:

After all, it would be a lot more efficient to divide the values by 15
once rather than force everyone who visits the page to do so on their
own. This is particularly annoying if one wishes to copy and paste the
values directly into a web site site as SIMBAD or NED. But whatever.

Actually, both SIMBAD and NED accept positions in decimal degrees,
as well as in sexigesimal notation.

Railing against the transition of coordinates from HMS to decimal
degrees is like complaining about the switch from photographic film to
CCDs. The change is going to happen, because searches in databases
are much easier to write (correctly) when one uses decimal degrees,
just as images of the sky are much easier to take when one uses CCDs
than photographic plates.

Like Greg, I was part of the SDSS team when the decision was made to
put decimal degrees into the project standards instead of hours,
minutes and seconds. And, like Greg, I'm an astronomer, not a
physicist. The decision wasn't made by physicists, but by
astronomers. Please don't make authoritative statements about
subjects in which you are ignorant.

Stupendous_Man wrote:
Greg Crinklaw wrote:

After all, it would be a lot more efficient to divide the values by 15
once rather than force everyone who visits the page to do so on their
own. This is particularly annoying if one wishes to copy and paste the
values directly into a web site site as SIMBAD or NED. But whatever.

Actually, both SIMBAD and NED accept positions in decimal degrees,
as well as in sexigesimal notation.

Railing against the transition of coordinates from HMS to decimal
degrees is like complaining about the switch from photographic film to
CCDs. The change is going to happen, because searches in databases
are much easier to write (correctly) when one uses decimal degrees,
just as images of the sky are much easier to take when one uses CCDs
than photographic plates.

That's utter nonsense unless you consider it "progress" to become
completely out of touch with the sky. The choice to use RA in hours
wasn't some stupid mistake made in ignorance by generations past. It is
a useful convention for those who's relationship to the sky goes beyond
a computer screen in some basement somewhere.

Like Greg, I was part of the SDSS team when the decision was made to
put decimal degrees into the project standards instead of hours,
minutes and seconds. And, like Greg, I'm an astronomer, not a
physicist. The decision wasn't made by physicists, but by
astronomers. Please don't make authoritative statements about
subjects in which you are ignorant.

I'm having some difficulty parsing this paragraph. I don't recall
anyone named "Stupendous Man" being associated with the project. What
you say does not gel with what I have been told. If that makes me
ignorant so be it.

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://comets.skyhound.com

To reply take out your eye

Chris L Peterson wrote:
On Sat, 01 Dec 2007 20:42:20 -0700, Greg Crinklaw
wrote:

While it might be argued that hours of R.A. are not
necessary, there are very few (if any) reasons why degrees would be any
more useful. Very few, if any, astronomers do spherical trigonometry on
their calculators and it is trivial to do unit conversions on the more
sophisticated math applications.

Well, the example I gave was meteor science. We do spherical trig
calculations all the time. And while there was a time that using hours
for RA made good sense, that time has mostly passed. These days, the
choice of units in general isn't all that important, since we usually
have tools of one kind or another handling things for us. I do think it
is rather ugly to have a spatial coordinate system that uses different
units for each axis.

Of course it makes sense for meteor science to use degrees--for altitude
and azimuth. But I fail to see the relevance to publishing catalog
positions on the celestial sphere. And you conveniently ignored the
rest of my argument...

It's also interesting how you have ignored the points I have made
regarding the usefulness of RA in hours. Just because your connection
to the sky does not include the traditional conventions does not
invalidate their utility to the rest of us, nor does it imply that we
are dinosaurs who don't get the modern world. If the modern world means
calling oneself and astronomer without having any real connection to the
sky then I say, "No Thanks." I don't believe that losing the connection
to the sky is something to be proud of.

And of course computers use radians
internally anyhow.

Radians would be okay, but IMO degrees are better, since one degree
provides a very convenient level of integer granularity for so many
applications.

If course hours do an even better job of this in most applications.

But my main complaint with using hours for RA
isn't specifically that unit, but the fact that it is a different unit
than is used for declination.

Of course it is. But that objection is difficult to justify. It is
more useful to specify RA in hours for the reasons I have listed
previously (I see no reason to repeat them). Utility trumps aesthetics.

Besides, if you tell me something is near 5 hours and the celestial
equator I know you mean in Orion. That's from a lifetime of association
with the "conventional" units. Why on earth would I wish to part with
that just so someone could feel a tiny bit more smug about the (still
very arbitrary) units matching?

That's fine. As I pointed out recently to a SkyTools beta tester,
however, my decision whether to purchase the next version will hinge on
whether or not it provides me- the user- with a global option allowing
me to use the units of my choice, and the format of my choice. As you
said, computers don't care, but humans do. No modern astronomical
software should lock its users into the favorite units of the developer.

I'm not sure if my tester knows this or not, because I haven't gotten
around to wiring the selection on the configuration dialog, but the
display format of coordinates will be globally selectable by the user.
Typical input formats are already recognized. As I said the last time
we had this discussion, at least we agree about the role of software in
accommodating the quirks of human interaction.

Not to restart the argument--I just feel that it should be pointed out
that your views do not necessarily reflect those of other astronomers.

Clearly. But they obviously do reflect the views of quite a few.

Not among the ones I know. Oh well.

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://comets.skyhound.com

To reply take out your eye

On Sun, 02 Dec 2007 21:49:38 -0700, Greg Crinklaw
wrote:

Of course it makes sense for meteor science to use degrees--for altitude
and azimuth. But I fail to see the relevance to publishing catalog
positions on the celestial sphere.

We don't normally use altaz for meteor work. Any altaz measurements are
usually converted immediately to RA/dec. And since we are doing great
circle calculations and other trig operations, and area calculations, it
would make absolutely no sense to use hour units for RA. The radiant
chart at http://www.cloudbait.com/science/showers.html is an example of
how things are typically presented in meteoritics. Shower radiants are
given in degrees of RA and degrees of declination, which makes
calculations simple.

It's also interesting how you have ignored the points I have made
regarding the usefulness of RA in hours. Just because your connection
to the sky does not include the traditional conventions does not
invalidate their utility to the rest of us, nor does it imply that we
are dinosaurs who don't get the modern world.

I didn't say otherwise, or even imply it. I said that an increasing
number of applications are finding it useful to simply work in degrees.
I'd much rather have ease of calculations than some mythical "connection
with the sky". In the end, they're just units. Why do you care what
other people choose to use? Lots of astronomers still use angstroms. I
only use nanometers. Angstroms has "historical value", but is a little
harder to work with (not as hard as hours, though). I can read either
just fine, though. Of course, as you pointed out earlier, units need to
be used if there's any possibility of confusion.

Radians would be okay, but IMO degrees are better, since one degree
provides a very convenient level of integer granularity for so many
applications.

If course hours do an even better job of this in most applications.

No, they don't. Having different units on different axes is awkward,
pure and simple. The second you want to do any calculations you have to
start doing conversions. Hours are useful for doing a few rule-of-thumb
sort of calculations about what is crossing the zenith, that's about it.
And you can come up with equally simple rules that work with degrees.

Besides, if you tell me something is near 5 hours and the celestial
equator I know you mean in Orion. That's from a lifetime of association
with the "conventional" units. Why on earth would I wish to part with
that just so someone could feel a tiny bit more smug about the (still
very arbitrary) units matching?

Nobody is suggesting you part with it. You seem pretty comfortable
telling everybody doing arithmetic astronomy to "just multiply by 15".
Well, I'll suggest you just divide by 15.

I'm not sure if my tester knows this or not, because I haven't gotten
around to wiring the selection on the configuration dialog, but the
display format of coordinates will be globally selectable by the user.
Typical input formats are already recognized. As I said the last time
we had this discussion, at least we agree about the role of software in
accommodating the quirks of human interaction.

Excellent. It is very important to me to be able to produce charts with
RA in degrees.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Greg Crinklaw wrote:

Stupendous_Man wrote:
Greg Crinklaw wrote:

After all, it would be a lot more efficient to divide the values by 15
once rather than force everyone who visits the page to do so on their
own. This is particularly annoying if one wishes to copy and paste the
values directly into a web site site as SIMBAD or NED. But whatever.

Actually, both SIMBAD and NED accept positions in decimal degrees,
as well as in sexigesimal notation.

Railing against the transition of coordinates from HMS to decimal
degrees is like complaining about the switch from photographic film to
CCDs. The change is going to happen, because searches in databases
are much easier to write (correctly) when one uses decimal degrees,
just as images of the sky are much easier to take when one uses CCDs
than photographic plates.

That's utter nonsense unless you consider it "progress" to become
completely out of touch with the sky. The choice to use RA in hours
wasn't some stupid mistake made in ignorance by generations past. It is
a useful convention for those who's relationship to the sky goes beyond
a computer screen in some basement somewhere.


ah but that "is" their relationship to the sky and the only one
they will ever have, and damned well paid for it! There's the rub...



Like Greg, I was part of the SDSS team when the decision was made to
put decimal degrees into the project standards instead of hours,
minutes and seconds. And, like Greg, I'm an astronomer, not a
physicist. The decision wasn't made by physicists, but by
astronomers. Please don't make authoritative statements about
subjects in which you are ignorant.

I'm having some difficulty parsing this paragraph. I don't recall
anyone named "Stupendous Man" being associated with the project. What
you say does not gel with what I have been told. If that makes me
ignorant so be it.

Greg

--
Greg Crinklaw
Astronomical Software Developer
Cloudcroft, New Mexico, USA (33N, 106W, 2700m)

SkyTools: http://www.skyhound.com/cs.html
Observing: http://www.skyhound.com/sh/skyhound.html
Comets: http://comets.skyhound.com

To reply take out your eye

On 2007-12-03, Greg Crinklaw wrote:
That's utter nonsense unless you consider it "progress" to become
completely out of touch with the sky. The choice to use RA in hours
wasn't some stupid mistake made in ignorance by generations past.

The choice to use RA and Dec in different units was done in the days
before computers. Now there are different reasons.

Most astronomers use computers more than they use telescopes these
days.

I'm having some difficulty parsing this paragraph. I don't recall
anyone named "Stupendous Man" being associated with the project.

Who do you recall being involved with the project?

I can assure you that Dr Stupendeous Man was heavily involved in the
setup of SDSS.

What
you say does not gel with what I have been told. If that makes me
ignorant so be it.

So be it.