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Main
Date: 28 Nov 2006 04:18:40
From: Emanuele D'Arrigo
Subject: Tycho Catalogue
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Greetings everybody!
There're a couple of issues with the fields of
the Tycho Catalogue I'd need some help with...
I'm looking at the searchable data available through this page
http://www.rssd.esa.int/hipparcos_scripts/hipMultiSearch.pl
The resulting fields of a search are documented in this PDF file:
http://www.rssd.esa.int/Hipparcos/pstex/sect2_02.pdf
Q1 - At page 150 it says:
"Field T11: Trigonometric parallax
The trigonometric parallax, p, is expressed in units of milliarcsec.
The estimated parallax is given for every star, even if it appears to
be insignificant or negative (which may arise when the true parallax
is smaller than its error)."
Am I correct to interpret a negative parallax as a star that is
-likely- to be very far away?
Q2 - And in the same same page:
"Fields T12-13: Proper motion components (epoch J1991.25, ICRS)
The proper motion components, ua* = ua cos d and ud , are
expressed in milliarcsec per Julian year (mas/yr), and are given
with respect to the reference system ICRS.
I don't quite understand that "cosin" based notation. It seems to hint
to the first proper motion component being related and not immediately
orthogonal to the second component.
Can anybody shed some light on these issues?
Thank you!
Manu
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Date: 28 Nov 2006 06:31:28
From: Emanuele D'Arrigo
Subject: Re: Tycho Catalogue
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Stupendous_Man wrote:
> The (RA, Dec) coordinate system we use in space
> is similar to the (longitude, latitude) system we use
> on Earth. Both have the same feature: differences of
> a fixed amount -- say, one degree -- in one of the coordinates
> (Dec in space, latitude on Earth) always correspond to the
> same actual separation. However, in the other coordinate
> (RA in space, longitude on Earth), a difference of one degree
> corresponds to a smaller and small actual separation as one
> approaches the poles.
>
> The factor of cosine(Dec) corrects for this factor; thus, if the
> Tycho table lists proper motions of 20 and 10 mas/yr, it
> really means that the star is moving twice as fast in one
> direction as the other.
Aaaaahhh!!! Thank you! That explains it! The cosinus
formula -compensate- for the diminishing dimension of the
milliarcseconds, relative to the declination. Ok, that makes
sense! =)
Thank you again, much appreciated!
Ciao ciao!
Manu
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Date: 28 Nov 2006 05:23:06
From: Stupendous_Man
Subject: Re: Tycho Catalogue
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Emanuele D'Arrigo wrote:
> Q1 - At page 150 it says:
> "Field T11: Trigonometric parallax
> The trigonometric parallax, p, is expressed in units of milliarcsec.
> The estimated parallax is given for every star, even if it appears to
> be insignificant or negative (which may arise when the true parallax
> is smaller than its error)."
>
> Am I correct to interpret a negative parallax as a star that is
> -likely- to be very far away?
Yes.
> Q2 - And in the same same page:
> "Fields T12-13: Proper motion components (epoch J1991.25, ICRS)
> The proper motion components, ua* = ua cos d and ud , are
> expressed in milliarcsec per Julian year (mas/yr), and are given
> with respect to the reference system ICRS.
>
> I don't quite understand that "cosin" based notation. It seems to hint
> to the first proper motion component being related and not immediately
> orthogonal to the second component.
The (RA, Dec) coordinate system we use in space
is similar to the (longitude, latitude) system we use
on Earth. Both have the same feature: differences of
a fixed amount -- say, one degree -- in one of the coordinates
(Dec in space, latitude on Earth) always correspond to the
same actual separation. However, in the other coordinate
(RA in space, longitude on Earth), a difference of one degree
corresponds to a smaller and small actual separation as one
approaches the poles.
The factor of cosine(Dec) corrects for this factor; thus, if the
Tycho table lists proper motions of 20 and 10 mas/yr, it
really means that the star is moving twice as fast in one
direction as the other.
For more details, see some of my class notes at
http://spiff.rit.edu/classes/phys301/lectures/coords/coords.html
http://spiff.rit.edu/classes/phys301/lectures/precession/precession.html
Good luck!
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