NAME

Astro::Constants - Physical constants for use in Astronomy

SYNOPSIS

  use Astro::Constants::CGS qw(:long);
  print "The Schwarzschild radius of the sun is ",
        2 * GRAVITATIONAL * SOLAR_MASS / LIGHT_SPEED ** 2, 
        " centimetres\n";

or

  use Astro::Constants::MKS qw(:short);
  print "The Schwarzschild radius of the sun is ",
        2 * $A_G * $A_msun / $A_c ** 2, 
        " metres\n";

DESCRIPTION

This module provides a large number of physical constants which are useful to Astronomers. The module itself is essentially a wrapper around the astroconst package of Jeremy Balin.

It allows you to choose between constants in units of centimetres/grams/seconds with Astro::Constants::CGS and metres/kilograms/seconds with Astro::Constants::MKS. It also allows you to select two different methods of refering to the constants using the import tags :long and :short.

The :short tag refers to the constants in the normal perl variable style, such as $A_G for the gravitational constant and $A_c for the speed of light in a vaccuum. The naming convention for the :short constant is A_ prepended to the symbol character.

The :long tag refers to the constants with longer descriptive names in ALL_CAPS that have been created with the use constant pragma. As in the example, GRAVITATIONAL is the gravitational constant and LIGHT_SPEED is the speed of light. This is a new addition to the Astroconst package and, at present, we are following a naming convention of OBJECT_PROPERTY and SYMBOL_SUBSCRIPT. Opinions on this matter will be listened to and if we decline to agree, you can always change it yourself, as described in "EXTENDING THE DATA SET".

The :long and the :short tags import the same constants. You may choose individual constants and even mix and match (I haven't tested this) by importing them in the use statement.

One of the problems with the long constants is that they are not interpolated in double quotish situations because they are really inlined functions.

The Function List

list_constants()

returns 2 references to lists of constants, the first one for the short names, the second one for the long names. (untested)

describe_constants()

returns the description of a constant. remember to enclose the constant in single quotes and remove the $ from the beginning of the short constants. If no argument is given, it returns a reference to the underlying hash.

precision()

returns the relative precision to which that constant is know in the file. If no argument is given, it returns a reference to the underlying hash.

pretty()

runs a number through sprintf "%1.5e" to tidy up values

The Constants List

Now if I list all the constants here, this becomes a very long document and the information is already available in the *.dat files and via the list_constants() function.

EXTENDING THE DATA SET

If you want to add in your own constants or override the factory defaults, run make, edit the site_const.dat file and then run make again. If you have a pre-existing site_const.dat file, drop it in place before running make.

EXPORT

The only function automatically exported is list_constants().

right now Astro::Constants defaults to Astro::Constants::CGS qw(:long) (is that a good thing? - moot point 'cuz it doesn't work.)

ASTROCONST

(Gleaned from the Astroconst home page - http://web.astroconst.org )

Astroconst is a set of header files in various languages (currently C, Fortran, Perl, Java, IDL and Gnuplot) that provide a variety of useful astrophysical constants without constantly needing to look them up.

The generation of the header files from one data file is automated, so you can add new constants to the data file and generate new header files in all the appropriate languages without needing to fiddle with each header file individually.

This package was created and is maintained by Jeremy Bailin. It's license states that it is completely free, both as in speech and as in beer.

DISCLAIMER

Since this package uses the values provided by the astroconst package we include the disclaimer from that package here:

The Astroconst values have been gleaned from a variety of sources, and have quite different precisions depending both on the known precision of the value in question, and in some cases on the precision of the source I found it from. These values are not guaranteed to be correct. Astroconst is not certified for any use whatsoever. If your rocket crashes because the precision of the lunar orbital eccentricity isn't high enough, that's too bad.

COMPATIBILITY

(Post test results here)

Astro::Constants has been tested under:

Perl Platform Results

5.6.0 Solaris 8 Works for me (BD)

BUGS

The Author needs to check the documentation yet again.

I haven't tested the list_constants() function. I'm not really bothered. Let me know if you use it.

"use Astro::Constants" doesn't work. I'd like it to provide a default behaviour, but for now you need to be explicit and "use Astro::Constants::CGS qw( :long )" instead.

ACKNOWLEDGEMENTS

Jeremy Balin, for writing the astroconst package and helping test and develop this module.

Doug Burke, for giving me the idea to write this module in the first place, tidying up Makefile.PL, testing and improving the documentation.

-> Put Jeremy's constant references here <-

AUTHOR

Boyd Duffee, perl@cs.keele.ac.uk

Copyright 2003, Boyd Duffee

All rights reserved. There is no warranty. This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

SEE ALSO

perl(1), Doug Burke's Astro::Cosmology module, PerlDL, the National Institute of Standards and Technology.

AVAILABILITY

try the astroconst web site currently at U of Arizona which has been relocated to Astroconst.org

there might be something at astro.keele.ac.uk

cpanm Astro::Constants

perl -MCPAN -e shell
install Astro::Constants