.. _constants:
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.. role:: gold

.. create a role that makes bolded text colored blue as well
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=================
:gold:`Constants`
=================
Last revised by M. Pugliatti on 2025 FEB 4

In SCB, constants are defined in the **constants.py** library and are divided into:

#. **Universal constants**: contains values related to universal constants such as the speed of light **c**, the universal gravitational constant **G**, etc. 
#. **Planetary bodies**: contains values related to mass, GM, radius, spice ID, etc. of planetary bodies

Values can be either defined by hand or from the **scipy.constants** method and are saved in dictionaries.


----------------------
:bold:`Load constants`
----------------------

There are 3 methods to load constants in a script: 

Method (1): Load all constants
==============================

.. code-block:: python

    import scarabaeus as scb
    # Import costants
    from scarabaeus import Constants

Method (2): Load only the universal constants
=============================================


.. code-block:: python

    from scarabaeus.Constants import CONSTANTS

this is used to load only the universal constants.

Method (3): Load constants running an initialization script
===========================================================


Constants are automatically loaded when running an initialization script

.. code-block:: python
    
    from scarabaeus.scb_initialize import * 


------------------------
:bold:`Access constants`
------------------------

Once loaded, constants can be accessed in this way:

Universal constants
===================


To access the universal constants saved in the dictionary you can run: 

.. code-block:: python

    Constants.CONSTANTS["G"]

The dictionary is made of 3 values: 
- [0] The value, saved as an ArrayWUnits
- [1] The name, saved as an string
- [2] The reference, saved as an string, that indicates the origin of the value.

You can access these as: 

.. code-block:: python

    Constants.CONSTANTS["G"][0]
    Constants.CONSTANTS["G"][1]
    Constants.CONSTANTS["G"][2]

Planetary constants
===================


Planetary constants are called by planetary body's name as dictionary: 

.. code-block:: python

    Constants.earth

You can access the various fields as: 

.. code-block:: python

    Constants.earth["name"]
    Constants.earth["spice_name"]    
    Constants.earth["ref_name"]
    Constants.earth["SPICE_ID"]
    Constants.earth["mass"]
    Constants.earth["mu"]
    Constants.earth["radius"]