Constants

Constants

The constants subpackage defines physical constants related to the terrestrial planets, moons, and asteroids. Each constant is an instance of an astropy Constant class, which has the attributes name, value, uncertainty, unit, and reference.

Each body has several attributes, including

• gm
• mass
• mean_radius (aliased as r)
• volume_equivalent_radius
• volume
• mean_density
• gravity_mean_radius
• angular_velocity
• rotational_period
• orbit_angular_velocity
• orbit_semimajor_axis
• orbit_eccentricity
• orbit_inclination
• orbit_period

Additional parameters are defined when appropriate. To see all information about an individual constant, it is only necessary to use the print function:

In [1]: print(pysh.constants.Mars.mean_radius)
  Name   = Mean radius of Mars
  Value  = 3389500.0
  Uncertainty  = 0.0
  Unit  = m
  Reference = MOLA_shape: Wieczorek, M. (2024). Spherical harmonic models of the shape of Mars (1.0.0) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.10794059

To use the value of a constant in a calculation, such as in this simple calculation of the circumference in kilometers, it is only necessary to access its value attribute:

In [2]: 2 * np.pi * pysh.constants.Mars.mean_radius.value / 1000
21296.856598685208

To convert to a different set of units, one only needs to use the to method. For example, this converts the rotational period of Callisto from seconds to days

In [3]: pysh.constants.Callisto.rotational_period.to('day')
<Quantity 16.68901797 d>

and this computes the gravitational acceleration on the mean planetary radius of Mercury and then returns the value as a simple float in mGals:

In [4]: (Mercury.gm / Mercury.mean_radius**2).to_value('mGal')
370218.70697392424

Physical constants from the Committee on Data for Science and Technology are provided in the submodule codata, and a few of these (such as G and mu0) are referenced in the main constants namespace.