# EarTh's NutAtion computation packages (ETNA)

ETNA is coded with python language; it can compute the theoretical Earth’s nutation values based on the current nutation model, such as IAU2000A or any future upgraded one. It works for all operation system (MacOs, Windows, Linux), which has a python installed. ETNA uses matrix computation, and the executing time is at least ten times shorter than the iterative based FORTRAN code for multiple entries modeling. The model (nutation series) is separated from the computation program, easy to update when the model is upgraded.

ETNA_v01.py, is the main function, it has four subroutines:

- Lharest_v01.py Compute the angles of all lunisolar components
- Pharest_v01.py Compute the angles of all planetary components
- Luso_nut_v01.py Compute the nutation induced by the lunisolar gravitational attraction
- Plan_nut_v01.py compute the nutation induced by the planetary attractions

And two parameter files:

- luso_nut.tab, the lunisolar nutation model values, currently IAU2000A values
- plan_nut.tab, the planetary nutation model values, currently IAU2000A values

To compile ENTA_v01.py, for python version (<3):

>>>execfile (‘ENTA_v01.py’)

Will do the job, but for python version (3 or later), it works with

>>>exec(open('ETNA v01.py').read())

The default compile command setting is w.r.t. Python version 3 or later. If your machine is under 3, please open the ENTA_v01.py with any text editor and make the necessary changes to the python version of your computer. Once the ENTA_v01 is successfully compiled, it is ready to compute the synthetic nutation values at any given timestamp. Please note that the input 'date' is required to be in Julian date or Modified Julian date. The ENTA_v01.py has been tested under MacOs, Windows and Linux system, the complete package, including the sample files could be downloaded here.

A sample file can be found in the software package; the figure shows the results computed from this file. For the usage of ETNA, please check the ‘Readme’ file.

Wants to know more, here are some selected references:

- Dehant, V. and Mathews, P.M., Precession, Nutation, and Wobble of the Earth, (2015), Cambridge University Press. 2015, 554 p. ISBN 9781107092549.
- Herring, T., P. Mathews, and B. Buffett (2002), Modeling of nutation-precession: very long baseline interferometry results, Journal of Geophysical Research: Solid Earth, 107 (B4), doi:10.1029/2001JB000165.
- Souchay, J., B. Loysel, H. Kinoshita, and M. Folgueira (1999), Corrections and new developments in rigid earth nutation theory-III. Final tables, Astronomy & Astrophysics Supplement Series, 135 (1), 111-131.
- Roosbeek, F., and V. Dehant (1998), RDAN97: An analytical develop ment of rigid Earth nutation series using the torque approach, Celestial Mechanics and Dynamical Astronomy, 70 (4), 215-253.