Add IERS DUT1 to Skyfield internal array builder

For #452.

.gitignore

@@ -12,6 +12,7 @@
 .settings
 .tox/
 /dist/
+/finals2000A.all
 /skyfield/documentation/_build/
 /tmp*
 CometEls.txt

builders/build_arrays.py

@@ -1,13 +1,18 @@
+#!/usr/bin/env python3
+
 import argparse
 import sys
 
+from skyfield.api import load
+from skyfield.data import iers
 from numpy import array, savez_compressed
 
 def main(argv):
     parser = argparse.ArgumentParser(description='Save arrays to a file')
     parser.parse_args(argv)
+
     g = globals()
-    savez_compressed('nutation', **{key: g[key] for key in [
+    savez_compressed('skyfield/data/nutation', **{key: g[key] for key in [
         'ke0_t',
         'ke1',
         'lunisolar_longitude_coefficients',
@@ -20,6 +25,18 @@ def main(argv):
         'se0_t_1',
     ]})
 
+    f = load.open(iers.FINALS_URL)
+    mjd, dut1 = iers.parse_dut1_from_finals_all(f)
+    delta_t_recent, leap_dates, leap_offsets = (
+        iers.build_timescale_arrays(mjd, dut1)
+    )
+    savez_compressed(
+        'skyfield/data/iers',
+        delta_t_recent=delta_t_recent,
+        leap_dates=leap_dates,
+        leap_offsets=leap_offsets,
+    )
+
 # ----------------------------------------------------------------------
 # START of arrays that used to be inline in nutation.py
 # ----------------------------------------------------------------------

skyfield/data/iers.py

@@ -1,9 +1,14 @@
-"""Parse data files from https://www.iers.org/"""
+"""Parse data files from the International Earth Rotation Service.
 
+See:
+https://datacenter.iers.org/eop.php
+ftp://ftp.iers.org/products/eop/rapid/standard/
+
+"""
 import numpy as np
 import re
 
-FINALS_ALL_URL = 'ftp://ftp.iers.org/products/eop/rapid/standard/finals.all'
+FINALS_URL = 'ftp://ftp.iers.org/products/eop/rapid/standard/finals2000A.all'
 _DUT1 = re.compile(b'^......(.........) ' + b'.' * 42 + b'(.\d........)', re.M)
 inf = float('inf')
 
@@ -16,12 +21,12 @@ def parse_dut1_from_finals_all(f):
 
 def build_timescale_arrays(mjd, dut1):
     big_jumps = np.diff(dut1) > 0.9
-    leap_second_indices = np.concatenate([[False], big_jumps])
-    delta_t = np.cumsum(leap_second_indices) - dut1 + 32.184 + 12.0
+    leap_second_mask = np.concatenate([[False], big_jumps])
+    delta_t = np.cumsum(leap_second_mask) - dut1 + 32.184 + 12.0
     delta_t_recent = np.array([mjd + 2400000.5, delta_t])
 
     leap_dates = 2400000.5 + np.concatenate([
-        [-inf], [41317.0, 41499.0, 41683.0], mjd[leap_second_indices], [inf],
+        [-inf], [41317.0, 41499.0, 41683.0], mjd[leap_second_mask], [inf],
     ])
     leap_offsets = np.arange(8.0, len(leap_dates) + 8.0)
     leap_offsets[0] = leap_offsets[1] = 10.0