Fix badly rounded constellation boundaries (#548)

Fixes #547.

builders/build_constellations.py

@@ -146,14 +146,11 @@ def compute_constellation(ra, dec, sorted_ra, sorted_dec, sorted_consts, grid):
 def extend(s):
     """Return a float for `s` extended to machine precision.
 
-    Takes a string like '13.6667', extends it to >30 digits by
-    duplicating the second-to-last character ('6' and not '7' because
-    the '7' in many cases will have been rounded), and passes it to
-    `float()`.
+    Takes a string like '13.6667', passes it to `float()`,
+    and snaps it to the nearest whole second.
 
     """
-    s = s[:-2] + 30 * s[-2:-1] + s[-1:]
-    return float(s)
+    return round(3600 * float(s)) / 3600.
 
 # Some discarded code that I might want to revive someday: how to grow
 # and shrink a list of segments as new ones supersede old ones on the

skyfield/tests/test_constellations.py

@@ -1,4 +1,5 @@
 from skyfield.api import load_constellation_map, position_of_radec
+from skyfield.timelib import Time, julian_date_of_besselian_epoch
 
 def test_constellations():
     lookup = load_constellation_map()
@@ -10,6 +11,11 @@ def test_constellations():
     assert lookup(position_of_radec(4.65, 0)) == 'Eri'
     assert lookup(position_of_radec(4.75, 0.3)) == 'Ori'
 
+    # exploit extend() bug, issue 547
+    B1875 = Time(None, julian_date_of_besselian_epoch(1875))
+    assert lookup(position_of_radec(18.1747, 39., epoch=B1875)) == 'Her'
+    assert lookup(position_of_radec(18.1753, 39., epoch=B1875)) == 'Lyr'
+
     # Test vectorization.
     assert list(
         lookup(position_of_radec([4.65, 4.75], [0, 0.3]))