Fix or remove several old broken links

ephem/doc/catalogs.rst

@@ -8,35 +8,18 @@ of comets or asteroids that interest you,
 or perhaps coordinates for stars and other fixed objects.
 Here are a few sources from which such data may be obtained:
 
-http://www.cfa.harvard.edu/iau/Ephemerides/Soft03.html
-
- The International Astronomical Union's
- *Central Bureau for Astronomical Telegrams and Minor Planet Center*
- is where comet and asteroid discoveries are traditionally reported,
- and is a source of ephemerides and orbital elements for these objects.
- They provide updated orbital elements, in ephem format,
- for many currently visible comets and asteroids.
-
-http://celestrak.com/NORAD/elements/
+https://celestrak.org/NORAD/elements/
 
  Orbital elements for man-made earth satellites are available from NORAD,
  whose Two-Line Element format can be understood
  by PyEphem's ``readtle()`` function.
 
-http://www.maa.mhn.de/Tools/Xephem/
-
- This directory has not been updated for several years,
- but has quite extensive collections of objects in ephem format.
- Though many of its comet and asteroid elements are doubtless out of date,
- its files of fixed objects like galaxies to pulsars
- should remain valuable.
-
 http://toyvax.glendale.ca.us/
 
  This page has elements in ephem format
  for several of our longer-lived space probes.
 
-http://www.clearskyinstitute.com/xephem/help/xephem.html#mozTocId468501
+https://xephem.github.io/XEphem/Site/help/xephem.html#mozTocId468501
 
  If you need more information about the XEphem database format itself,
  here is the link where the format is defined

ephem/doc/newton.rst

@@ -103,4 +103,4 @@ that mark the solstices and equinoxes
     # 2015/6/16 15:03:15 0:00:00.0
     # 2015/12/31 21:12:07 90:00:00.0
 
-.. _a Stack Overflow question: http://stackoverflow.com/questions/25538926/solar-longitude-from-pyephem/
+.. _a Stack Overflow question: https://stackoverflow.com/questions/25538926/solar-longitude-from-pyephem/

ephem/doc/quick.rst

@@ -9,7 +9,7 @@ PyEphem Quick Reference
 .. _Apparent geocentric: radec
 .. _Apparent topocentric position: radec
 .. _Apparent position: radec
-.. _XEphem format: http://www.clearskyinstitute.com/xephem/help/xephem.html#mozTocId468501
+.. _XEphem format: https://xephem.github.io/XEphem/Site/help/xephem.html#mozTocId468501
 
 Those experienced with both Python and astronomy
 should be able to start using PyEphem

ephem/doc/tutorial.rst

@@ -13,9 +13,9 @@ for PyEphem to be built upon his work.
 After installing the module,
 you can use it in Python with the statement:
 
-.. _PyEphem library: http://rhodesmill.org/pyephem/
-.. _XEphem: http://www.clearskyinstitute.com/xephem/
-.. _Elwood Charles Downey: http://www.clearskyinstitute.com/resumes/ecdowney/resume.html
+.. _PyEphem library: https://rhodesmill.org/pyephem/
+.. _XEphem: https://xephem.github.io/XEphem/Site/xephem.html
+.. _Elwood Charles Downey: https://www.clearskyinstitute.com/
 
 >>> import ephem
 
@@ -485,7 +485,7 @@ C/2002 Y1 (Juels-Holvorcem)
 >>> print("%s %s" % (ephem.constellation(yh), yh.mag))
 ('And', 'Andromeda') 5.96
 
-.. _ephem database format: http://www.clearskyinstitute.com/xephem/help/xephem.html#mozTocId468501
+.. _ephem database format: https://xephem.github.io/XEphem/Site/help/xephem.html#mozTocId468501
 
 (Unfortunately, the library upon which PyEphem is build
 truncates object names to twenty characters, as you can see.)