#######################
PyEphem Quick Reference
#######################

.. _Coordinate Transformations: coordinates
.. _Angle: angle
.. _Date: date
.. _Astrometric geocentric: radec
.. _Apparent geocentric: radec
.. _Apparent topocentric position: radec
.. _Apparent position: radec
.. _XEphem format: http://www.clearskyinstitute.com/xephem/help/xephem.html#mozTocId468501

Those experienced with both Python and astronomy
should be able to start using PyEphem
using only the notes and examples shown below!
There are two ways to begin using PyEphem in a Python program.
One is to import the module by name,
and then to prefix everything you want to use from the module
with the qualifier ``ephem``;
this is the way the code snippets below are written,
which hopefully makes it clear
which variables are coming from PyEphem itself
and which are being created in the course of each example.

>>> import ephem
>>> m = ephem.Mars('1970')
>>> print(ephem.constellation(m))
('Aqr', 'Aquarius')

But to avoid typing the module name over and over again,
you can tell Python to import everything from the module
right into your namespace,
where you can then use them without further qualification:

>>> from ephem import *
>>> m = Mars('1970')
>>> print(constellation(m))
('Aqr', 'Aquarius')

To understand each of the following examples,
first read the source code snippet carefully,
and only then dive into the explanations beneath it.

Bodies
======

 >>> m = ephem.Mars()
 >>> m.name
 'Mars'
 >>> a = ephem.star('Arcturus')
 >>> a.name
 'Arcturus'

 * The Sun, Moon, planets, and major planet moons each have their own class.
 * PyEphem includes a modest catalog of famous bright stars.
 * Body instances know their ``name``
   (which you can set to whatever you want).

..

 >>> m = ephem.Mars('2003/8/27')
 >>> print('%s %s %.10f' % (m.name, m.elong, m.size))
 Mars -173:00:34.2 25.1121063232

 * Extra arguments when you create a Body
   are used to perform an initial ``compute()``
   (see the next section).

body.compute(date)
------------------

 >>> j = ephem.Jupiter()
 >>> j.compute('1986/2/8')
 >>> print('%s %s' % (j.ra, j.dec))
 21:57:50.46 -13:17:37.2
 >>> j.compute('1986/2/9', epoch='1950')
 >>> print('%s %s' % (j.a_ra, j.a_dec))
 21:56:50.83 -13:22:54.3

 * Computes the position of the ``body``.
 * The date if omitted defaults to ``now()``.
 * The epoch if omitted defaults to ``'2000'``.
 * Date and epoch arguments can be anything acceptable to ``Date()``.
 * Sets the following ``body`` attributes:

   |  ``a_ra`` — `Astrometric geocentric`_ right ascension for the ``epoch`` specified
   |  ``a_dec`` — `Astrometric geocentric`_ declination for the ``epoch`` specified

   |  ``g_ra`` and ``ra`` — `Apparent geocentric`_ right ascension for the epoch-of-date
   |  ``g_dec`` and ``dec`` — `Apparent geocentric`_ declination for the epoch-of-date

   | ``elong`` — Elongation (angle to sun)
   | ``mag`` — Magnitude
   | ``size`` — Size (diameter in arcseconds)
   | ``radius`` — Size (radius as an angle)

   | ``circumpolar`` — whether it stays above the horizon
   | ``neverup`` — whether is stays below the horizon

 * On Solar System bodies, also sets:

   | ``hlon`` — Heliocentric longitude (see next paragraph)
   | ``hlat`` — Heliocentric latitude (see next paragraph)
   | ``sun_distance`` — Distance to Sun (AU)
   | ``earth_distance`` — Distance to Earth (AU)
   | ``phase`` — Percent of surface illuminated

   Both ``hlon`` and ``hlat`` have a special meaning
   for the Sun and Moon.
   For a ``Sun`` body,
   they give the *Earth’s* heliocentric longitude and latitude.
   For a ``Moon`` body,
   they give the Moon’s *geocentric* longitude and latitude.

 * On planetary moons, also sets:

   | Position of moon relative to planet
   | (measured in planet radii)
   |  ``x`` — offset +east or –west
   |  ``y`` — offset +south or –north
   |  ``z`` — offset +front or –behind

   | Whether the moon is visible...
   |  ``earth_visible`` — from the Earth
   |  ``sun_visible`` — from the Sun

 * On artificial satellites, also sets:

   | Geographic point beneath satellite:
   |  ``sublat`` — Latitude (+N)
   |  ``sublong`` — Longitude (+E)
   |
   | ``elevation`` — Height above sea level (m)
   | ``range`` — Distance from observer to satellite (m)
   | ``range_velocity`` — Range rate of change (m/s)
   | ``eclipsed`` — Whether satellite is in Earth's shadow

 * On ``Moon`` bodies, also sets:

   | Current libration:
   |  ``libration_lat`` — in Latitude
   |  ``libration_long`` — in Longitude
   |
   | ``colong`` — Selenographic colongiude
   | ``moon_phase`` — Percent of surface illuminated
   | ``subsolar_lat`` — Lunar latitude that the Sun is standing above

 * On ``Jupiter`` bodies,
   also determines the longitude of the central meridian facing Earth,
   both in System I (which measures rotation at the Jovial equator)
   and System II (which measures rotation at temperate latitudes).

   | ``cmlI`` — Central meridian longitude in System I
   | ``cmlII`` — Central meridian longitude in System II

 * On ``Saturn`` bodies,
   also sets the tilt of the rings,
   with southward tilt being positive, and northward, negative:

   | ``earth_tilt`` — Tilt towards Earth
   | ``sun_tilt`` — Tilt towards Sun

body.compute(observer)
----------------------

 >>> gatech = ephem.Observer()
 >>> gatech.lon = '-84.39733'
 >>> gatech.lat = '33.775867'
 >>> gatech.elevation = 320
 >>> gatech.date = '1984/5/30 16:22:56'
 >>> v = ephem.Venus(gatech)
 >>> print('%s %s' % (v.alt, v.az))
 72:19:44.8 134:14:25.3

 * Computes the position of the ``Body``.
 * Uses the ``date`` of the observer.
 * Uses the ``epoch`` of the observer.
 * Sets all of the ``Body`` attributes listed in the previous section
   (but ``ra`` and ``dec`` will get different values;
   see below).
 * Also computes where the body appears in the sky
   (or below the horizon) for the observer,
   and sets four more ``Body`` attributes:

   | `Apparent topocentric position`_
   |  ``ra`` — Right ascension
   |  ``dec`` — Declination
   |
   | `Apparent position`_ relative to horizon
   |  ``az`` — Azimuth east of north
   |  ``alt`` — Altitude above horizon

 * These apparent positions
   include an adjustment to simulate atmospheric refraction
   for the observer's ``temp`` and ``presure``;
   set the observer's ``pressure`` to zero to ignore refraction.

 * For earth satellite objects,
   the astrometric coordinates ``a_ra`` and ``a_dec`` are topocentric
   instead of geocentric,
   since there is little point in figuring out where the satellite
   would appear on a J2000 (or whatever epoch you are using) star chart
   for an observer sitting at the center of the earth.

catalog format
--------------

 >>> line = "C/2002 Y1 (Juels-Holvorcem),e,103.7816,166.2194,128.8232,242.5695,0.0002609,0.99705756,0.0000,04/13.2508/2003,2000,g  6.5,4.0"
 >>> yh = ephem.readdb(line)
 >>> yh.compute('2007/10/1')
 >>> print('%.10f' % yh.earth_distance)
 14.8046731949
 >>> print(yh.mag)
 23.96

 * Bodies can be imported and exported
   in the popular `XEphem format`_.
 * When you deal with asteroids and comets,
   whose orbital parameters are subject to frequent revision,
   you will usually find yourself downloading an XEphem file
   and reading its contents.
 * To interpret a line in XEphem format,
   call the ``readdb()`` function::

    halley = ephem.readdb(line)

 * To export a body in XEphem format,
   call the ``writedb()`` method of the body itself::

    print(halley.writedb())

..

 >>> line1 = "ISS (ZARYA)"
 >>> line2 = "1 25544U 98067A   03097.78853147  .00021906  00000-0  28403-3 0  8652"
 >>> line3 = "2 25544  51.6361  13.7980 0004256  35.6671  59.2566 15.58778559250029"
 >>> iss = ephem.readtle(line1, line2, line3)
 >>> iss.compute('2003/3/23')
 >>> print('%s %s' % (iss.sublong, iss.sublat))
 -76:24:18.3 13:05:31.1

 * Satellite elements often come packaged in a format called TLE,
   that has the satellite name on one line
   and the elements on the following two lines.
 * Call the ``readtle()`` function to turn a TLE entry
   into a PyEphem ``Body``.

bodies with orbital elements
----------------------------

 * When you load minor objects like comets and asteroids,
   the resulting object specifies the *orbital elements*
   that allow XEphem to predict its position.
 * These orbital elements are available for you to inspect and change.
 * If you lack a catalog from which to load an object,
   you can start by creating a raw body of one of the following types
   and filling in its elements.
 * Element attribute names start with underscores
   to distinguish them from the normal ``Body`` attributes
   that are set as the result of calling ``compute()``.
 * Each ``FixedBody`` has only three necessary elements:

   | ``_ra``, ``_dec`` — Position
   | ``_epoch`` — The epoch of the position

   The other ``FixedBody`` elements store trivia about its appearance:

   | ``_class`` — One-character string classification
   | ``_spect`` — Two-character string for the spectral code
   | ``_ratio`` — Ratio between the major and minor diameters
   | ``_pa`` — the angle at which the major axis lies in the sky,
     measured east of north (°)

 * ``EllipticalBody`` elements:

   | ``_inc`` — Inclination (°)
   | ``_Om`` — Longitude of ascending node (°)
   | ``_om`` — Argument of perihelion (°)
   | ``_a`` — Mean distance from sun (AU)
   | ``_M`` — Mean anomaly from the perihelion (°)
   | ``_epoch_M`` — Date for measurement ``_M``
   | ``_size`` — Angular size (arcseconds at 1 AU)
   | ``_e`` — Eccentricity
   | ``_epoch`` — Epoch for ``_inc``, ``_Om``, and ``_om``
   | ``_H``, ``_G`` — Parameters for the H/G magnitude model
   | ``_g``, ``_k`` — Parameters for the g/k magnitude model

 * ``HyperbolicBody`` elements:

   | ``_epoch`` — Equinox year for ``_inc``, ``_Om``, and ``_om``
   | ``_epoch_p`` — Epoch of perihelion
   | ``_inc`` — Inclination (°)
   | ``_Om`` — Longitude of ascending node (°)
   | ``_om`` — Argument of perihelion (°)
   | ``_e`` — Eccentricity
   | ``_q`` — Perihelion distance (AU)
   | ``_g``, ``_k`` — Magnitude model coefficients
   | ``_size`` — Angular size in arcseconds at 1 AU

 * ``ParabolicBody`` elements:

   | ``_epoch`` — Epoch for ``_inc``, ``_Om``, and ``_om``
   | ``_epoch_p`` — Epoch of perihelion
   | ``_inc`` — Inclination (°)
   | ``_Om`` — Longitude of ascending node (°)
   | ``_om`` — Argument of perihelion (°)
   | ``_q`` — Perihelion distance (AU)
   | ``_g``, ``_k`` — Magnitude model coefficients
   | ``_size`` — Angular size in arcseconds at 1 AU

 * ``EarthSatellite`` elements:

   | ``_epoch`` — Reference epoch
   | ``_n`` — Mean motion, in revolutions per day
   | ``_inc`` — Inclination (°)
   | ``_raan`` — Right Ascension of ascending node (°)
   | ``_e`` — Eccentricity
   | ``_ap`` — Argument of perigee at epoch (°)
   | ``_M`` — Mean anomaly from perigee at epoch (°)
   | ``_decay`` — Orbit decay rate in revolutions per day, per day
   | ``_drag`` — Object drag coefficient in per earth radii
   | ``_orbit`` — Integer orbit number of epoch

------------

Other Functions
===============

 >>> m = ephem.Moon('1980/6/1')
 >>> print(ephem.constellation(m))
 ('Sgr', 'Sagittarius')

 * The ``constellation()`` function returns a tuple
   containing the abbreviated name and full name
   of the constellation in which its argument lies.
 * You can either pass a ``Body`` whose position is computed,
   or a tuple ``(ra, dec)`` of coordinates —
   in which case epoch 2000 is assumed
   unless you also pass an ``epoch=`` keyword argument
   specifying another value.

..

 >>> print(ephem.delta_t('1980'))
 50.54

 * The ``delta_t()`` function
   returns the difference, in seconds, on the given date
   between Terrestrial Time and Universal Time.
 * Takes a ``Date`` or ``Observer`` argument.
 * Without an argument, uses ``now()``.

..

 >>> ephem.julian_date('2000/1/1')
 2451544.5

 * The ``julian_date()`` function
   returns the official Julian Date of the given day and time.
 * Takes a ``Date`` or ``Observer`` argument.
 * Without an argument, uses ``now()``.

..

 >>> ra, dec = '7:16:00', '-6:17:00'
 >>> print(ephem.uranometria(ra, dec))
 V2 - P274
 >>> print(ephem.uranometria2000(ra, dec))
 V2 - P135
 >>> print(ephem.millennium_atlas(ra, dec))
 V1 - P273

 * Take an ``ra`` and ``dec`` as arguments.
 * Return the volume and page on which that coordinate lies
   in the given star atlas:
 
   | *Uranometria* by Johannes Bayer.
   | *Uranometria 2000.0* edited by Wil Tirion.
   | *Millennium Star Atlas* by Roger W. Sinnott and Michael A. C. Perryman.

..

 >>> m1 = ephem.Moon('1970/1/16')
 >>> m2 = ephem.Moon('1970/1/17')
 >>> s = ephem.separation(m1, m2)
 >>> print("In one day the Moon moved %s" % s)
 In one day the Moon moved 12:33:28.5

 * The ``separation()`` function
   returns the angle that separates two positions on a sphere.
 * Each argument can be either a ``Body``,
   in which case its ``ra`` and ``dec`` are used,
   or a tuple ``(lon, lat)`` giving a pair of spherical coordinates
   where ``lon`` measures angle around the sphere's equator
   and ``lat`` measures the angle above or below its equator.

------------

Coordinate Conversion
=====================

 >>> np = Equatorial('0', '90', epoch='2000')
 >>> g = Galactic(np)
 >>> print('%s %s' % (g.lon, g.lat))
 122:55:54.9 27:07:41.7

 * There are three coordinate classes,
   which each have three properties:

   | ``Equatorial``
   |  ``ra`` — right ascension
   |  ``dec`` — declination
   |  ``epoch`` — epoch of the coordinate

   | ``Ecliptic``
   |  ``lon`` — ecliptic longitude (+E)
   |  ``lat`` — ecliptic latitude (+N)
   |  ``epoch`` — epoch of the coordinate

   | ``Galactic``
   |  ``lon`` — galactic longitude (+E)
   |  ``lat`` — galactic latitude (+N)
   |  ``epoch`` — epoch of the coordinate

 * When creating a new coordinate,
   you can pass either a body,
   or another coordinate,
   or a pair of raw angles
   (always place the longitude or right ascension first).

 * When creating a coordinate,
   you can optionally pass an ``epoch=`` keyword
   specifying the epoch for the coordinate system.
   Otherwise the epoch is copied
   from the body or other coordinate being used,
   or J2000 is used as the default.

 * See the `Coordinate Transformations`_ document for more details.

Observers
=========

 >>> lowell = ephem.Observer()
 >>> lowell.lon = '-111:32.1'
 >>> lowell.lat = '35:05.8'
 >>> lowell.elevation = 2198
 >>> lowell.date = '1986/3/13'
 >>> j = ephem.Jupiter()
 >>> j.compute(lowell)
 >>> print(j.circumpolar)
 False
 >>> print(j.neverup)
 False
 >>> print('%s %s' % (j.alt, j.az))
 0:57:44.7 256:41:01.3

 * Describes a position on Earth's surface.
 * Pass to the ``compute()`` method of a ``Body``.
 * These are the attributes you can set:

   | ``date`` — Date and time
   | ``epoch`` — Epoch for astrometric RA/dec

   | ``lat`` — Latitude (+N)
   | ``lon`` — Longitude (+E)
   | ``elevation`` — Elevation (m)

   | ``temp`` — Temperature (°C)
   | ``pressure`` — Atmospheric pressure (mBar)

 * The ``date`` defaults to ``now()``.
 * The ``epoch`` defaults to ``'2000'``.
 * The ``temp`` defaults to 25°C.
 * The ``pressure`` defaults to 1010mBar.
 * Other attributes default to zero.

 >>> lowell.compute_pressure()
 >>> lowell.pressure
 775.6025138640499
 
 * Computes the pressure at the observer's current elevation,
   using the International Standard Atmosphere.

 >>> boston = ephem.city('Boston')
 >>> print('%s %s' % (boston.lat, boston.lon))
 42:21:30.4 -71:03:35.2

 * XEphem includes a small database of world cities.
 * Each call to ``city()`` returns a new ``Observer``.
 * Only latitude, longitude, and elevation are set.
 * XEphem can also perform Google geocoding lookups:

 >>> from ephem import cities
 >>> ven = cities.lookup('Ven, Sweden')

 * To avoid Google rate limits,
   avoid performing any ``lookup()`` more than once —
   instead, print the result to your screen
   and then cut-and-paste the latitude and longitude into your code.
 * A ``ValueError`` signals a non-existent place.

 >>> cities.lookup('nonsense string')
 Traceback (most recent call last):
   ...
 ValueError: Google cannot find a place named 'nonsense string'

transit, rising, setting
------------------------

 >>> sitka = ephem.Observer()
 >>> sitka.date = '1999/6/27'
 >>> sitka.lat = '57:10'
 >>> sitka.lon = '-135:15'
 >>> m = ephem.Mars()
 >>> print(sitka.next_transit(m))
 1999/6/27 04:22:45
 >>> print('%s %s' % (m.alt, m.az))
 21:18:33.6 180:00:00.0
 >>> print(sitka.next_rising(m, start='1999/6/28'))
 1999/6/28 23:28:25
 >>> print('%s %s' % (m.alt, m.az))
 -0:00:05.8 111:10:41.6

 * Eight ``Observer`` methods are available
   for finding rising, transit, and setting times::

    previous_transit()
    next_transit()

    previous_antitransit()
    next_antitransit()

    previous_rising()
    next_rising()

    previous_setting()
    next_setting()

 * Each takes a ``Body`` argument,
   which can be any body except an ``EarthSatellite``
   (for which the ``next_pass()`` method below should be used).
 * Starting at the Observer’s ``date``
   they search the entire circuit of the sky
   that the body was making from its previous anti-transit to the next.
 * If the search is successful, returns a ``Date`` value.
 * Always leaves the ``Body`` at its position on that date.
 * Always leaves the Observer unmodified.
 * Takes an optional ``start=`` argument
   giving the date and time
   from which the search for a rising, transit, or setting should commence.
 * We define the meridian as the line
   running overhead from the celestial North pole to the South pole,
   and the anti-meridian as the other half of the same great circle;
   so the transit and anti-transit methods always succeed,
   whether the body crosses the horizon or not.
 * But the rising and setting functions raise exceptions
   if the body does not to cross the horizon;
   the exception hierarchy is::

    ephem.CircumpolarError
     |
     +--- ephem.AlwaysUpError
     +--- ephem.NeverUpError

 * Rising and setting are defined
   as the moments when the upper limb of the body touches the horizon
   (that is, when the body's ``alt`` plus ``radius`` equals zero).
 * Rising and setting
   are sensitive to atmospheric refraction at the horizon,
   and therefore to the observer's ``temp`` and ``pressure``;
   set the ``pressure`` to zero to turn off refraction.
 * Rising and setting pay attention
   to the observer's ``horizon`` attribute;
   see the next section.

 >>> line1 = "IRIDIUM 80 [+]"
 >>> line2 = "1 25469U 98051C   09119.61415140 -.00000218  00000-0 -84793-4 0  4781"
 >>> line3 = "2 25469  86.4029 183.4052 0002522  86.7221 273.4294 14.34215064557061"
 >>> iridium_80 = ephem.readtle(line1, line2, line3)
 >>> boston.date = '2009/5/1'
 >>> info = boston.next_pass(iridium_80)
 >>> print("Rise time: %s azimuth: %s" % (info[0], info[1]))
 Rise time: 2009/5/1 00:22:15 azimuth: 104:36:16.0

 * The ``next_pass()`` method takes an ``EarthSatellite`` body
   and determines when it will next cross above the horizon.
 * It returns a six-element tuple giving::

    0  Rise time
    1  Rise azimuth
    2  Maximum altitude time
    3  Maximum altitude
    4  Set time
    5  Set azimuth

 * Any of the tuple values can be ``None`` if that event was not found.

observer.horizon
----------------

 >>> sun = ephem.Sun()
 >>> greenwich = ephem.Observer()
 >>> greenwich.lat = '51:28:38'
 >>> print(greenwich.horizon)
 0:00:00.0
 >>> greenwich.date = '2007/10/1'
 >>> r1 = greenwich.next_rising(sun)
 >>> greenwich.pressure = 0
 >>> greenwich.horizon = '-0:34'
 >>> greenwich.date = '2007/10/1'
 >>> r2 = greenwich.next_rising(sun)
 >>> print('Visual sunrise: %s' % r1)
 Visual sunrise: 2007/10/1 05:59:30
 >>> print('Naval Observatory sunrise: %s' % r2)
 Naval Observatory sunrise: 2007/10/1 05:59:50

 * The ``horizon`` attribute defines your *horizon*,
   the altitude of the upper limb of a body
   at the moment you consider it to be rising and setting.
 * The ``horizon`` defaults to zero degrees.
 * The United States Naval Observatory,
   rather than computing refraction dynamically,
   uses a constant estimate of 34' of refraction at the horizon.
   So in the above example,
   rather than attempting to jury-rig values
   for ``temp`` and ``pressure``
   that yield the magic 34',
   we turn off PyEphem refraction entirely
   and define the horizon itself as being at 34' altitude instead.
 * To determine
   when a body will rise “high enough” above haze or obstacles,
   set ``horizon`` to a positive number of degrees.
 * A negative value of ``horizon`` can be used
   when an observer is high off of the ground.

other Observer methods
----------------------

 >>> madrid = ephem.city('Madrid')
 >>> madrid.date = '1978/10/3 11:32'
 >>> print(madrid.sidereal_time())
 12:04:28.09

 * Called without arguments.
 * Returns the sidereal time for the observer's circumstances.

..

 >>> ra, dec = madrid.radec_of(0, '90')
 >>> print('%s %s' % (ra, dec))
 12:05:35.12 40:17:49.8

 * Called like ``radec_of(az, alt)``.
 * Returns the apparent topocentric coordinates
   behind the given point in the sky.

---------------------

Equinoxes & Solstices
=====================

 >>> d1 = ephem.next_equinox('2000')
 >>> print(d1)
 2000/3/20 07:35:17
 >>> d2 = ephem.next_solstice(d1)
 >>> print(d2)
 2000/6/21 01:47:51
 >>> t = d2 - d1
 >>> print("Spring lasted %.1f days" % t)
 Spring lasted 92.8 days

 * Functions take a ``Date`` argument.
 * Return a ``Date``.
 * Available functions::

    previous_solstice()
    next_solstice()

    previous_equinox()
    next_equinox()

    previous_vernal_equinox()
    next_vernal_equinox()

-----------

Phases of the Moon
==================

 >>> d1 = ephem.next_full_moon('1984')
 >>> print(d1)
 1984/1/18 14:05:10
 >>> d2 = ephem.next_new_moon(d1)
 >>> print(d2)
 1984/2/1 23:46:25

 * Functions take a ``Date`` argument.
 * Return a ``Date``.
 * Available functions::

    previous_new_moon()
    next_new_moon()

    previous_first_quarter_moon()
    next_first_quarter_moon()

    previous_full_moon()
    next_full_moon()

    previous_last_quarter_moon()
    next_last_quarter_moon()

-----------

Angles
======

 >>> a = ephem.degrees('180:00:00')
 >>> print(a)
 180:00:00.0
 >>> a
 3.141592653589793
 >>> print("180° is %f radians" % a)
 180° is 3.141593 radians
 >>> h = ephem.hours('1:00:00')
 >>> deg = ephem.degrees(h)
 >>> print("1h right ascension = %s°" % deg)
 1h right ascension = 15:00:00.0°

 * Many ``Body`` and ``Observer`` attributes
   return their value as ``Angle`` objects.
 * Most angles are measured in degrees.
 * Only right ascension is measured in hours.
 * You can also create angles yourself through two ``ephem`` functions:

   | ``degrees()`` — return an ``Angle`` in degrees
   | ``hours()`` — return an ``Angle`` in hours

 * Each angle acts like a Python ``float``.
 * Angles always store floating-point radians.
 * Only when printed, passed to ``str()``, or formatted with ``'%s'``
   do angles display themselves as degrees or hours.
 * When setting an angle attribute in a body or observer,
   or creating angles yourself,
   you can provide either floating-point radians
   or a string with degrees or hours.
   The following angles are equivalent::

    ephem.degrees(ephem.pi / 32)
    ephem.degrees('5.625')
    ephem.degrees('5:37.5')
    ephem.degrees('5:37:30')
    ephem.degrees('5:37:30.0')
    ephem.hours('0.375')
    ephem.hours('0:22.5')
    ephem.hours('0:22:30')
    ephem.hours('0:22:30.0')

 * When doing math on angles,
   the results will often exceed the normal bounds for an angle.
   Therefore two attributes are provided for each angle:

   | ``norm`` — returns angle normalized to [0, 2π).
   | ``znorm`` — returns angle normalized to [-π, π).

 * For more details see the Angle_ document.

-----

Dates
=====

 >>> d = ephem.Date('1997/3/9 5:13')
 >>> print(d)
 1997/3/9 05:13:00
 >>> d
 35496.717361111114
 >>> d.triple()
 (1997, 3, 9.21736111111386)
 >>> d.tuple()
 (1997, 3, 9, 5, 13, 2.3748725652694702e-07)
 >>> d + ephem.hour
 35496.75902777778
 >>> print(ephem.date(d + ephem.hour))
 1997/3/9 06:13:00
 >>> print(ephem.date(d + 1))
 1997/3/10 05:13:00

 * Dates are stored and returned as floats.
 * Only when printed, passed to ``str()``, or formatted with ``'%s'``
   does a date express itself as a string
   giving the calendar day and time.
 * The modern Gregorian calendar is used for recent dates,
   and the old Julian calendar for dates before October 15, 1582.
 * Dates *always* use Universal Time, *never* your local time zone.
 * Call ``.triple()`` to split a date into its year, month, and day.
 * Call ``.tuple()`` to split a date into its year, month, day,
   hour, minute, and second.
 * You can create ``ephem.Date()`` dates yourself
   in addition to those you will be returned by other objects.
 * Call ``ephem.now()`` for the current date and time.
 * When setting a date attribute in a body or observer,
   or creating angles yourself,
   you can provide either floating-point radians, a string, or a tuple.
   The following dates are equivalent::

    ephem.Date(35497.7197916667)
    ephem.Date('1997/3/10.2197916667')
    ephem.Date('1997/3/10 05.275')
    ephem.Date('1997/3/10 05:16.5')
    ephem.Date('1997/3/10 05:16:30')
    ephem.Date('1997/3/10 05:16:30.0')
    ephem.Date((1997, 3, 10.2197916667))
    ephem.Date((1997, 3, 10, 5, 16, 30.0))

 * Dates store the number of days that have passed
   since noon Universal Time on the last day of 1899.
   By adding and subtracting whole numbers from dates,
   you can move several days into the past or future.
   If you want to move by smaller amounts,
   the following constants may be helpful::

    ephem.hour
    ephem.minute
    ephem.second

 * For more details see the Date_ document.

local time
----------

 >>> d = ephem.Date('1997/3/9 5:13')
 >>> ephem.localtime(d)
 datetime.datetime(1997, 3, 9, 0, 13, 0, 6)
 >>> print(ephem.localtime(d))
 1997-03-09 00:13:00.000006

 * The ``localtime()`` function converts a PyEphem date
   into a Python ``datetime`` object expressed in your local time zone.

----

Stars and Cities
================

 >>> rigel = ephem.star('Rigel')
 >>> print('%s %s' % (rigel._ra, rigel._dec))
 5:14:32.30 -8:12:06.0

 * PyEphem provides a catalog of bright stars.
 * Each call to ``star()`` returns a new ``FixedBody``
   whose coordinates are those of the named star.

..

 >>> stuttgart = ephem.city('Stuttgart')
 >>> print(stuttgart.lon)
 9:10:50.8
 >>> print(stuttgart.lat)
 48:46:37.6

 * PyEphem knows 122 world cities.
 * The ``city()`` function returns an ``Observer``
   whose longitude, latitude, and elevation
   are those of the given city.

----

Other Constants
===============

 * PyEphem provides constants
   for the dates of a few major star-atlas epochs::

    B1900
    B1950
    J2000

 * PyEphem provides, for reference,
   the length of four distances, all in meters::

    ephem.meters_per_au
    ephem.earth_radius
    ephem.moon_radius
    ephem.sun_radius

 * PyEphem provides the speed of light in meters per second::

    ephem.c
