Observing excercise with Skyfield

observe_skyfield.py

#!/usr/bin/env python3.4

"""
observe_skyfield.py

Observing exercise with Skyfield.
"""

import numpy as np

from skyfield.api import JulianDate, earth, venus, jupiter, saturn, Star

from datetime import datetime
from pytz import timezone

###################
# Set Dates & Times
###################

# Central timezone
central = timezone('US/Central')

# Skyfield can make date/time objects as arrays (for instance, 
#     if want info for multiple times in one night), but 
#     not clear how would handle higher order arrays 
#     (eg., multiple times in multiple nights).

# What time to start/end observing in local timezone
local_start_hour = 20 	# 8:00 pm
#local_start_min = 30
local_end_hour = 23  	# 11:00 pm
#local_end_min = 30

# Observing in
year = 2015
month = 5
day = 15
hour = range(local_start_hour, local_end_hour + 1)

observe_date = []
for i in range(0, len(hour)):
    observe_date.append(central.localize(datetime(year, month, day, hour[i])))

jd_1 = JulianDate(observe_date)

###################
# Find Observer-centered Coords
###################

# Fayetteville, AR
fville = earth.topos('36.0764 N', '94.1608 W')

# Define star targets.
# This should instead be input as lists or dictionaries.
# Because of the tendency to create "shortcut" objects for the planets,
#     creating observation objects for planets within the same loop 
#     that creates observation objects for stars does not have an 
#     obvious solution for the novice or casual programmer.
Sirius = Star(ra_hours=(6, 45, 08.9173), dec_degrees=(-16, 42, 58.017))
Pollux = Star(ra_hours=(7, 45, 19), dec_degrees=(28, 1.584, 0))
Castor = Star(ra_hours=(7, 34, 36), dec_degrees=(31, 53, 18))
Polaris = Star(ra_hours=(2, 31, 48.7), dec_degrees=(89, 15, 51))
Betelgeuse = Star(ra_hours=(5, 55, 10.3053), dec_degrees=(7, 24, 25.426))
Rigel = Star(ra_hours=(5, 14, 32.272), dec_degrees=(-8, 12, 05.91))
Altair = Star(ra_hours=(19, 50, 47), dec_degrees=(8, 52.098, 0))
Deneb = Star(ra_hours=(20, 41, 25.9), dec_degrees=(45, 16, 49))
Vega = Star(ra_hours=(18, 36, 56.3364), dec_degrees=(38, 47, 01.291))
Arcturus = Star(ra_hours=(14, 15, 39.7), dec_degrees=(19, 10, 56))
Spica = Star(ra_hours=(13, 25, 11.5793), dec_degrees=(-11, 9, 40.759))
# Define Messier object targets.
M31 = Star(ra_hours=(0, 42, 44.3), dec_degrees=(41, 16, 10))
M33 = Star(ra_hours=(1, 33, 50.9), dec_degrees=(30, 39, 36))
M13 = Star(ra_hours=(16, 5, 15), dec_degrees=(17, 44, 55))
M1 = Star(ra_hours=(5, 34, 32), dec_degrees=(22, 0.870, 0))
M51 = Star(ra_hours=(13, 29, 52.7), dec_degrees=(47, 11, 43))

# List of targets
# Clear that target objects need to include a name attribute as well that can be used to refer
#     to the variables, in order to avoid a situation like this, with separate target & target 
#     name lists.
# Also, dictionaries would be more flexible than arrays when putting together varying data 
#     types, as may be necessary when putting together observation tables. 
targets = [Sirius, Pollux, Castor, Polaris, Betelgeuse, Rigel, Altair, Deneb, Vega, Arcturus, Spica, M31, M33, M13, M1, M51]
target_names = ['Sirius', 'Pollux', 'Castor', 'Polaris', 'Betelgeuse', 'Rigel', 'Altair', 'Deneb', 'Vega', 'Arcturus', 'Spica', 'M31', 'M33', 'M13', 'M1', 'M51']

# Get alt/az for all objects from apparent positions.
for target in range(0, len(targets)):
    print(target_names[target])
    ## Numpy array to hold output data
    ## Date/Time   Alt     Az
    #output_array = np.zeros((len(hour), 3), dtype=str)
    for date in range(0, len(hour)):
        alt, az = fville(jd_1[date]).observe(targets[target]).apparent().altaz()[:2]
        #output_array[date:0] = str(jd_1[date].astimezone(central))
        print(jd_1[date].astimezone(central))
        #output_array[date:1] = str(alt.dstr())
        print(alt.dstr())
        #output_array[date:2] = str(az.dstr())
        print(az.dstr())
    #np.savetxt(target_names[target]+'--alt_az.txt', output_array, fmt="%s", delimiter="\t")
    ## This savetxt statement doesn't work.
    print("\n")

from skyfield.api import JulianDate

throws an error...