moonphase cron job for Facebook page The Moon

#!/usr/bin/python
'''
create webpage of moonphase

update Facebook "The Moon" page when it's a major phase
'''
import sys, os, math, ephem, netrc, urllib, urllib2, logging, httplib
from datetime import datetime
logging.basicConfig(level = logging.DEBUG)
IMAGE_PATTERN = 'moons/m%03d.gif'  # relative to images/
FACEBOOK = 'graph.facebook.com'
NETRC = netrc.netrc()
THE_MOON_ID, APP, KEY = NETRC.authenticators(FACEBOOK)
MOONPAGE = 'http://unixshell.jcomeau.com/luna/index.html'
DOCUMENT_ROOT = os.getenv('DOCUMENT_ROOT', '/var/www/jcomeau/luna')
GRAPH = 'https://%s' % FACEBOOK
TIMESTAMP = datetime.utcnow()
MESSAGE = {
    0: 'Beginning a new cycle!',
    90: 'First quarter happening now!',
    180: 'Full moon happening now!',
    270: 'Last quarter happening now!',
}

def update_image():
    '''
    update symlink images/phase.gif

    use trig function guessed at by observing how new and full moons seem
    to last several days, whereas first and last quarter zoom by quickly
    '''
    os.chdir(DOCUMENT_ROOT)
    utcnow = ephem.Date(TIMESTAMP)
    last_new_moon = ephem.previous_new_moon(utcnow)
    next_new_moon = ephem.next_new_moon(utcnow)
    lunar_cycle = next_new_moon - last_new_moon
    full_moon_offset = lunar_cycle / 2
    current_phase = utcnow - last_new_moon
    if current_phase >= full_moon_offset:
        degrees, image = image_url((math.cos(
            (current_phase / lunar_cycle) * 2 * math.pi) + 3) * 90)
    else:
        degrees, image = image_url((-math.cos(
            (current_phase / lunar_cycle) * 2 * math.pi) + 1) * 90)
    try:
        old_image = os.readlink('images/phase.gif')
    except OSError:
        old_image = ''
    if old_image != image:
        try:
            os.unlink('images/phase.gif')
        except OSError:
            pass
        os.symlink(image, 'images/phase.gif')
    if degrees in [0, 90, 180, 270, 360]:
        update_facebook_page(degrees, text = MESSAGE(degrees % 360))

def init_https():
    '''
    enable debugging of HTTPS connection if run from command-line
    '''
    handler = urllib2.HTTPSHandler(debuglevel = sys.stdin.isatty())
    opener = urllib2.build_opener(handler)
    urllib2.install_opener(opener)

def update_facebook_page(degrees, link = MOONPAGE, text = 'My current phase'):
    '''
    make a POST to The Moon page

    notifying followers of major phase
    '''
    init_https()
    arguments = {
        'link': '%s?cachebreaker=%s' % (link, TIMESTAMP),
        'message': text,
        'access_token': KEY,
    }
    if not text:
        arguments.pop('message')
    logging.debug('notifying followers of %d phase' % degrees)
    response = urllib2.urlopen('%s/%s/feed' % (GRAPH, THE_MOON_ID),
        urllib.urlencode(arguments))
    page = response.read()
    logging.debug(page)

def image_url(number):
    '''
    images are only in multiples of 2 degrees, such as moons/m002.gif,
    moons/m356.gif, so round accordingly and construct relative URL
    '''
    rounded = int((number / 2) + .5) * 2
    return rounded, IMAGE_PATTERN % rounded

if __name__ == '__main__':
    update_image()