Local solar time function from UTC and longitude

Question

I'm wondering if there is a python function/module that calculates the local time after midnight (or local solar time) given the UTC time and longitude? It doesn't need to take into account daylight saving time.

Thanks in advance.

Answer 1

Using ephem's sidereal_time() method:

import ephem # pip install pyephem (on Python 2)
             # pip install ephem   (on Python 3)

def solartime(observer, sun=ephem.Sun()):
    sun.compute(observer)
    # sidereal time == ra (right ascension) is the highest point (noon)
    hour_angle = observer.sidereal_time() - sun.ra
    return ephem.hours(hour_angle + ephem.hours('12:00')).norm  # norm for 24h

Note: ephem.hours is a float number that represents an angle in radians and converts to/from a string as "hh:mm:ss.ff".

For comparison, here's the "utc + longitude" formula:

import math
from datetime import timedelta

def ul_time(observer):
    utc_dt = observer.date.datetime()
    longitude = observer.long
    return utc_dt + timedelta(hours=longitude / math.pi * 12)

Example

from datetime import datetime

# "solar time" for some other cities
for name in ['Los Angeles', 'New York', 'London',
             'Paris', 'Moscow', 'Beijing', 'Tokyo']:
    city = ephem.city(name)
    print("%-11s %11s %s" % (name, solartime(city),
                             ul_time(city).strftime('%T')))

# set date, longitude manually
o = ephem.Observer()
o.date = datetime(2012, 4, 15, 1, 0, 2) # some utc time
o.long = '00:00:00.0' # longitude (you could also use a float (radians) here)
print("%s %s" % (solartime(o), ul_time(o).strftime('%T')))

Output

Los Angeles 14:59:34.11 14:44:30
New York    17:56:31.27 17:41:27
London      22:52:02.04 22:36:58
Paris       23:01:56.56 22:46:53
Moscow       1:23:00.33 01:07:57
Beijing      6:38:09.53 06:23:06
Tokyo        8:11:17.81 07:56:15
1:00:00.10 01:00:01