Why is the sunrise and sunset time returned by pyephem incorrect for a given location?

Question

I am using the ephem module in Python to calculate the next sunrise and sunset time for a given location (Boston). However, the results returned by the next_rising and next_setting methods are incorrect.

Here is the code I am using:

import ephem
import datetime

Boston=ephem.Observer()
Boston.lat='42.3462'
Boston.lon='-71.0978'
Boston.date = datetime.datetime.now()
Boston.elevation = 3 # meters
Boston.pressure = 1010 # millibar
Boston.temp = 25 # deg. Celcius
Boston.horizon = 0

sun = ephem.Sun()

print("Next sunrise in Boston will be: ",ephem.localtime(Boston.next_rising(sun)))
print("Next sunset in Boston will be: ",ephem.localtime(Boston.next_setting(sun)))

The output I get is:

Next sunrise in Boston will be:  2023-04-27 10:45:21.307728
Next sunset in Boston will be:  2023-04-27 00:38:21.498500

However, these times are not correct!

Is there an error in my code, or is there something else I need to consider when using the ephem module to calculate sunrise and sunset times for a given location?

Answer 1

The above code gets the sunrise/sunset times in the local time. To get these in the observer's timezone, we can use a library like timezonefinder.

import ephem
import datetime
import pytz
from timezonefinder import TimezoneFinder

# set observer coordinates
lat = '42.3462'
lon = '-71.0978'

# create observer
observer = ephem.Observer()
observer.lat = lat
observer.lon = lon
observer.date = datetime.datetime.utcnow()

# define observer parameters
observer.elevation = 3  # meters
observer.pressure = 1010  # millibar
observer.temp = 25  # deg. Celcius
observer.horizon = 0

# define sun
sun = ephem.Sun()

# get sunrise and sunset times in UTC
next_rising = observer.next_rising(sun)
next_setting = observer.next_setting(sun)

# get observer timezone
tf = TimezoneFinder()
timezone_str = tf.timezone_at(lat=float(lat), lng=float(lon))
tz = pytz.timezone(timezone_str)

# convert UTC times to local time
next_rising_local = pytz.utc.localize(next_rising.datetime()).astimezone(tz)
next_setting_local = pytz.utc.localize(next_setting.datetime()).astimezone(tz)

# print results
print("Next sunrise will be at: ", next_rising_local)
print("Next sunset will be at: ", next_setting_local)

Answer 2

I believe you need to compute the position of the sun using: Boston.compute(sun) You need to recalculate for each date you wish to calculate.