I was learning to calculate local solar time with ephem. I must admit, unit conversion between radians, hours and days, which are all represented by a float in the program can be somewhat confusing for first-timers.
import ephem
from math import pi
sun = ephem.Sun()
loc = ephem.city("Taipei")
# Calculate the value of base_date,
# which is the start of a new (solar) day at 0:00 hrs
# to which solar time hours can be added
ref = round(loc.date)
if ref > loc.date:
base_date = ref - 1.5
else:
base_date = ref - 0.5
sun.compute(loc)
# Calculate solar time:
# (A) Using the sun's hour angle
# solar time = hour angle + 12 hrs
# because ephem dates are stored as a float number
# indicating the number of (sidereal?) days counting from 1900/1/1 12:00:00,
# I divide the radian value by 2pi (rad/day) to convert its unit to day
# and add that to the base date.
loc_time_a = ephem.date(base_date + (sun.ha + ephem.hours('12:00')) / (2 * pi))
# (B) Using UTC
# solar time = UTC + longitude
# Longitude is in radians,
# so we need to divide by pi and multiply by 12 hrs to convert its unit to hours.
# Finally, we multiply by `ephem.hour` to convert the unit to days
# to add to the date.
loc_time_b = ephem.date(loc.date + (loc.long / pi * 12) * ephem.hour)
print(str(loc_time_a), str(loc_time_b), sep="\n")
Output:
2022/6/15 10:53:00
2022/6/15 10:53:26
As can be seen from the output above, there is a discrepancy of about half a minute between the two calculations.
I was wondering how best to make sense of this difference: Is it safe to say that loc_time_a is the apparent solar time while loc_time_b is the mean?
I don't know how sun.ha is calculated in the ephem library behind the scene. I presume the above claim would stand, to the extent which sun.ha can represent the actual position of the sun at a given time, in a specific location. Am I missing something?
