PyEphem reporting different latitude and longitude than input values

Question

I have used Pyephem to calculate the latitude and longitude of the sun given date, latitude and longitude of an observer at sea level. I get results I do not understand. The code I ran follows (on Ipython notebook in windows 7):

import ephem
date = '2015-04-17 12:30:00'

Amundsen = ephem.Observer()
Amundsen.lat = '46.8'
Amundsen.lon = '-71.2'
Amundsen.date = date

sun = ephem.Sun(Amundsen)
sun.compute(Amundsen)

print Amundsen
print "sun latitude: {0}, sun longitude: {1}".format(sun.hlat,sun.hlon)

The resulst I obtained are the following:

<ephem.Observer date='2015/4/17 12:30:00' epoch='2000/1/1 12:00:00'lon='-71:12:00.0' lat='46:48:00.0' elevation=0.0m horizon=0:00:00.0 temp=15.0C pressure=1010.0mBar>
sun latitude: -0:00:00.1, sun longitude: 207:11:10.2

As you can see, when printing the input data, the latitude and the longitude of my obesrver have been changed from 46.8 and -71.2 to 46.48 to -71.12. This is maybe a basic fact, but why does this happen? and how to correct for it?

Thanks in advance,

Grégory

Answer 1

The values, happily, are not changing. You have entered them as decimals with a decimal point . separating each degree into ten tenths, but that is not how longitude and latitude are traditionally expressed — they are usually written as a whole number, then sixtieths called “minutes” and then sixtieths-of-sixtieths called “seconds” which PyEphem separates with the : character.

So 46:48 means “46 degrees 48 minutes latitude” because 48/60 = 0.8.

The libsastro use of : is a compromise to the limitations of ASCII. Traditionally, degrees minutes and seconds would be delimited with a degree symbol, a prime, and a double prime, which is now possible in Unicode but not widespread in programming languages:

46°48′00″

I note that in the academic papers of Bernard R. Goldstein, on the astronomy of the late Middle Ages and Renaissance, that an academic notation using a semicolon for the decimal point and a comma between the minutes and seconds is used, that looks like:

46;48,00°