def field_elongation(ra, dec, date):
    """
    For a given field, calculate the solar elongation at the given date.
    :param ra: field's right ascension. unit="h"   format="RAh:RAm:RAs"
    :param dec: field's declination. degrees
    :param date: date at which to calculate elongation
    :return: elongation from the Sun in degrees
    """
    sun = ephem.Sun()
    sun.compute(date)
    sep = ephem.separation((ra, dec), sun)
    retval = 180. - math.degrees(sep)

    return retval