python – 根据纬度和日出时间计算经度？

我正在计算当前太阳高度当前为~0.0的位置的经度.这是通过迭代一系列纬度来计算,计算日出时间(0.0,纬度)然后通过将时间差(小时数)乘以15(太阳“移动”的度数来计算经度)地球表面).

当从计算的坐标元组计算日出时间时,最低纬度显示具有最高纬度的几分钟的时间差.如何解释这种差异？

在：

points=walk_the_earth()

日期：

[-66.53673944994807,-65.0] 2012-08-21 12:07:04.748893
[-67.13184367865324,-64.5] 2012-08-21 12:07:05.666852
[-67.70314011722803,-64.0] 2012-08-21 12:07:06.541521
...
[-119.24775995314121,64.0] 2012-08-21 12:08:45.536679
[-119.93103107437491,64.5] 2012-08-21 12:08:47.770382
[-120.64480075612664,65.0] 2012-08-21 12:08:50.152224

(时间是UTC).代码在?秒下运行.

造成这种差异的原因是什么？

码

import math
import xephem

def longitude_from_latitude(lat):
    """
    Calculate the longitude at which Sun altitude is ~0.0.

    Args:
        lat: A float indicating the latitude to calculate longitude
            for.

    Returns:
        float
    """
    now = xephem.julianday.now()
    meridian = xephem.Observer(now.midnight.dublin,0.0,lat)
    sun = xephem.Sun.fromobserver(meridian)
    transit = sun.transit(-1)
    # Calculate time difference between sun position and local time.
    delta_t = ((now - transit['rs_risetm']) * 24.0) * 15.0
    return delta_t


def walk_the_earth(resolution=0.5,minlat=-65.0,maxlat=65.0):
    """
    Calculate the coordinate at which Sun altitude is ~0.0 for
    a given range of latitudes.

    Args:
        resolution: A float indicating the number of points to
            return for the specified range of latitudes. 1.0 means
            that 1 longitude will be calculated for each real
            latitude,0.5 means 2,etc.
        minlat: A float indicating the lowest latitude to start
            calculating.
        maxlat: A float indicating the highest latitude to 
            calculate up to.

    Returns:   
        list of longitude,latitude,xephem.Sun tuples.
    """
    now = xephem.julianday.now()
    lat = minlat
    points = []
    while True:
        if lat > maxlat:
            break
        lng = longitude_from_latitude(lat)
        # Create an Observer for longitude and latitude
        obs = xephem.Observer(now.dublin,lng,lat)
        sun = xephem.Sun.fromobserver(obs)
        points.append([lng,lat,sun])
        # sun.transit() calculates the rising,transit and setting times
        # of the sun at Observers location. The -1 argument specifies
        # that we consider sunrise to occur when the upper limb touches
        # the horizon (0 indicates center,1 indicates lower limb).
        print points[-1],sun.transit(-1)['rs_risetm'].datetime()
        lat += resolution
    return points