Eclipse Functionality

I noticed in the file TODO.rst adding the functionality for lunar/solar eclipses.

I am not sure if you intend to calculate this information or rather draw from a table. For what it’s worth, below is a code sample I use in my application to extract eclipse information from tables I took from NASA. If any of the code is useful, please use it in Skyfield.

#!/usr/bin/env python3
# -*- coding: utf-8 -*-


# Lunar/solar eclipse tables up to 2099.
# Courtesy of Fred Espenak and Jean Meeus.


import datetime


# Eclipse Types.
ECLIPSE_TYPE_ANNULAR = "A"
ECLIPSE_TYPE_HYBRID = "H"
ECLIPSE_TYPE_PARTIAL = "P"
ECLIPSE_TYPE_PENUMBRAL = "N"
ECLIPSE_TYPE_TOTAL = "T"


# Gets the upcoming eclipse, lunar or solar.
#
# dateTimeUTC - Current date/time in UTC as a python datetime.datetime object.
# isLunar - If True, finds the next lunar eclipse; otherwise finds the next solar eclipse.
#
# Returns a tuple of strings describing the next eclipse:
#    dateTime (format of YYYY-MM-DD HH:MM:SS)
#    eclipseType ("A" = Annular, "H" = Hybrid, "N" = Penumbral, "P" = Partial, "T" = Total)
#    latitude (south is negative)
#    longitude (east is negative).
def getEclipse( dateTimeUTC, isLunar ):
    if isLunar:
        eclipseData = __lunarEclipseData

    else:
        eclipseData = __solarEclipseData

    eclipseInfo = None
    for eclipse in eclipseData:
        dateTime = datetime.datetime.strptime( eclipse[ 0 ] + ", " + eclipse[ 1 ] + ", " + eclipse[ 2 ] + ", " + eclipse[ 3 ], "%Y, %m, %d, %H:%M:%S" )
        dateTime = dateTime - datetime.timedelta( seconds = int( eclipse[ 4 ] ) ) # Need to subtract delta T (https://eclipse.gsfc.nasa.gov/LEcat5/deltat.html).
        if dateTimeUTC <= dateTime:
            latitude = eclipse[ 6 ][ 0 : len( eclipse[ 6 ] ) - 1 ]
            if eclipse[ 6 ][ -1 ] == "S":
                latitude = "-" + latitude

            longitude = eclipse[ 7 ][ 0 : len( eclipse[ 7 ] ) - 1 ]
            if eclipse[ 7 ][ -1 ] == "E":
                longitude = "-" + longitude

            eclipseInfo = ( str( dateTime ), eclipse[ 5 ], latitude, longitude )
            break

    return eclipseInfo


# https://eclipse.gsfc.nasa.gov/5MCLE/5MKLEcatalog.txt
#
# Title: Five Millennium Catalog of Lunar Eclipses: -1999 to [phone] BCE to 3000CE)
# Authors: Fred Espenak and Jean Meeus
# Source: NASA Technical Publication NASA/TP-2009-214173
# On Web: https://eclipse.gsfc.nasa.gov/SEpubs/5MKLE.html
# Catalog Key: https://eclipse.gsfc.nasa.gov/LEcat5/LEcatkey.html
# Date: 2011 May 23
#
#      Year   Month  Day   HH:MM:SS    DT   Type  Lat   Long
__lunarEclipseData = [
    [ "2020", "11", "30", "09:44:01", "72", "N", "21N", "148W" ],
    [ "2021", "05", "26", "11:19:53", "72", "T", "21S", "170W" ],
    [ "2021", "11", "19", "09:04:06", "73", "P", "19N", "139W" ],
    # <snip>
    [ "2099", "09", "29", "10:36:38", "202", "N", "3N", "161W" ] ]


# https://eclipse.gsfc.nasa.gov/SEpubs/5MKSE.html
# https://eclipse.gsfc.nasa.gov/5MCSE/5MKSEcatalog.txt
#
# Title: Five Millennium Catalog of Solar Eclipses: -1999 to [phone] BCE to 3000CE)
# Authors: Fred Espenak and Jean Meeus
# Source: NASA Technical Publication NASA/TP-2008-214170
# Catalog Key: https://sunearth.gsfc.nasa.gov/eclipse/SEcat5/catkey.html
# Date: 2008 10 07
#
#      Year   Month  Day   HH:MM:SS    DT   Type  Lat      Long
__solarEclipseData = [
    [ "2020", "12", "14", "16:14:39", "72", "T", "40.3S", "67.9W" ],
    [ "2021", "06", "10", "10:43:07", "72", "A", "80.8N", "66.8W" ],
    [ "2021", "12", "04", "07:34:38", "73", "T", "76.8S", "46.2W" ],
    # <snip>
    [ "2099", "09", "14", "16:57:53", "202", "T", "23.4N", "62.8W" ] ]

[Bernmeister]

I’ll have a crack at it…

[brandon-rhodes]

Only noticed it after you’ve pointed it out…oh well.

If you were following the issue but GitHub didn‘t light up your notifications, I’ll be happy in the future to make a comment after a commit to try to draw attention to it.

… in the end, I wrote code to implement the formulae in the book(s).

My approach with the commit above was similar: reading, followed by just converting their formulae into Python, with a focus on getting the right answer rather than understanding immediately. 🙂

Only once it worked did I review the text, figure out what the symbols meant, and substituted in some cases meaningful names for what had been plain variables in the original formulae.

Hopefully your code helps other folks who need eclipse data before solar eclipses are implemented in Skyfield, which probably won’t be for a few weeks, even if I put it at the front of my to-do list! My guess is that it would be hard to incorporate into official Skyfield code, because I think many of those equations are approximations for things Skyfield does at high precision, like TDB time, and the Sun and Moon positions — but to untangle which are which, we would need to hunt down the source of all of the coefficients in your example code. Of which, wow, there are a lot! You did a lot of work to successfully bring them into Python, and I’m glad for the moment that they provide a good match for at least the times of both lunar and solar eclipses.