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Astronomical calculations in MicroPython

This module enables sun and moon rise and set times to be determined at any geographical location. Times are in seconds from midnight and refer to any event in a 24 hour period starting at midnight. The midnight datum is defined in local time. The start is a day being the current day plus an offset in days.

A moonphase function is also provided enabling the moon phase to be determined for any date.

Caveat. I am not an astronomer. If there are errors in the fundamental algorithms I am unlikely to be able to offer an opinion, still less a fix.

The code is currently under development: the API may change.

Licensing and acknowledgements

The code was ported from C/C++ as presented in "Astronomy on the Personal Computer" by Montenbruck and Pfleger, with mathematical improvements contributed by Raul Kompaß and Marcus Mendenhall. The sourcecode exists in the book and also on an accompanying CD-R. The file CDR_license.txt contains a copy of the license file on the disk, which contains source, executable code, and databases. This module (obviously) only references the source. I am not a lawyer; I have no idea of the legal status of code translated from sourcecode in a published work.

Installation

Installation copies files from the astronomy directory to a directory \lib\sched on the target. This is for optional use with the schedule module. This may be done with the official mpremote:

$ mpremote mip install "github:peterhinch/micropython-samples/astronomy"

On networked platforms it may alternatively be installed with mip.

>>> mip.install("github:peterhinch/micropython-samples/astronomy")

Currently these tools install to /lib on the built-in Flash memory. To install to a Pyboard's SD card rshell may be used. Move to micropython-samples on the PC, run rshell and issue:

> rsync astronomy /sd/sched

mip installs the following files in the sched directory.

sun_moon.py
sun_moon_test.py A test/demo script. After installation the RiSet class may be accessed with

from sched.sun_moon import RiSet

The RiSet class

This holds the local geographic coordinates and the localtime offset relative to UTC. It is initialised to the current date and can provide the times of rise and set events occurring within a 24 hour window starting at 00:00:00 local time. The RiSet instance's date may be changed allowing rise and set times to be retrieved for other 24 hour windows.

Rise and set times may be retrieved in various formats including seconds from local midnight: this may be used to enable the timing of actions relative to a rise or set event.

Constructor

Args (float):

lat=LAT Latitude in degrees (-ve is South). Defaults are my location. :)
long=LONG Longitude in degrees (-ve is West).
lto=0 Local time offset in hours to UTC (-ve is West).

Methods:

set_day(day: int = 0) day is the offset in days from the current system date. If day is changed compared to the object's currently stored value its rise and set times are updated. Returns the RiSet instance.
sunrise(variant: int = 0) See below for details and the variant arg.
sunset(variant: int = 0)
moonrise(variant: int = 0)
moonset(variant: int = 0)
moonphase() Return current phase as a float: 0.0 <= result < 1.0. 0.0 is new moon, 0.5 is full.
set_lto(t) Set localtime offset in hours relative to UTC. Primarily intended for daylight saving time. Rise and set times are updated if the lto is changed.

The return value of the rise and set method is determined by the variant arg. In all cases rise and set events are identified which occur in the current 24 hour period. Note that a given event may be absent in the period: this can occur with the moon at most locations, and with the sun in polar regions.

Variants:

0 Return integer seconds since midnight local time (or None if no event).
1 Return integer seconds since since epoch of the MicroPython platform (or None).
2 Return text of form hh:mm:ss (or --:--:--) being local time.

Example constructor invocations:

r = RiSet()  # UK near Manchester
r = RiSet(lat=47.609722, long=-122.3306, lto=-8)  # Seattle 47°36′35″N 122°19′59″W
r = RiSet(lat=-33.87667, long=151.21, lto=11)  # Sydney 33°52′04″S 151°12′36″E

The moonphase function

This is a simple function whose provenance is uncertain. I have a lunar clock which uses the original C code. This has run for 14 years without issue, but I can't vouch for its absolute accuracy over long time intervals. The Montenbruck and Pfleger version is very much more involved but they claim accuracy over centuries.

Args:

year: int 4-digit year
month: int 1..12
day: int Day of month 1..31
hour: int 0..23

Return value:
A float in range 0.0 <= result < 1.0, 0 being new moon, 0.5 being full moon.

Utility functions

now_days() -> int Returns the current time as days since the platform epoch. abs_to_rel_days(days: int) -> int Takes a number of days since the Unix epoch (1970,1,1) and returns a number of days relative to the current date. Platform independent. This facilitates testing with pre-determined target dates.

Demo script

This produces output for the fixed date 4th Dec 2023 at three geographical locations. It can therefore be run on platforms where the system time is wrong. To run issue:

import sched.sun_moon_test

Expected output:

>>> import sched.sun_moon_test
4th Dec 2023: Seattle UTC-8
Sun rise 07:40:09 set 16:18:15
Moon rise 23:38:11 set 12:53:40

4th Dec 2023: Sydney UTC+11
Sun rise 05:36:24 set 19:53:21
Moon rise 00:45:55 set 11:27:14

From 4th Dec 2023: UK, UTC
Day: 0
Sun rise 08:04:34 set 15:52:13
Moon rise 23:03:15 set 13:01:04
Day: 1
Sun rise 08:05:54 set 15:51:42
Moon rise --:--:-- set 13:10:35
Day: 2
Sun rise 08:07:13 set 15:51:13
Moon rise 00:14:40 set 13:18:59
Day: 3
Sun rise 08:08:28 set 15:50:49
Moon rise 01:27:12 set 13:27:08
Day: 4
Sun rise 08:09:42 set 15:50:28
Moon rise 02:40:34 set 13:35:56
Day: 5
Sun rise 08:10:53 set 15:50:10
Moon rise 03:56:44 set 13:46:27
Day: 6
Sun rise 08:12:01 set 15:49:56
Moon rise 05:18:32 set 14:00:11
>>>

Code comments show times retrieved from timeanddate.com.

Scheduling events

A likely use case is to enable events to be timed relative to sunrise and set. In simple cases this can be done with asyncio.

from sched.sun_moon import RiSet
import time
rs = RiSet()
tsecs = time.time()  # Time now in secs since epoch
tsecs -= tsecs % 86400  # Last midnight in secs since epoch
tmidnight = tsecs
async def do_sunrise():
    while True:
        toff = time.time() - tmidnight  # Seconds relative to midnight
        if toff > 0:  # Midnight has passed, wait for sunrise
            twait = rs.sunrise() - toff  # Assumes a latitude where sun must rise
        else:  # Wait for tomorrow
            twait = tmidnight + 86400 + toff
        await asyncio.sleep(twait)
        if toff > 0:
            # Turn the lights off, or whatever

An alternative, particularly suited to more complex cases, is to use the schedule module. This allows more intuitive coding without the epoch calculations. The following is a minimal example:

import uasyncio as asyncio
from sched.sched import schedule
from sched.sun_moon import RiSet

async def turn_off_lights(rs):  # Runs at 00:01:00
    rs.set_day()  # Re-calculate for new daylight
    asyncio.sleep(rs.sunrise() - 60)
    # Actually turn them off

async def main():
    rs = RiSet()  # May need args for your location
    await schedule(turn_off_lights, rs, hrs=0, mins=1)  # Never terminates

try:
    asyncio.run(main())
finally:
    _ = asyncio.new_event_loop()

This approach lends itself to additional triggers and events:

import uasyncio as asyncio
from sched.sched import schedule, Sequence
from sched.sun_moon import RiSet

async def turn_off_lights(t):
    asyncio.sleep(t)
    # Actually turn them off

async def main():
    rs = RiSet()  # May need args for your location
    seq = Sequence()  # A Sequence comprises one or more schedule instances
    asyncio.create_task(schedule(seq, "off", hrs=0, mins=1))
    # Can schedule other events here
    async for args in seq:
        if args[0] == "off":  # Triggered at 00:01 hrs (there might be other triggers)
            rs.set_day()  # Re-calculate for new day
            asyncio.create_task(turn_off_lights(rs.sunrise() - 60))

try:
    asyncio.run(main())
finally:
    _ = asyncio.new_event_loop()

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