micropython-samples/astronomy

Astronomical calculations in MicroPython

1. Overview
   1.1 Applications
   1.2 Licensing and acknowledgements
   1.3 Installation
2. The RiSet class
   2.1 Constructor
   2.2 Methods
   2.3 Effect of local time
3. The moonphase function
4. Utility functions
5. Demo script
6. Scheduling events
7. Performance and accuracy

1. Overview

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.

1.1 Applications

There are two application areas. Firstly timing of events relative to sun or moon rise and set times, discussed later in this doc. Secondly constructing lunar clocks such as this one - the "lunartick":

1.2 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.

1.3 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

2. 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. In continuously running applications which must access current rise and set times the application should re-calculate (by issuing .set_day()) prior to retrieving that day's data.

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.

2.1 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); the value is checked to ensure -12 < lto < 12. See section 2.3.

2.2 Methods

• set_day(day: int = 0) day is an offset in days from the current system date. The number of days from the specified day to a fixed epoch is calculated and compared to that stored in the instance. If there is a change the new value is stored and the rise and set times are updated - otherwise return is "immediate". 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)
• is_up(sun: bool) Returns True if the selected object is above the horizon. This calls .set_day() to ensure the current day is selected.
• moonphase() Return current phase as a float: 0.0 <= result < 1.0. 0.0 is new moon, 0.5 is full. See section 3 for observations about this.
• 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 value is checked to ensure -12 < lto < 12. See section 2.3.

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

2.3 Effect of local time

MicroPython has no concept of local time. A hardware platform has a clock; depending on application this might be permanently set to local winter time, or it might be adjusted twice per year for local daylight saving time. It is the responsibility of the application to do this if it is considered necessary.

Rise and set times are computed relative to UTC and then adjusted using the RiSet instance's local time offset before being returned (see .adjust()). This applies to all variants - note that the local platform epoch is on a fixed date at 00:00:00 local time.

If the machine clock has a fixed relationship to UTC, RiSet instances should have a corresponding fixed local time offset: rise and set times will be relative to that time. If the application implements daylight saving time, the local time offsets should be adjusted accordingly.

The constructor and the set_day() method set the instance's date relative to the machine clock. They use only the date component of system time, hence they may be run at any time of day. In continuously-running applications, set_day() may be run each day just after midnight to keep a RiSet instance up to date.

3. The moonphase function

This is a simple function whose provenance is uncertain. It appears to produce valid results but I plan to implement a better solution.

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.

4. 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.

5. 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
Maximum error 0. Expect 0 on 64-bit platform, 30s on 32-bit
>>>

Code comments show times retrieved from timeanddate.com.

6. 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. This coroutine will execute a payload at sunrise every day. A similar coroutine might handle sunsets.

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

async def tomorrow():  # Wait until 1 minute past midnight
    now = round(time.time())
    tw = 86400 + 60 - (now % 86400)  # Time from now to one minute past next midnight
    await asyncio.sleep(tw)

async def do_sunrise():
    rs = RiSet()  # May need args
    while True:
        if (now := round(time.time())) < rs.sunrise(1):  # Sun has not yet risen
            await asyncio.sleep(rs.sunrise(1) - now)  # Wait for it to rise
            # Sun has risen, execute payload
        await tomorrow()
        rs.set_day()  # Update to new day

This code assumes that .sunrise() will never return None. At polar latitudes waiting for sunrise in winter would require changes.

Code may be simplified by using the schedule module. This may be installed with

$ mpremote mip install "github:peterhinch/micropython-async/v3/as_drivers/sched"

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()

7. Performance and accuracy

A recalculation is triggered whenever the 24 hour local time window is changed, such as calling .set_day() where the stored date changes. Normally two days of data are calculated, except where the local time is UTC where only one day is required. The time to derive one day's data on RP2040 was 707μs.

The accuracy of rise and set times was checked against online sources for several geographic locations. The online data had 1 minute resolution and the checked values corresponded with data computed on a platform with 64 bit floating point unit. The loss of precision from using a 32 bit FPU was no more than 30s.

For reasons which are unclear, the is_up() method is less precise, showing incorrect results when within a few minutes of the rise or set time.