micropython-samples/astronomy/README.md

# 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.

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.

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.

# The RiSet class

## Constructor

Args (float):
* `lat=LAT` Latitude in degrees. Defaults are my location. :)
* `long=LONG` Longitude in degrees (-ve is West).
* `lto=0` Local time offset in hours (-ve is West).

Methods:
* `set_day(day: int = 0)` The arg is the offset from the current system date.
Calling this with a changed arg causes the rise and set times to be updated.
* `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.

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.

# The moonphase function

This is a simple function whose provenance is uncertain. I have a lunar clock
which uses a C version of this which 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 (all integers):
* `year` 4-digit year
* `month` 1..12
* `day` Day of month 1..31
* `hour` 0..23

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