Overview

Earth Moon Model (EMM) is a tabletop digital art project that combines a Raspberry Pi computer with sensors and actuators to create a realistic model of the Earth and the Moon in their orbits.

EMM can orient the Earth in its orbit and rotation and the Moon in its orbit at any target date-time. In normal operating mode (main.py), EMM is a timepiece that reflects the positions of the Earth and Moon at the present clock time. EMM can also operate in a demonstrate mode (time_warp.py) that accepts ISO-date time entries in standard input and adjusts Earth and Moon positions to reflect them.

EMM utilizes Skyfield library for all astronomical calculations and Adafruit CircuitPython Motor Kit library for stepper motor controls.

A video of the final assembled model is available on YouTube.

Design

The image on the left is a rendering of EMM. The design source is freely available on TinkerCAD.

The image below illustrates the perspective of the EMM observer. The observer follows the Earth as it travels around the Sun, with the Sun always due west of the observer.

CAD Model	Assembled Model

Model Perspective

Parts

• Raspberry Pi Zero W
• Adafruit Stepper Motor, NEMA-17 size, 200 steps/rev, 12V 350mA (x3)
• Adafruit Stepper Motor HAT (x2)
• Hall effect sensor (x3)
• Tiny round disk magnet (x3)
• 5V 2A power supply

Software

EMM is running on a Raspberry Pi Zero W with DietPi OS. DietPi is a highly optimized and minimal Debian-based Linux distribution.

At the time of this writing, DietPi version 8.9.2 is installed. It runs atop Linux kernel version 5.15.

# uname -a
Linux DietPi 5.15.61+ #1579 Fri Aug 26 11:08:59 BST 2022 armv6l GNU/Linux

Python3 is required.

apt-get install python3
apt-get install python3-pip

The following Python package dependencies are required:

• skyfield - library for calculating astronomical predictions
• adafruit_motorkit - library for controlling stepper motors
• RPi.GPIO - library for interacting with Raspberry Pi GPIO pins

pip3 install skyfield
pip3 install adafruit-circuitpython-motorkit
pip3 install RPi.GPIO

The following was also necessary with my setup. See numpy/numpy#14553.

apt-get install libatlas-base-dev

Assembly

A small amount of soldering is required to assemble the components.

Neither the Raspberry Pi Zero W nor the Adafruit Stepper Motor HAT have header pins pre-installed. The first step in preparing the electronics is to solder those headers to allow basic connectivity.

• Solder male pin headers to Raspberry Pi Zero W
• Solder stacking headers to lower Stepper Motor HAT
• Solder female pin headers to upper Stepper Motor HAT
• Solder terminal blocks to lower Stepper Motor HAT
• Solder terminal blocks to upper Stepper Motor HAT

Current Draw

The Raspberry Pi Zero W alone draws about 113 mA of current.

Each motor draws roughly an additional 140 mA of current.

Together, the system draws about 538 mA of current as measured with a multimeter.

Tests were conducted by activating the stepper motors one at a time via a simple Python REPL.

from adafruit_motorkit import MotorKit
kit = MotorKit()
kit.stepper1.onestep() # engage 1st motor
kit.stepper2.onestep() # engage 2nd motor
kit2 = MotorKit(address=0x61)
kit2.stepper1.onestep() # engage 3rd motor

RPi	1 Motor	2 Motors	3 Motors

Modules

earth.py

• Uses skyfield library to calculate earth orbit, earth rotation, and moon orbit given a point in time
• All rotation values range from 0 to 360
• Test module earth_test.py
• Defines Earth class

sensor.py

• Uses RPi.GPIO library to yield a sensor signal True or False
• Defines Sensor class

motor.py

• Uses sensor input to orient motor shafts to reference positions
• Implements a simple scan algorithm to discover sensor region and to move to the midpoint
• Test module motor_test.py
• Defines Motor class

steps.py

• Defines Steps class that eases arithmetic and normalization of motor steps
• Used by Model

model.py

• Governs trio of motors in coordination
• Includes awareness that earth-rotation and moon-orbit motors are mounted on earth-orbit motor
• Test module model_test.py
• Defines Model class

main.py

• Main application entry point
• Utilizes modules above
• Does the following:
  • Moves motor shafts to reference positions
  • Moves motor shafts to target position based on earth and moon positions
  • Check positions minutely, moving motor shafts as needed

time_warp.py

• Application that demonstrates model capabilities
• Goes to the reference position on startup
• Subsequently waits for ISO-formatted date entries on standard input
• Model will adjust to reflect each entry in turn

The following entries were submitted for the demo video linked above.

2022-06-21T09:13:51+00:00
2022-09-23T01:03:42+00:00
2022-12-21T21:48:13+00:00
2022-12-22T00:54:17+00:00
2022-12-22T15:21:57+00:00
2022-12-23T10:16:53+00:00
2023-01-06T23:07:54+00:00
2023-05-05T17:22:54+00:00

Wood Working

The incredible Lance Barlas constructed wood components based on the 3D model above.