Put the solar system in your pocket! Orrery is an astronomy/astrology app written for the PlayDate handheld (https://play.date/). Crank through time to see where the planets were or will be on specific dates. Includes:

• Scientific mode: a 3D view with realistic orbits
• Diagrammatic mode: a simple, compass-like view of the solar system
• Astrological mode, including:
  • Zodiac annotation
  • Retrograde planet detection
  • Lunar phase
    
• Controls for easily jumping forward or backward in time by decades or centuries at a time, and the ability to jump to the current date as well
• A save system for bookmarking specific dates
  
• Delightful sound effects!

View Modes

Scientific

A 3D render of the solar system. Orbital sizes are correct relative to each other, with the exception of the moon, which has been exaggerated for visibility. Planet sizes are also exaggerated for visibility, but are correct relative to each other. The size of the sun isn't proportional to anything else, and is also exaggerated for visibility.

Diagrammatic

A simplified view of the solar system. Orbits are rendered on a single plane with equal spacing. When hiding planetoids, their orbits are left blank. This coincidentally leaves space where you'd find the asteroid belt.

Astrological

A geocentric view of the solar system. Our sun, Sol, is rendered on an inner ring orbiting the earth, with all other planets, planetoids, and the moon (Luna) orbiting on an outer ring. The geocentric angle of each orbital body places it on one of 12 astrological segments corresponding to the Western Zodiac. For example, you can find out your sun and moon signs by setting the date to your birthday and looking at which segments Sol and Luna fall into.

Orbital bodies that are in retrograde are rendered as black with a halo. 

The phase of the moon is shown in the bottom right corner.

Controls

Open the system menu to view camera controls.

• Cranking will move forward and backward through time
• Holding B while cranking will zoom in and out
• Using the D-Pad will pan the camera
• Holding B while using the D-Pad will rotate the camera
• Pressing A will open the menu

Menu Options

Press A to open the in-app menu.

The following options are available:

• mode: Cycle between the 3 view modes
• jump bar: Jump to the current date, or jump forward/back in time by increments of 10 or 100 years

• Saves: Opens the save menu
  • To save: Press A on an empty save slot
  • To load: Press A on an occupied save slot
  • To delete: Press A on the "X" to the right of the save slot you'd like to clear.

• planetoids: Toggles the visibility of planetoids like Eris, Pluto, and Makemake

The final option is Settings.

Settings

• label style: Adjust verbosity of planetary labels, using Earth as an example. Supports: 
  • full (Earth, Mars, Jupiter)
  • one-letter (E, M, J)
  • two-letter (Ea, Ma, Ju)
  • no labels
    

• hemisphere: Set whether you're viewing the solar system from the northern or southern hemisphere. Mirrors the image horizontally.

• date format: Adjust the date format. Supports: 
  • MDY (May 28, 2022 C.E.)
  • DMY (28 May, 2022 C.E.)
  • YMD (2022 C.E. May 28)
• epoch: Change the annual notation. Supports:
  • CE/BCE
  • AD/BC
  • HE/BHE (Human Era notation. Adds 10000 to the year)

Data

Overrides for solar system data are saved in your device/simulator's data directory under "customSolarSystems". You may adjust them as you see fit, or even add and remove orbital bodies. To edit the orbital data, connect to a computer with your device in disc mode. If using the simulator, find your user data directory with File -> Reveal Data Folder. To reset to defaults, simply delete your "customSolarSystems" folder or delete the overrides individually via the Planetary System menu.

Default values are based on data provided by SpaceReference.org, NASA and Princeton University. Parameters are calculated for 2000 Jan 0.0 TDT with deltas given per day. Angular parameters are given in degrees, radius is given in 1000s of miles, and the semi-major axis is given in AU. Any value listed as a set of 2 numbers uses the first number as the value at the reference date and the second number as the rate of change per day.

For example, here is the default data for Mercury:

{
    "argumentOfPerihelion": [
        29.124099731445,
        1.0144400221179e-05
    ],
    "eccentricity": [
        0.20563499629498,
        5.5950001294518e-10
    ],
    "incline": [
        7.0047001838684,
        5.0000000584305e-08
    ],
    "isPlanetoid":false,
    "longitudeOfAscendingNode": [
        48.331298828125,
        3.2458701753058e-05
    ],
    "meanAnomaly": [
        168.65620422363,
        4.0923342704773
    ],
    "name":"Mercury",
    "radius":1.5160000324249,
    "semimajorAxis": [
        0.38709801435471,
        0
    ]
},

Moons can be added to any orbital body with the "moons" attribute:

            "moons": [
                 {
                    "argumentOfPericenter": [
                        318.06338500977,
                        0.16435730457306
                    ],
                    "eccentricity": [
                        0.054950001734495,
                        0
                    ],
                    "incline": [
                        5.1454000473022,
                        0
                    ],
                    "isPlanetoid":false,
                    "longitudeOfAscendingNode": [
                        125.12280273438,
                        -0.052953809499741
                    ],
                    "meanAnomaly": [
                        115.36540222168,
                        13.064992904663
                    ],
                    "name":"Luna",
                    "radius":1.0795999765396,
                    "semimajorAxis": [
                        0.002542698988691,
                        0
                    ]
                }
            ],

Calculations

Calculations used in this simulation are based on this paper: https://www.stjarnhimlen.se/comp/tutorial.html#2, while omitting interplanetary perturbations to preserve on-device performance.