# Built-In Physical Constants

    library(orbitr)

`orbitr` ships a set of real-world masses, distances, and orbital speeds so you don’t have to Google them every time. All values are in SI units (kg, meters, m/s).

## Masses

    mass_sun          # 1.989e30 kg
    mass_earth        # 5.972e24 kg
    mass_moon         # 7.342e22 kg
    mass_mars         # 6.417e23 kg
    mass_jupiter      # 1.898e27 kg
    mass_saturn       # 5.683e26 kg
    mass_venus        # 4.867e24 kg
    mass_mercury      # 3.301e23 kg

## Orbital Distances (Semi-Major Axes)

    distance_earth_sun    # 1.496e11 m  (~149.6 million km)
    distance_earth_moon   # 3.844e8  m  (~384,400 km)
    distance_mars_sun     # 2.279e11 m
    distance_jupiter_sun  # 7.785e11 m
    distance_venus_sun    # 1.082e11 m
    distance_mercury_sun  # 5.791e10 m

## Mean Orbital Speeds

    speed_earth       # 29,780 m/s
    speed_moon        #  1,022 m/s
    speed_mars        # 24,070 m/s
    speed_jupiter     # 13,060 m/s
    speed_venus       # 35,020 m/s
    speed_mercury     # 47,360 m/s

## Using the Constants

This means the Earth-Moon example can be written as:

    create_system() |>
      add_body("Earth", mass = mass_earth) |>
      add_body("Moon",  mass = mass_moon, x = distance_earth_moon, vy = speed_moon) |>
      simulate_system(time_step = seconds_per_hour, duration = seconds_per_day * 28) |>
      plot_orbits()

## Why “Distance” Constants Are Semi-Major Axes

Orbital distances are not truly constant — every orbit is an ellipse, so the separation between two bodies changes continuously throughout each revolution. The values provided here are semi-major axes: the average of the closest approach (periapsis) and the farthest point (apoapsis).

The semi-major axis is the single most characteristic length scale of an elliptical orbit. It determines the orbital period via Kepler’s Third Law, and when paired with the mean orbital speed, it produces a near-circular trajectory that closely approximates the real orbit. For example, the Earth-Sun distance varies from about 147.1 million km in January (perihelion) to 152.1 million km in July (aphelion). The semi-major axis of 149.6 million km sits right in the middle and gives the correct one-year orbital period.

If you want an elliptical orbit instead, start the body at periapsis with a faster-than-mean velocity, or at apoapsis with a slower one.