Semi-Major Axis

In an elliptical orbit, the orbital parent is located at one of the focus points (F₁ or F₂). The semi-major axis, a, is half of the longest diameter of the ellipse. Source: Wikimedia Commons

The Semi-Major Axis of an object's elliptical orbit is the distance from the center of the ellipse to the most distant point in the orbit. In other words, the Semi-Major Axis is half the distance between the pericenter and apocenter of the orbit. Semi-Major Axis is a way to represent the size of an orbit; objects with a smaller Semi-Major Axis will orbit closer to their Orbital Parent than objects with a larger Semi-Major Axis.

The Semi-Major Axis of an object with a hyperbolic trajectory is negative.

Semi-Major Axis is one of the six orbital elements that described an object's orbit.

Property Details

Location

The Semi-Major Axis property is located in the Orbital Elements section of the Motion tab of an object's properties panel, if that object has an Orbital Parent.

Units

Semi-Major Axis can be viewed and edited using the following units:

• Milky Way Galaxy radius (milky way) = 5.5×10⁴ light year
• Parsec (parsec) = 3.08568025×10¹⁶ m
• Light-year (light year) = 9.46073047×10¹⁵ m
• Astronomical Unit (AU) = 1.495978707×10¹¹ m
• Solar radius (sun) = 6.955×10⁸ m
• Lunar distance (lunar dist) = 3.844×10⁸ m
• Light-second (light sec) = 2.99792458×10⁸ m
• Jupiter radius (jupiter) = 6.9911×10⁷ m
• Earth radius (earth) = 6.37101×10⁶ m
• Moon radius (moon) = 1.7371×10⁶ m
• Kilometer (km) = 1000 m
• Meter (m) is the SI base unit for length
• Centimeter (cm) = 0.01 m
• Millimeter (mm) = 0.001 m