Solstice and Equinox - Credit: NASA
The major axis of an ellipse is its longest diameter. So, it is a line that runs through the center and both foci. It is a line through the widest points of the ellipse. The semi-major axis is one half of the major axis, so it runs from the center to a single focus. That makes it a method to measure the radius of an orbit when taken from the points of that orbit’s two most distant points.
The semi-major axis is one of the most important orbital elements in astronomy. For solar system objects, the semi-major axis is related to the period of the orbit through Kepler’s third law and is a simplification of the two-body problem. The orbiting body’s path around the barycenter and its relative path to the primary are both ellipses. The semi-major axis used in astronomy is always the primary-to-secondary distance. Because of this, the parameters are always given in heliocentric terms.
A component that has to be considered when determining a semi-major axis is the gravitational constant. The gravitational constant is an empirical physical constant in the calculation of gravitational attraction between objects with mass. It is a factor in Newtonian law and Einstein’s theory of general relativity. It should not be confused with the local gravitational field.
The two-body problem is to determine the motion of two point particles that interact only with each other. It is applied to satellites around a planet, planets around a star, binary systems, and even electrons orbiting a nucleus.
It may be easier to understand the semi major axis if you do a little additional research and discover all of the mathematical formulas needed to derive it.
We have written many articles about the semi major axis for Universe Today. Here’s an article about the distance from the Earth to the Moon, and here’s an article about the Kepler’s Laws.
If you’d like more info on the orbit, check out The Laws of Planetary Motion. And here’s a link to an article about what an orbit is.
We’ve also recorded a Question Show regarding the Orbit of Planets. Listen here, Question Show: Orbit of the Planets, Green Stars, and Oort Cloud Contamination.
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