Tuesday, September 14, 2021

6 Basic orbital elements – an overview

1:15 AM | ,

If you like astronomy, you have to start learnig from basic notioms such as the position of any celestial objiect in the sky. The path followed by any solar system body circling another is defined by 6 basic orbital elements. Let us have a look at each of them one by one.

Mean anomaly

Among the 6 basic orbital elements, the first one is “Mean anomaly”. It is defined as the angular distance from perihelion to the present position of the body measured in the direction of motion. Mean anomaly is denoted by ‘M’. The mean anomaly is defined as M=ω (t-τ), where ω is average rate of sweep given as ω=2π/T , τ is the time at which body is at perihelion

The mean anomaly M can be computed from the eccentric anomaly E and the eccentricity e using the formula :- M = E – esinE

Semi-major axis

For any ellipse, the semi-major axis is defined as one-half the distance between perihelion and the aphelion. It is denoted by ‘a’.


The orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. An eccentricity value of 0 represents a circular orbit, values between 0 and 1 form an elliptic orbit, eccentricity of 1 is a parabolic escape orbit, and greater than 1 is a hyperbola. It is denoted by ‘e’.


It is defined as the angle between the plane of the orbit of the celestial body and the ecliptic. If the inclination is more than 90° then the motion of the object is considered to be retrograde. Inclination is denoted by ‘i’.

Longitude of the ascending node

It is defined as the direction in space of the line where the orbital plane intersects the plane of the ecliptic. It is always measured eastwards (increasing RA) from the vernal equinox, which is defined as first point of Aries. Longitude of ascending node is denoted by ‘Ω’.

Argument of perihelion

Argument of perihelion defines how the major axis of the orbit is oriented in the orbital plane. It is the angle between the ascending node and the perihelion point measured in the direction of motion. Argument of perihelion is denoted by ‘ω’.

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