Question

What is meant by apogee and perigee, eccentricity? How do we calculate them?

Answer 1

Introduction:

1. Planets, the moon, and satellites all have elliptic orbits. There are several parameters that can be used to describe an orbit.
2. A minimum of two parameters are required which are as follows:

1. The semi-major axis $\left(a\right)$ is half of the greatest width of the ellipse.
2. The eccentricity $\left(0\le e<1\right)$ describes the shape of the ellipse. For instance, the eccentricity of a perfect circle is zero.

An illustrative diagram of an elliptical path is given below.

Explanation:

1. Apogee: Apogee is the maximum distance of the Moon or a satellite gets away from the Earth within its orbit. The semi-major axis and eccentricity are related to apogee $\left(A\right)$ as follows:

$A=a\left(1+e\right)$, where $e$ is the eccentricity, $a$ is the length of the semi-major axis.

2. Perigee: Perigee means the closest distance the Moon or a satellite gets to Earth in its orbit. The semi-major axis and eccentricity are related to Perigee $\left(P\right)$ by the following relation given as:

$P=a\left(1-e\right)$, where $e$ is the eccentricity, $a$ is the length of the semi-major axis.

An illustrative diagram given below shows the apogee and perigee.