For example , suppose a Planet is orbiting around a Star in perfect circular orbit , then the Circumference is orbital path of the planet and area of circle is ` orbital plane ' of that Planet . By definition the inclination of a planet in the solar system is the angle between its orbital plane and the orbital plane of the Earth ( the ecliptic ) . In other cases , for instance a moon orbiting another planet , it is convenient to define the inclination of the moon 's orbit as the angle between its orbital plane and the planet 's equator .