Plane of Ecliptic. Plane of celestial sphere passing through centres of Earth and Sun.
Plane Sailing. That method of solving certain navigational problems on assumption that Earth's surface, in the area concerned, is a plane.
Planet. Satellite of Sun that reflects Sun's light. Excluding minor planets there are eight, Earth not being reckoned. Those revolving inside Earth's orbit are termed 'minor planets', those outside Earth's orbit being 'major planets'.