Due to gravitational forces exerted by their host star, planets orbit around it in elliptical paths called orbits. Each orbit of a planet has a fixed shape and duration before one cycle (orbit) completes.
All planets in our solar system, along with moons, asteroids and comets found within an asteroid belt and even our Earth’s moon, generally follow this same general path known as the Ecliptic.
If you venture outside in a clear sky after sunset this week, look out for Venus riding low in the west just above the setting sun – marking one point on the ecliptic. Just a few hours later, Mars will rise higher in the eastern sky to mark another. If the night is clear, you will also be able to observe Saturn rising in the east as part of an ecliptic point. Venus, Mars and Saturn often appear at various points across its path in the sky due to how their Earth-Moon barycenter shifts slightly under influence of other planets in our solar system affecting its position.
The ecliptic intersects the celestial sphere, an imaginary globe representing our view of the sky from Earth, at two points: at mid-March equinox and mid-September equinoxes respectively – these marking spring’s start and autumn’s arrival respectively.
Most planets in our solar system, including the Moon and all major moons of inner planets, travel in an orbit that follows along the ecliptic. There are however a few minor planets and comets whose orbits diverge significantly from this plane, meaning some bodies can be seen by naked eye while others cannot.
Johannes Kepler was an 18th Century astronomer who was responsible for clarifying the elliptical orbits of planets, known as Keplers Laws of Planetary Motion. Keplers Laws explain that any object orbiting around another more massive one will be drawn by gravity into an elliptical path and will cover equal portions of sky at regular intervals over time. Mercury can always be found in its usual place at dusk and dawn, taking less than 24 hours for one complete orbit around our solar system. Unfortunately, however, why planets follow elliptical orbits instead of circular ones is still unknown.