Motion of Planets
Two Minute Astronomy, 3
Last week, we explored the concept of the celestial sphere and gathered that unlike stars, Solar System objects (we’ve been using this term for a while; it includes planets, moons, the Sun, asteroids, comets…basically all celestial bodies that are bound by the Sun’s gravitational force) are close enough to us that they move across our sky independent of each other.
Before we begin, let’s go through a short and oversimplified take on the formation of the Solar System. A gas cloud that existed in space was bombarded with heavy elements from a nearby dying star (we shall talk about the life cycle of stars — yes, they do have one! — in a later episode, but for now what you need to know is that when stars convert all their fuel to heavier elements, they become unstable, blow up, and “die,” sending said heavier elements catapulting into space). This enriched the cloud both matter- and energy-wise, making it spin at a high velocity and thus flattening it into a disk. Stuff cooled down a bit and matter clumped together, forming the Solar System.
Of course, all of this is just a theory, but it explains one peculiar thing about the Solar System: it is almost flat, just like the disk it formed from! All planets’ orbits are in nearly the same plane, which is something you’ll want to keep in mind for a while.
The actual movement of individual planets requires a bit more explanation, which we will undertake later. But one key takeaway from the preceding paragraphs is that all planets (and our Moon) exist in roughly the same plane. Because of this, while viewing the Sun, Moon and the planets from the Earth, you’ll see that they appear to move only within a ‘band’ across the sky (similar to the above diagram). They don’t move in the exact same line because they are only roughly in the same plane, but often in sky maps this band is denoted by a line called the ecliptic. It is implied that major Solar System objects can be found near the ecliptic, which actually represents the plane of the Earth’s orbit around the Sun (and hence, the Sun is always right on this line at any given point of time).
Remember the diagram of the celestial sphere we saw last time? Here’s a version of it with the ecliptic highlighted. The places where the celestial equator crosses the ecliptic correspond to the Sun’s position on the days of the two equinoxes (Happy Fall/Spring Equinox, depending on where you live!), which is denoted by the two red Xs.
All this might seem like a slight information overload, but as we start using these terms and concepts more often, everything will fall into place. So, stay tuned for our next episode, where we’ll talk about the celestial coordinate system!
