Every one of us is taught that all planets revolve around the sun. But is that actually true?
No, it isn’t. Now, don’t get panicked and start blaming your Physics Teacher for teaching you wrong. Let’s go through this article together.
Before going any further we need to know what is ‘centre of mass’. Simply stating; it is a point on which a particular body can be balanced. Seems confusing but it isn’t actually. For example–when we take a ruler and try to balance it on our finger, we keep the finger at the centre of the ruler in order to balance it; the specific point on which we can balance the ruler is the centre of mass of that particular ruler.
But it is not always the case, that the centre of mass of a particular body will always be at its centre. It may or may not lie at its centre in every case. For example – in the case of a hammer, it will lie somewhere near the metal piece. From this, we even concluded that in case of a body in which the mass of the body is unevenly distributed; the centre of mass of that body will lie near the heavier part of the body, not at the centre.
In case of two bodies, we call it the common centre of mass also known as Barycentre. This is the point around which both the stars ( or the Sun and the planets ) revolve.
But where does it actually lie? It doesn’t have a particular position. It depends on the masses of both the celestial bodies. If the masses of the bodies are equal it will lie in the centre of the distance between the bodies and the bodies will revolve around this point. (The the distance between the bodies is measured from the centre of one body to the centre of the other)
But in the case of the Earth and Sun, as the mass of the sun is much more than that of the mass of the Earth, the Barycentre will lie near the centre of the sun, in fact somewhere in the sun. Both, the Sun and the Earth, revolve around this Barycentre. Since the Barycentre is near the centre of the sun, when we observe this from the earth in normal everyday life it seems us that earth is revolving around. The same happens in the case of the Moon and the Earth.
But in the case of Jupiter and the Sun, it becomes noticeable . As the mass of Jupiter is way more. The Barycentre lies near the sun but somewhere outside it. So when we observe this by a strong telescope we see that both sun and Jupiter are revolving around a single common point i.e. the Barycentre. The concept of Barycentre is true for any two bodies in space.
Written by: Aaryan Salgania
Aaryan Salgania is volunteering for Voyager Space Outreach to write Blog Posts on STEM/Space Topics.