# Why does the earth revolve around the sun? [duplicate]

I want to know why the Earth rotates around the sun. Why doesn't it rotate around the moon or a different star?

## marked as duplicate by rob♦Dec 15 '17 at 5:24

It doesn't. All the planets and the Sun, along with asteroids and comets and multiple other objects all revolve around an effective center of mass known as the barycenter of the solar system. Because the mass of the Sun is so much larger than everything else, that barycenter is close to the Sun.

If you cconsider an isolated Earth-Sun system, the revolution would be around a point about 450 km from the center of the Sun. The Sun itself is about 696000 km in radius, so the Sun would be "wobbling" around, but the Earth is taking a very long trip around that point, from 150 Gm away. So, the primary appearance is that the Earth is travelling around the Sun. In reality, the Sun is moving, too, but due to the other planets, it's not a purely elliptical motion.

For the Earth-Moon system, the barycenter is about 4600 km from the center of the Earth, which is 6380 km in diameter. That's a much bigger "wobble" but the Moon is still a long distance away, 384000 km.

The Sun's mass is so larger that the Earth-Moon revolutionary system orbits the Sun together, but the Earth is also revolving around the Earth-Moon barycenter.

• The planets do not orbit the barycenter. What is true is that the equations of motion take on their simplest form when expressed with respect to the barycenter. – David Hammen Dec 15 '17 at 12:03

The basic reason why the earth circles the sun instead of another star or the moon is that that reflects the way the planets formed, from a gas and dust cloud that was circling around the sun. For better understanding we need to consider how gravity operates.

Gravity works according to Newton's law of gravitation:$$F=G\frac{M_1M_2}{r^2}$$ Here $M_1$ and $M_2$ are the masses of the 2 bodies involved, r is the distance between them, and $G$ is the gravitational constant. Its value depends on the units used for the other terms.

It follows from this that, if one of the masses is 10 times larger, the force is also 10 times larger. A 10 times larger distance, however, reduces the force by a factor of $100$ ($10^2$). Now, the earth is 81 times more massive than the moon, and the sun is $333,000$ times heavier still, so the sun equals $26,973,000$ moons. On the other hand, the moon is "only" $380,000\space km$ away, while the distance to the sun is $150,000,000 \space km$, a factor of $395$ further.

Using these figures we can see that the attractive force between earth and sun is $$26973000/{395}^2=173$$ times larger than that between earth and moon.

As far as other stars are concerned, they are so far away that their gravitational attraction is minuscule. Consider that the nearest star, Proxima Centauri is more than $4\space ly$ away, or approximately $40,000,000,000,000\space (40\space trillion)\space km$. That is $268,000$ time further than the sun, making its attraction $268000^2=71\space billion$ times weaker. On top of that, Proxima is a red dwarf star, with a mass of only $0.12$ suns, making its effect another $8$ times weaker.

So while the moon, the other planets and all the stars do affect the earth's orbit, their combined effects are far less than that of the sun.

• Gravitational acceleration is not a good metric. Look to the Moon. This metric says that the Moon orbits the Sun rather than the Earth. (The gravitational acceleration of the Moon toward the Sun is more than twice that of the Moon toward the Earth.) – David Hammen Dec 15 '17 at 12:05

The sun has more gravitational pull then the moon so it pulls the Earth towards it!

In our solar system the object that is formed earlier,is the Sun.Now according to cosmological big bang theory first our sun is formed, after very long time i.e. few billion years, the matter which is remains vast from the sun, is collected gravitationally due to gradual decrease in temperature of the system. In this way our earth is formed having age 4.5 billion years, while on the other hand our moon is the sub plant of the earth, having age much much smaller then the earth.Remember that the formation of moon is also take place just like the earth around the sun.

• The entire post is confusing. The only part confusing part that is understandable is your confusion of millions and billions. – hdhondt Dec 15 '17 at 4:45