# HOW PLANETS REVOLVE AROUND THE SUN | EXPLAINATION | APHELION AND PERIHELION

HOW PLANETS REVOLVE AROUND THE SUN

Have you ever think that how planets revolve around the sun in an elliptical orbit? And how moon and other satellites revolve around the planets?
Millions of people saw the apple fall but only newton asked “why”.
Sir Isaac Newton descovered Gravity when he saw a falling apple. But do you know what is GRAVITY?

Why can’t we fly in the air when we try to jump upwards ? Why did the apple fall from the tree ? And when we throw something, why does it come down and fall?
Answer is GRAVITY.

## Gravity :

Gravity is the force by which a planet or other body attracts objects toward its center.
Every object has its own weight and mass. And if the object has mass then it also has gravity. Object with more mass has more gravity but gravity gets weaker with distance.

``                     W = m x surface gravity ``

Where W = weight of the object
m = mass of the object
Everyone knows their weight on Earth and the surface gravity of earth. So they can easily calculate their mass.
Then you can also calculate your weight on other planets because your mass doesn’t change on other planets but the gravity and size of the planets definitely affects on your weight that’s the reason why astronauts are able to jump so high on the moon.

Due to gravity, heavy objects pull lighter objects towards its center, just like our earth means the lighter object revolves around the havier object.
In the same way , sun has the larger mass in our solar system as compared to planets , satellites and other ovjects ( sun is the haviest object in our solar system ).
So this is the main reason why all the planets and other lighter object revolves around the the sun.

Do you know our sun(or solar system)also revolves around the center of milky way galaxy at the speed of 800,000 km/hr. Because milky way galaxy is a collection of about 100 –400 billion stars , planets , dwarf planets , gases , dust etc. And has mass of about 1.5 trillion solar masses ( one solar mass = mass of the sun ) because lighter object revolves around the havier object due to gravity.

As we discuss above that sun is the haviest object so its gravity attracts other lighter object towards its center. But if sun attracts towards its center then why should not earth(& other planets) fall into the sun?

The answer is Linear Velocity of planets.
So sun will try to pull planets but planets will try to move linearly. Therefore due to this resultant, planets revolve around the sun in an elliptical orbit.
Lets take an example :
Suppose you are revolving a stone tied with a string and if you swing it around then you will constantly pulling it towards your hand just like sun’s gravity pulls the planet in, but the linear velocity of that stone keeps stone swinging around.
Without that motion, it would fall to the center.
Exactly in same way , if earth (& other planets or sattelite which revolve around the sun) does not have linear velocity then they would fall into the sun because sun has the strongest gravity in our solar system.
Mass is directly proportional to gravitational force. If mass is greater then gravitational force will be strongest. And gravitational force will be weak if mass is smaller.

And If we remove sun from the solar system then all the planets will go on a straigt path with their velocity.

• Gravitational pull of sun
• Velocity of planets
These are the reasons how all the planets revolve around the sun.

## Why planets move faster when they are closer to sun?

But do you know that planets move faster when they are closer to the sun?
As we know that all the planets revolve around the sun in an elliptical orbit. So In this whole elliptical path there is one point which is the farthest from our sun which is called aphelion and the point which is closest to our sun which is called perihelion.

Now when the planet cross aphelion , at that time planet moves with the slowest speed and when the planet cross perihelion , at that time planets moves with the fastest speed.

But still the question is WHY? Why is this happen?

For this we have to understand the concept of kepler’s planetary motion.

There are two laws:

• The orbit of a planet is an ellipse with the sun at one of the foci.

•  The line joining the planet and the sun sweep equal areas in equal intervals of time.

At one of the foci we have to consider the position of the sun.

Now let us first see how much distance this planet will cover in 45 days.

When planet cross aphelion, it covers less distance  (A) but when planet cross perihelion, it covers more distance(B).

As we observe in this image

Distance covered at perihelion > distance covered at aphelion

But

Area (A) = Area (B)

l = length

b= breadth

l(A) > b(B)

Because planet move faster when it cross perihelion so distance will be greater at that point.

But why all the planets change their speed when they cross aphelion or perihelion.

Let us consider planet X at aphelion.

As we observed planet in this image, the potential energy will be greater because of the larger distance from the sun.

But when the planet moves towards perihelion, its potential energy will continue to descrease.

Now when planet cross aphelion with more potential energy and less kinetic energy (means speed will also descrease) then the speed of that planet will be slowest.

But when planet cross perihelion with low potential energy and more kinetic energy(means speed will also increase) then the speed of that planet will be faster.

Thank you.