So let's talk about the earth centered inertial frame or the ECI frame. Now this frame has got its origin at the center of the earth and it has got three mutually perpendicular axis, the x axis, the y axis and the z axis. The z axis points towards the geographical north pole. The x axis it points towards the warmer equinox. And the y axis it completes the right hand coordinate system. So here you notice that the z axis it coincides with the spin axis of the earth. By spin axis of the earth I mean the axis about which the earth spins. This axis passes through the north pole and the south pole of the earth, the geographical north pole and the geographical south pole. Now you'll see that the earth keeps on spinning about its spin axis which coincides with the z axis. However the ECI frame the x, y and z they will not spin. So the ECI frame is perfectly inertial frame. To make it more clear let me give you an example. Suppose this is an x degree longitude. This is the position of x degree longitude at time t. Let's call the time t0. At time t equals to t0 this is the position of x degree longitude. At time equals to t0 plus delta t the position of x degree longitude won't be here. It will be somewhere over here. Suppose this is the position 1 at time t then this will be the position 2 at time t0 plus delta t. Let's call this t. So at time equals to t0 the x degree longitude will be over here. At time equals to t0 plus delta t the x degree longitude will come somewhere over here because of the earth spin. But the x, y and z axis will still be pointing towards the same point. x will always be pointing towards the vernal equinox. z will always be pointing towards the north pole and y will be making the right hand coordinate system. So I have been mentioning this word quite frequently the vernal equinox. So let's see what this vernal equinox is. Suppose this is our earth now this will be our zero degree latitude which is the equator. The plane containing the equator is all the equatorial plane. Now if you observe from the earth then the sun will appear to orbit the earth. So this is the position of the sun at time t0 then at time t0 plus delta t this will be the new position of the sun. So let's see on going like this. It will come down and down and down and down again and then it will stop descending at this point and from here on it will go up up up up up up and over here. It will start descending again. So now you observed that the sun pierces the equatorial plane at two points. The first one is over here the second one somewhere over here. This point where the sun passes through the equatorial plane while going up is called the ascending mode of the sun. And this point where the sun goes down and passes through the equatorial plane is called the descending mode. Now this ascending mode of the sun has got another name and that name is vernal equinox. So I hope I have made this clear and if you guys have any queries just drop us a mail and we will try to get back to you and thank you for watching the video.