NCERT Class 6 Geography Chapter 3: Motions of the Earth Complete Notes Part 2

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Motions of the Earth

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Orbital Plane

  • Plane formed by the orbit

  • Axis of earth makes 66 ½ degree with orbital plane

Image of Orbital Plane

Image of Orbital Plane

Now understanding what is an orbital plane. The plane which is from by the orbital of the earth is known as an orbital plane.

Now the axis of the earth makes an angle of degree from the orbital plane. So you have orbital plane and you have axis of the earth and angle tilted at degrees. This axis which is the axis of the earth makes degree with the orbital plane. So that is way we understand the orbital plane.

Now we have to understand what is orbital plane and the next important concept is circle of Illumination.

Circle of Illumination – Does Not Coincide with Axis

Image of Circle of Illumination

Image of Circle of Illumination

When we talk about circle of Illumination considered this to be the region of sunrays falling. Now what would happen if sunrays are falling, that region will have light or day. Now this half region would have day and the other half region would have night and this line which divided earth in two that is day and night would be considered as the circle of illumination.

So this is the three dimension picture of the earth. If we cut it round the center. I would have two hemisphere, that hemisphere would circular boundary and that would be circle of Illumination. What belong to this side will be day and other side would be a dark side or it would be the night.

Now when this earth is taking a revolution it’s going on to other side of the sun what would happen is slowly and gradually it is having rotation and revolution taking place together. So this position which was day on the side when on the west it turn into night. So that is how we understand the circle of Illumination. Now it’s important this circle of Illumination does not coincide with the axis of the earth.

As a result you have North Pole which is here and the North Pole you would have 6 month of day and 6 months of night. However you would South Pole here. You have 6 month of day and 6 month of night. And this region when it is Illumination would be 6 months day here and 6 months of night and also if you have considered along the equator you would have kind of equal distribution of sunlight and the inclination would change as you move towards the poles.

That is how we try to explain circle of Illumination. But there is one important concept that you need to understand while working about circle of Illumination. That would be what happened if earth does not rotate? If I’m saying earth is not rotating on axis. I said sun is here and you have earth which move here to here and then I explain the day and night. But I can explain that along point is here because the rotation causes the day and night what would happen. You would see sun here and this would be day the similar side would be day and another side would be a night. Now what would happen when rotation takes place. You would see this side after 12 hours, it would be day and other side would be the night and this would continue to happen as when it is rotating. That is taking every day for 365 days. So every day you would have phenomena of day and night however there would be change of season only when you have revolution that is occuring.

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