NCERT Class 6 Geography Chapter 3: Motions of the Earth Complete Notes Part 4

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Motions of the Earth

Perihelion Precession

That next precession that explain there was perihelion precession or what we call as orbital precession. Orbital precession takes 71,000 years to complete and this believes you have the orbital of the earth that changes. That is orbit of the earth that is changing and new orbit taking place. And because of this two precession, the perihelion precession and the axial precession. There is a kind of important feature that happened in a solar system.

One of that which is known as Milkanvotich cycle. And this milkanvotich cycle talks about the climatic contrast between the season and glaciation, so mainly we talk about glaciation and change contrast in the season.

Image of Perihelion Precession

Image of Perihelion Precession

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Now this milkanvotich cycle focuses on three steps (1) Eccentricity (shape) (2) Axial tilt that’s the obliquity or the obliquity nature (3) Precession.

When we talk about eccentricity the most important thing is the eccentricity that takes about one hundred thousand years or 1, 00,000 years. Which you have eccentricity that is completed. Then you have, Axial Tilt that takes about 41,000 years and that is average of that is calculated by the orbital precession as well as the axial precession. Finally you have concept of precession. These three together govern milkanvotich cycle which affix the glaciation and contrast of the season that exists. Now this something to do with the perihelion precession and we talked about two types of precession. We will consider milkanvotich cycle in details in further lectures.

Now next important thing is due to the revolution of the earth, you have seasons that come up and you have Solstice and Equinox that happen.

Solstice and Equinox

Image of Solstice and Equinox

Image of Solstice and Equinox

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Now solstice can be divided as summer solstice and winter Solstice. Under summer solstice you have 21st June as a summer so what would happen in a north hemisphere. North hemisphere would have the longest day and short’s night on 21st June. Coming back on the winter solstice you have 22nd December as a winter solstice and what would happen in the north hemisphere would be shortest day and longest night and reverse happens in a south hemisphere for both the solstice.

Now coming to the Equinox you have the Autumnal Equinox and the vernal Equinox. When we talk about 21st March and 23rd September these are the Equinoxes. Now 21st March in the north hemisphere is vernal Equinox as spring Equinox. However, North hemisphere it is 23rd September which becomes the Autumnal Equinox. So you have concept of solstice and equinox explained by this diagram. You have Equinox where you have equal days and night.

The solstice summer or winter you have the maximum difference in the day and night conditions. So you have maximum difference, shortest day and longest night that’s happen during the solstice. Towards the equinox you have equal day and equal night.