Understand precession with the help of simple model.

Earth has 4 kinds of movements.

Earth revolves around the SUN.

Earth also spins around its axis as it revolves. It spins around its axis in approximately 24 hours which creates day and night.

Remaining two movements are related to the Earth's axis.

Wobbling and change in tilt angle of the axis.

Just like the top, Earth’s axis wobbles.

Let us understand the third movement with the help of a model.

This is the Northern Hemisphere and this is the Southern Hemisphere of Earth

Imaginary line dividing these two hemispheres is known as the equator.

This is the axis of the Earth around which it rotates.

Let us place this disc between two hemispheres. It represents Earth's equatorial plane.

We will mount this assembly on this platform.

I can rotate it by hand like this.

This is the North Pole and if we extend it further, the tip of this axle can be considered as the Celestial North Pole.

Somewhere here will be the celestial South Pole.

Extension of the Earth's equatorial plane defines the celestial equator.

Let us insert the axle in this hole so that the axis is now tilted by an angle of 23 and half .

But with respect to which axis it is tilted ?

Let us add another disc to the setup.

This disc represents the ecliptic plane.

On this plane Earth goes around the SUN in one year.

In our model , with respect to Earth, SUN will be on this plane.

Let us also identify four cardinal points on the equatorial plane.

There are two points where the celestial equator and ecliptic intersect. On these days day and nights are of equal length. These are points where Equinoxes happen. Other two points are the Summer solstice and winter solstice.

On the summer solstice, Earth's maximum axial tilt toward the Sun is 23.44. We can mark this point here. Point exactly opposite the summer solstice is the winter solstice.

There are two points where the Celestial Equatorial plane and ecliptic plane interact. This is the point of the vernal equinox. 180 degrees from here is Autumn equinox.

We can better represent it with this dial. Let's place it on the equatorial plane.

Four cardinal points along with the Gregorian calendar are marked on the dial. This will help in our analysis later.

Rotating the axle holder like this enables us to explore the third movement. Wobbling of Earth’s axis. Earth’s axis is rotating about another axis. Axis of an ecliptic plane. This is known as the Precession of Earth’s axis.. It happens due to gravitational forces of the Sun and Moon on the Earth.

Earth goes through one such complete precessional cycle in a period of approximately 26,000 years or 1° every 72 years . The amount of precessional motion is 50.37" per year. But this
amount is not fixed and this value changes between 44.5" and 56"

.

Let us add this dial on the ecliptic plane. It represents almost steady positions of all the stars and constellations. There are 12 constellations and 27 Nakshatra we are familiar with.

Although the amount of motion is very small in a year, with the motion being unidirectional, the positions of the equinoxes change sufficiently over a long period of time. This positional shift of the equinoctial points is known as Precession of the Equinoxes.

Now the Vernal equinox happens near Aries and in approximately 1000 years in another zodiac.

There is another important change which happens due to this movement.

Let us place this transparent disc on the top.

Let us rotate the assembly slowly to visualize how orientation of the Celestial North Pole changes over time.

Small red bead represents the Ecliptic North Pole. The Celestial North Pole also rotates due to precession over the course of the year.

Precession changes the pole star as seen from Earth. [ Brightest star somewhere near the Celestial North Pole ]

Right now Polaris is the pole star. After around 8000 years, Deneb will be the pole star. In another 4000 years, Vega will be the new Pole Star. Another 5000 years later, Thuban will be the pole star.

Forth Motion is the obliquity of Earth’s axis. It moves between 22.5 degrees and 24 degrees.

The angle Earth’s axis of rotation is tilted as it travels around the Sun is known as obliquity. Obliquity is why Earth has seasons. Over the last million years, it has varied between 22.1 and 24.5 degrees with respect to Earth’s orbital plane.

In the next video, we will explore the connection between precession and two types of calendars, Tropical and sidereal.

Thank You.


#astronomy
#precession
#diy
#model