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Chapter: 11th Physics : Gravitation

Interesting Astronomical Facts

1. Lunar eclipse and measurement of shadow of Earth 2. Why there are no lunar eclipse and solar eclipse every month? 3. Why do we have seasons on Earth? 4. Star’s apparent motion and spinning of the Earth

Interesting Astronomical Facts

 

1. Lunar eclipse and measurement of shadow of Earth

On January 31, 2018 there was a total lunar eclipse which was observed from various places including Tamil Nadu. It is possible to measure the radius of shadow of the Earth at the point where the Moon crosses. Figure 6.31 illustrates this.


When the Moon is inside the umbra shadow, it appears red in color. As soon as the Moon exits from the umbra shadow, it appears in crescent shape. Figure 6.32 is the photograph taken by digital camera during Moon’s exit from the umbra shadow.


By finding the apparent radii of the Earth’s umbra shadow and the Moon, the ratio of the these radii can be calculated. This is shown in Figures 6.33 and 6.34.


The apparent radius of Earth’s umbra shadow = Rs = 13.2 cm

The apparent radius of the Moon = Rm= 5.15 cm

The ratio Rs/Rm ≈ 2.56

The radius of the Earth’s umbra shadow is Rs = 2.56 x Rm

The radius of Moon Rm = 1737 km


The radius of the Earth’s umbra shadow is Rs = 2.56x1737km ~≡4446 km.

The correct radius is 4610 km.

The percentage of error in the calculation = [ (4610 – 4446) / 4610 ] x 100 = 3.5% . 

The error will reduce if the pictures taken using a high quality telescope are used. It is to be noted that this calculation is done using very simple mathematics.

 Early astronomers proved that Earth is spherical in shape by looking at the shape of the shadow cast by Earth on the Moon during lunar eclipse.


2. Why there are no lunar eclipse and solar eclipse every month? 

If the orbits of the Moon and Earth lie on the same plane, during full Moon of every month, we can observe lunar eclipse.


If this is so during new Moon we can observe solar eclipse. But Moon’s orbit is tilted 5° with respect to Earth’s orbit. Due to this 5° tilt, only during certain periods of the year, the Sun, Earth and Moon align in straight line leading to either lunar eclipse or solar eclipse depending on the alignment. This is shown in Figure 6.35

 

3.  Why do we have seasons on Earth?

The  common  misconception  is  that ‘Earth revolves around the Sun, so when the Earth is very far away, it is winter and when the Earth is nearer, it is summer’.

Actually, the seasons in the Earth arise due to the rotation of Earth around the Sun with 23.5° tilt. This is shown in Figure 6.36


Due  to  this  23.5°  tilt,  when  the northern part of Earth is farther to the Sun, the southern part is nearer to the Sun. So when it is summer in the northern  hemisphere,  the  southern hemisphere experience winter.

 

4.  Star’s apparent motion and spinning of the Earth

The Earth’s spinning motion can be proved by observing star’s position over a night. Due to Earth’s spinning motion, the stars in sky appear to move in circular motion about the pole star as shown in Figure 6.37



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