Glossary

You may know that the Earth rotates around its axis. The axis is an imaginary line around which every point on the Earth, including air molecules in the atmosphere move. Let us understand this with the help of an activity.

(Note: This is an outdoor activity. Please conduct the activity preferably in the morning or late in the afternoon so that you don’t have to stand under the hot Sun.)

Material: Globe, magnetic compass, protractor, atta dough and matchsticks

Procedure:

1. Attach a globe to a mike stand.

2. Identify north using the Pole Star (If you are using the Pole Star, you will have to do it the previous night or with the help of a mobile app called ‘SkyView’) or a magnetic compass and align the axis of the globe in the north-south direction¹.

3. At any location on the Earth, the Pole Star is seen at an angle equal to its latitude. Incline the globe by an angle that is equal to the latitude of your location (See Figure 1). (Latitudes: Hyderabad: 17^°, Mumbai: 19^°, Jaipur: 27^°)

4. Place the geosynchron under sunlight and rotate it such that India is at the top.

5. Now take a matchstick. Consider that it represents a human being. The match head represents the head of the human being. Put the matchstick on the location closest to your location on the globe using some dough. Remember that the matchstick should be vertical with respect to the globe, not with respect to us (stick it radially).

6. Now observe the shadow of the matchstick on the surface of the globe and your shadows or shadows of nearby objects such as a pole, pillar or tree on the surface of the Earth. Are they parallel?

         

Figure 1: Geosynchron

Questions for Discussion:

1. The shadow of the matchstick on the globe and the shadows of poles or other vertical objects nearby are parallel. This means that the solar times at your actual location is same as that of your location on the geosynchron.

2. Which portion of the globe is lit up by sunlight? It is daytime in the places located in this portion. Can you name some countries where it is daytime? Can you name one country where it is noon now (where the Sun is directly above the country)? The Sun will appear to be overhead only from one point but it will be noon everywhere on the longitude passing through that point².

3. Now identify the part of the globe not lit by sunlight (i.e., which is dark). It is night in the places located in this part. You can see this part on the geosynchron because there is scattered light around. In the actual Earth, there is no scattered light. The only light the Earth might get is reflected sunlight from the Moon and other planets and starlight.  

4. When seen from above the North Pole, the Earth rotates in anticlockwise direction³. To remember this you can use Right hand Thumb rule. Right Hand Thumb Rule: When you align the thumb of your right hand with the axis pointing towards the North Pole, then the direction of the curled fingers of your right hand is the same as the direction of the rotation of the Earth.

5. Slowly rotate the globe from west to east and see how the countries that are located in the east and where it is daytime gradually move into night and the countries located in the west move into daylight and it becomes daytime for them. Complete one rotation to indicate the passage of one complete day.

^[1] The geographic North Pole and magnetic north of the Earth is not same. If you are standing on the geographic North Pole holding your compass, it would point towards northern Canada at Ellesmere Island. This is a difference of about 500 kilometres between the geographic north and the magnetic North Pole! Therefore it is best to identify north using the Pole Star. http://gisgeography.com/magnetic-north-vs-geographic-true-pole/                                                                         
^[2] The Sun does not come overhead everyday but only twice a year in the tropical region and never comes overhead in the  subtropical and polar regions.
^[3] Keep the globe moving in the same manner and you will notice that the Earth moves in clockwise direction if seen from above the South Pole.


This activity is adapted from  
Monteiro, V., Mahashabde, G., Barbhai, P. (2008). Sun-Earth Experiments: Activity Cards for Day Time Astronomy. Navnirmiti.