Glossary

If we go to the sea shore or to a field where there are no big buildings nearby, we see a part of the Earth like a disc and the dome of the sky above it. There is an imaginary circular line around us which separates the land from the sky, and this we call the horizon. The horizon is used as a reference line for most of the observations of the sky from the Earth. For example, when the Sun or any celestial object comes above the horizon, we say it 'rose'. Astronomers also specify the angle from the horizon at which a star or any celestial object is visible along with the direction (north/ south/ east/ west) in which it is seen. For example, one might say that tonight at 8 pm, the Moon is visible in the East at 30 degrees above the horizon. Zenith, the point exactly overhead makes an angle of 90 degrees from the horizon from all sides.

Let us see how can we decide where the horizon is for a person standing on the Earth.

Suppose a person is standing on the spherical Earth. How far would he see? He would see the part of the Earth encompassed by the tangents drawn from his eyes to the Earth (A). We just saw that the Earth is very big as compared to the height of a human being; so let us decrease the height of the person (B); now how far can the person see? What happens to the angle between the tangents as we increase the radius of the Earth? The angle between the lines increased.

Figure 3: Horizon

The radius of the Earth is 64,00,000 meters and the height of a tall person is about 2 meters. So aren't we like a dot on the Earth (C)? What will be the angle between the lines of sight in that case? Can we consider it 180°? That is just a tangent to the Earth at the point the person is standing.


You must have been to the sea shore or on a big flat field. If you look around you, you feel like you are standing on a slightly bulged disc. The edge of this disc is almost circular with some trees and other objects to make it a little uneven. This circular rim of this disc, or the line up to which we can see land or things on the land is called the horizon. So henceforth, we will refer to the tangent in the diagram as the 'line of horizon'.

Now consider a sailor travelling in his ship (Figure 4). See how his horizon changes and how the positions of the stars in the sky change as he travels. Which stars will be visible to the sailor from each position? Also, which among them will be closer to the horizon and which ones will be closer to zenith?
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Figure 4: Different parts of the sky is visible from different locations on the Earth

People at different locations on the Earth see different parts of sky. That is why, at one location, people might see that the Sun is just rising, at another location it is noon, from yet another location people will see the Sun setting and from yet another position the Sun is not at all visible in the sky and, instead, the stars will be visible in the dark sky. At any given time, the Sun is visible (above the horizon) from half the Earth, and it is day for that part of the Earth. From the remaining half  of the Earth, the Sun is not visible, and it is 'night' for that part of the Earth. Moreover, in Figure 5, although it is noon for persons A, B and C,  the Sun is directly overhead (at the zenith) only for person A. For person B, it is to the south of the zenith, and for person C, it is to the north of the zenith. Thus, the Sun doesn’t come overhead at noon for everyone. Check whether the Sun comes overhead at your location tomorrow.

 

Figure 5: The Sun is not at the Zenith at noon from every location on the Earth

You might have noticed that in Figure 5, the sunrays are shown parallel. This is an important approximation we make. The Sun is spherical in shape; its rays goes in all directions. But the Sun is very far away from the Earth (about 150 million km). So the angle between any two rays falling on the Earth is very small, practically zero. Which is why we consider sunrays falling on the Earth to be parallel.

In this lesson, you learned that the Earth is spherical in shape and we live all over its surface. You also learned about the horizon, a concept which we will use in many of the remaining lessons. Another important approximation we learned about is sunrays falling on the Earth can be considered parallel. In the next three lessons, we will discuss rotation and revolution of the Earth and their consequences in some detail. In Unit 2: Lessons 1 to 4, we will learn about the Moon: how it moves and, as a result, what we see. Unit 3: Lessons 1 to 4 are about our Solar System and the universe beyond it. Be prepared for a journey through the space!