My Science School

            Sea water is salty because it contains many dissolved salts, mostly sodium chloride with small proportions of potassium, magnesium, calcium and carbonates. The salt comes mainly with river water that flows through rocks eroded by frost and rain. The gradual wearing a way of mountains releases chemicals which are carried down by rivers to the ocean as dissolved salts. Some salt also enters sea water from the rocks beneath the sea bed. The river water carrying the salts to the sea does not taste salty because the salts are present in extremely small concentration. Whereas in the case of the oceans, water is continuously evaporating even as more and more salt continues to be added with river water. This, over millions of years has led to the high concentration of dissolved salts in sea water.

            During a total solar eclipse the Moon moves across the bright disc of the Sun from west to east. So, as the eclipse progresses, the Moon’s shadow too moves in the same direction. The Earth too spins from west to east. But the speed of movement of the Moon’s shadow is much greater than the speed at which the surface of the Earth moves from west to east. As a result, the Moon’s shadow still moves from west to east along the path of totality.

                                    

  All the planets of our solar system were formed out of the same cloud of gas and dust that gave rise to the Sun, some five billion years ago. As random collisions and gravitational forces compacted the gas and dust particles into the various planets, the heat generated brought the masses into a molten state. Since surface tension of a liquid tends to make the surface area minimum and the sphere has the minimum surface area for a given volume, all the planets in the molten state became spherical and have retained that shape as they cooled.

 The phase of the Moon as seen from the Earth arises because of the constantly changing position of the Moon relative to the Sun and the Earth. As the Moon goes round the Earth, one half of its surface that faces the Sun is always illuminated. But from Earth, the entire illuminated side is visible only when it faces us, that is, only at full Moon. At other times, depending on the position of the Moon in the sky relative to the Sun, only a part of the illuminated face becomes visible, which grows and shrinks in size, giving rise to the familiar phases of the Moon.

            Stars are giant glowing balls of fire situated far away from us. Many of them are several times larger than our sun, but being very far away, appear as points of light. Stars themselves don’t twinkle but appear as steady points of light when seen from space where there is no atmosphere. The twinkling of stars as seen from ground is caused by the unsteadiness of the Earth’s atmosphere. When light from an extended source such as the Sun, Moon or the planets pass through the atmosphere, it does not appear to twinkle because the slight changes in apparent brightness in one part is usually compensated by light coming from another part of the celestial object. But when light from a point source like a distant star passes through the atmosphere the fluctuations in the light intensity is not                                                                                       compensated and the star appears to twinkle.

            According to the plate tectonics theory, the Earth’s surface consists of huge, rocky plates that move slowly past one another. Known as tectonic plates, some of these carry continents, others the ocean floors and some a combination of the two. Most earthquakes take place along the boundary between two plates. Propelled by heat-driven currents in the softer rock beneath them, the plates constantly pull apart, collide, or grind against each other. When the strain becomes too great, the pent-up energy ruptures the rocks and creates a fracture known as a fault. This sudden release of energy unleashes the ground-shaking vibrations that constitute an earthquake.

  The sky as seen from the Earth appears blue because of the scattering of sunlight by the Earth’s atmosphere. When sunlight reaches the Earth’s atmosphere, it hits suspended molecules of air and dust and gets scattered in all directions. Sunlight consists of light waves of varying wavelengths each of which is seen as a different colour. The size of the scattering molecules decides which colour is scattered most. Gas molecules in the atmosphere scatter blue colour the most. So during the day the sky appears blue because more blue light is deflected towards the earth. However, on the Moon, since there is no atmosphere, scattering of sunlight is negligible. So the sky appears black from the Moon.

  The tides are caused by the gravitational pull of the Moon on Earth’s oceans. In fact, the Moon’s gravitational pull attracts everything, even the land and mountains, though being rigid and fixed they do not move. But not so with the oceans. Being an unbroken stretch of water, spread over almost 70% of the Earth’s surface, the Moon’s pull can easily draw water away from one part and pile it up at another which show up as tides. However, this is not possible in a small body of water such as a lake. Here the total surface area and volume of water is too small to produce any perceptible change in the level as happens in tides.

                                    

       

    Fog is a collection of tiny water droplets that float in the air near the ground. When there is a sudden fall in temperature, moisture present in the air condenses and begins to change into tiny droplets of water. The air holds these droplets as fog as long as the temperature remains low. But when the air temperature rises, the tiny droplets evaporate and the fog disappears.

                        

 

            Climate is the average weather condition of a place recorded over a long period. It depends mainly on the geographical location, but factors such as distance from the equator, proximity to the sea, altitude and topography influence climate of a given place. Places located near the equator are generally hotter than those located at higher latitudes. Places located at higher altitudes are usually cooler than the plains. Coastal areas generally have a more humid climate than places far inland.

            The waves are higher near beaches than in interior parts of the sea mainly because of the shallowness of the sea near beaches. The shape of the coast and surface features of the sea bottom near the coast also decide the height of the waves. In deep seas, the water in a wave moves in large vertical circles, without actually moving forward. Near the beaches such circulation is modified by the closeness of the bottom. This leads to increase in the height of the waves. Speed and direction of the wind, and the distance over which the wind is blowing are other factors that can further affect the height of waves. A                                                                                         strong wind can create huge waves even in mid sea.

            Rivers usually meander when they flow over the plains. On the plains, the gradient is usually low and river has no preferred direction of flow. Its course is usually determined by local irregularities in the terrain and soil condition. In such a situation when the river bends in one direction, it tends to scoop out its outer bank and deposit silt on the inner bend. After scouring one bank the water is deflected towards the opposite bank which now starts getting eroded. The process continues with the river changing direction over short distances, giving rise to the familiar meanders.

            Clouds are large masses of fine water droplets or ice particles formed by condensing water vapour. In spite of their great masses, clouds float in the sky because they are held up by the up drift caused by hot air rising from the ground below. Within the clouds themselves strong currents of upward rising air keep the droplets or ice crystals in constant motion which not only helps them keep afloat but also leads to the rapid changes in shapes of clouds.

            Ice skating is an act of gliding over the smooth surface of hard ice on ice skates – boots with attached metal blades. It is easy t skate on hard and smooth surface of ice because of the absence of frictional force between ice and the skating blade. Another reason is that ice melts under the pressure of the sharp blades of the skate and water thus formed acts as a lubricant which helps in skating. It is not possible to skate on snow or sand because the surface is not hard.

            The peculiar smell is produced by the soil inhabiting bacteria called streptomycetes. Streptomycetes are abundant in dry warm soil – a million of them are present in a pinch of soil. They release compounds such as geosmin and 2-methyl isoborneol, which vapourize easily, when wetted by rain water after a dry spell. That is why we get the musty odour only after the first rain of the rainy season. The smell can be detected in a newly ploughed field also.