The Earth’s axis – the imaginary line around which the Earth turns daily is titled at an angle of  to the plane of its orbit. The seasons are caused by this tilt and by the fact that the Earth’s axis always holds the same orientation in space. As the Earth travels round the Sun, the North Pole is sometimes directed towards the Sun and sometimes directed away from it. When the North Pole is directed towards the Sun, sunray’s strike the northern hemisphere almost directly. This causes summer in the northern hemisphere and the southern hemisphere has winter. Three months later, the North Pole begins to slant away from the Sun. This causes the northern autumn and southern spring. After another three months the North Pole it tilted away from the Sun, this causes winter in the north and summer in the south. Finally the North Pole tilts towards the Sun again, bringing spring to north and autumn to south.

The main reason why some trees shed leaves is to conserve water during the winter months. When root activity and water absorption decrease, deciduous trees drop their leaves. This helps them keep water loss at the minimum. Unlike deciduous trees, which lose their leaves all at the same time, evergreen trees, mostly conifers, shed and replace leaves continuously. Before the leaves fall, triggered by changes in temperature and daylight, groups of the soft cells from around the base of the leaf stalk. These cells produce an enzyme which dissolves the stalk cell walls and causes leaf to detach. Discarding their leaves also helps trees dispose of toxic and waste products which accumulate in the leaves before they fall.

            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.