Question

Chapter 17 stars and the solar system all notes from S. Chand for class 8

Answer 1

•   Celestial Objects: Objects, such as the stars, the planets, the moon and many other objects, in the sky are called celestial objects.

•   Phases of the Moon: The various shapes of the bright part of the moon as seen during a month are called phases of moon.

        The moon revolves round the earth. It also revolves round the sun along with the earth. The various stages of the moon during a month are shown in Fig. 17.1.

        In position 1, the part of the moon facing earth does not receive light from the sun.

        Hence it appears dark. The other part of the moon that is facing away from earth is lit by the sun light. This position of moon, when it appears dark on earth is known as new moon.

        In position 2, 3 and 4, we see the moon not as a full disc but as a cresent. Position 2 of the moon is known as cresent moon, Position 3 as first quarter and position 4 as Gibbous (bright part is greater than a semicircle) moon.

        In position 5, the Sun's rays fall directly on the part of the moon facing earth. In this position we see the moon as a full disc of light. This position of moon is known as full moon.



        Between the positions 1 and 5, the bright portion of moon increases. It is called waxing phase of the moon. We have "Amavasya" at position 1, and "Purnima" at position 5. During positions 6, 7, 8 and 1, the moon vanes, that is the bright portion of moon becomes smaller and smaller.

•   The time period between one full moon to the next full moon is slightly longer than 29 days. In many calenders this period is called a month.

•   How we see the moon: We see only that part of the moon from which the light of the sun is reflected towards us.



Fig. 17.2 Moon is visible due to reflected sunlight

•   The moon completes one rotation on its axis as it completes one revolution around the earth.

•   The moon has dusty and barren surface with craters, steep and high mountains. The moon has no atmosphere and no water.

•   On July 21, 1969 (Indian time), Neil Armstrong landed on the moon for the first time. He was followed by Edwin Aldrin.

•   Stars emit light of their own. Sun is also a star.

•   The stars appear to move from east to west because of the rotation of the earth. We know that our earth rotates about its axis from west to east. Therefore, the stars appear to move in the direction, opposite to rotation of earth, i.e. from east to west.

•   Light year: Distances between the stars and earth are so big that it is difficult to measure these distances in kilometers. For this we need a bigger unit. This unit is light year. A light year is the distance travelled by light in one year. The speed of light is 300,000 km/s. If we multiply this speed of light by the number of seconds in one year, we get the distance of about 9,460,800,000,000 kilometres. This distance is one light year.

        The distance between the sun and the earth is about 150,000,000 (15 crore) kilometres. Light takes about 8.3 minutes to reach earth from the sun.

•   Constellation: The stars forming a group that has a recognisable shape of animals, human beings or other objects is called a constellation.

        These constellations appear to move from east to west. The shape of all constellations seen in the sky always remain the same.

        Each constellation has been given the name of an animal or object with which it resembles.