People try to find a pattern in everything they see in nature. The ancient Greeks also did so when they observed the stars. They found that eight stars together looked like a whale, while some other stars seemed to resemble a lion or a bear. They put together these groups of stars as pictures, many of which we can hardly track these days. The reason is that the stars have changed positions in the 3000 years that followed after the Greeks first observed the constellations. However, the following constellations can be seen very clearly in the northern hemisphere: Pisces, Aries, Taurus, Cassiopeia, Perseus, Auriga, Orion, Gemini, Lynx, Cancer, Little Dipper, Draco, Herdsman, Hercules, and Ophiuchus.
Without water, there would be no animals or plants, which carry out photosynthesis and release oxygen, and consequently the oxygen content in the atmosphere would go down. Moreover, the absence of oceans would have serious consequences as they help to keep the Earth’s climate moderate by storing heat inside them. Without large water bodies, the temperatures of the sunlit areas and those away from the sunlight would change drastically. The temperature difference between day and night and between summer and winter would be very large. As a result, hurricanes would be a constant occurrence on the Earth.
Gaseous oxygen was created on the Earth about 3.5 billion years ago. At that time, the UV light of the sun decomposed the water vapour molecules present in the atmosphere, and released oxygen and hydrogen. But a major part of the oxygen immediately reacted with other substances, so that it was no longer available in the atmosphere as gas. Oxygen was also produced by the seas, where the ‘blue algae’ carried out photosynthesis. These bacteria converted sunlight and carbon dioxide into energy. In the process, gaseous oxygen was released as a ‘waste product’, which accumulated in the atmosphere. About one billion years ago, about one-twentieth (4%) of the atmosphere consisted of oxygen.
The Earth, along with the sun and the remaining planets of our solar system, originated around 4.8 billion years ago from dust particles, which agglomerated gradually. At that time, the young Earth was bombarded by countless meteorites from space. The Earth’s interior was molten and volcanoes erupted constantly. But over millions of years, the Earth’s surface cooled down. Around 3.8 billion years ago, the temperature decreased to about -100°C and the Earth’s crust slowly started becoming solid. At this time the gaseous cover on the Earth was mostly made up of water vapour.
At the beginning of the universe, there were only gas and dust particles in space. Since objects with physical mass have a mutual attraction, the dust particles agglomerated gradually to form larger bodies such as stars and planets. These bodies started rotating the moment they collided. After many collisions, the rotations got added up. For this reason, the planets, including the Earth, are in a constant state of motion in their respective orbits.
The North and South Poles have two seasons: summer and winter. Each season lasts for six months. The difference between the two seasons is more a matter of daylight than of temperature. Although both seasons are cold, the temperature is lower in winter than in summer at the poles. In summer, there is 24 hours of sunlight (no nights), whereas in winter there is 24 hours of darkness (no daylight).
The axis of rotation of the Earth is tilted, and this can be seen clearly in a globe. Around June 21, the axis points clearly towards the sun. As a result, a large part of the northern hemisphere is illuminated by a strong sun and daytime remains long till the rotation of the Earth causes this area to move away from the direct glare of sunlight. The place, thus, has long hours of sunlight and the days are long. In fact, the sun shines over the North Pole continuously for several weeks without setting even once. This period is known as the ‘polar day’. During Christmas, the axis of the Earth points away from the sun. During these days, the northern hemisphere remains dark for a longer period of time and a ‘polar night’ sets in at the North Pole. On the other hand, the southern sphere enjoys long hours of sun.
One rotation of the Earth takes one day. If at 12 noon the sun is at the highest point in the sky, then after 24 hours it will come back again to the same position. It was the Babylonians who first divided the day into 24 hours. They could have counted the hours from 1 to 10, then to 20, and so on, but instead they calculated from 1 to 60, then to 120, and so on. For this reason, time is divided into parts of 60 (1 hour has 60 minutes, 1 minute has 60 seconds) or in parts of 6 (one day has 24 hours, i.e., 4 X 6 hours).
Water evaporates when the sun shines on oceans, seas, or wet grounds or when wind blows over them. The water vapour rises up and gets converted again into tiny droplets on reaching the cold heights of the sky.
Accumulation of moisture in the atmosphere causes a thunderstorm. This occurs mostly in summer when the sun is hot and keeps soaking up water from the ground and water bodies. The moist air rises and forms small clouds that pile up gradually to form huge cauliflower-shaped clouds as high as 10 km. Inside the clouds, large water drops and ice crystals form due to extreme cold conditions. These remain afloat, being supported by powerful winds. When they collide and rub against one another, an electrical voltage is produced between the upper and lower layers of clouds. This voltage, once sufficiently high, is discharged in the form of lightning.
