What is the difference between mean local time and Indian Standard Time (IST)?
The actual time (mean local time) at any place varies depending on its longitude. However, for convenience one time (Indian Standard Time) has been adopted for the whole of India. Mean local time can be converted to IST as follows:
(Typical variation of IST from mean local time is depicted in minutes)
Port Blair -41
Guwahti -37
Patna -11
Lucknow +6
Delhi +21
Srinagar +31
Panaji +35
What is a civil day?
A civil day is the day of 24 hours used routinely on Earth for setting clocks i.e., 24 hours divided into two portions- day time and night time- depending on whether the Sun Shines in the sky or not.
Does the difference between the civil and astronomical days have any significance in astronomical 
observations?
Since the civil day is 3 minutes and 56 seconds longer than the astronomical day, any star in the sky will rise 3 minutes and 56 seconds earlier every day compared to the previous day.
What is the time difference between a lunar and a solar year?
A solar year has 365 days. A lunar year has 354 days, and is based on twelve complete moon phase cycles or lunation.
Are all calendars based on the solar year?
No. Some calendars (such as those of the Mohammed am era) are based on the lunar year.
What is an astronomical year?
An astronomical year (also known as sidereal year) for a planet is the time taken by the planet to make a complete revolution around the Sun. In the case of the Earth, the sidereal year is equal to 365.2564 mean solar days or 365 days 6 hours 9 minutes and 9.5 seconds.
What are the regions or zones of the Earth?
The regions north of the Arctic Circle and south of the Antarctic Circle are called the Polar Regions or frigid zones.
The region between the Tropic of Cancer and the Tropic of Capricorn is called the Torrid Zone. The regions between the Tropic of Cancer and the Arctic Circle and between the Tropic Capricorn and the Antarctic Circle are called the north and south temperate zones respectively.
What are the polar ice caps?
Extremely cold temperatures of the Polar Regions cause ice formation on land and sea, resulting in a permanent blanket of ice around the poles called the polar ice caps.
Can all spherical celestial bodies be imagined to have their own equator, poles, latitudes and longitudes?
Yes, all spherical celestial bodies are imagined to have their own equator, poles, latitudes and longitudes.
What are the tropics?
The latitude 23.50 North of the Earth’s equator is called the tropic of cancer.
The tropic of Capricorn is the latitude 23.50 south of the Earth’s equator. There are limits of the area of earth’s surface in which the Sun can be directly overhead.
The peak activity was, however, visible only in the north Pacific region.
What are latitudes?
Latitudes are imaginary lines between the equator and the poles which run parallel to the equator and divide the surface of the Earth into parallel segments. Latitudes are also called “Parallels” or “Parallels of Latitude” e.g. 20 degrees latitude.
What are longitudes?
Longitudes are imaginary-lines through the geographic poles which run circumferentially and intersect the surface of the Earth dividing it into segments e.g. 20 degree w longitudes. (For example, if the Earth is imagined as an orange, the longitudes would divide it into its natural “slices”). Longitudes are also called “Meridians” or “Meridians of longitude”. The zero degree meridians is also called the “Greenwich” meridian since it passes through the town of Greenwich in England.
What is the speed of rotation of the Earth?
The Earth spins faster during September than during March in any year.
What are poles?
All the bodies which rotate on their axes, such as the Earth, Sun, Moon, planets, etc. are said to have two poles, called North Pole and South Pole for connivance. They are the points at which the axis of rotation intersects the surface of the body. Hence, the Earth has a north pole and a south pole, and the line passing through the centre connecting the poles is the axis of rotation of the Earth.
How are the poles identified with respect to surface features on the Earth?
The poles are imaginary points and are never identified by any natural features on the Earth’s surface.
What is the shape of the Earth?
The Earth is not a mere sphere but an oblate spheroid i.e., a sphere flattened slightly at the top and bottom (poles) and bulging at the central region. There is a little more flattening at the bottom than at the trop.
Why is the Earth an oblate spheroid rather than a sphere?
The Earth’s spinning motion causes the central region to bulge leading to flattened top and bottom.
Why is the study of the Earth significant for astronomers?
The Earth, the third planet from the Sun, is only planet that can be easily studied. A study of the Earth (as a planet) helps astronomers to understand the other planets better.
Earth is perhaps the only planet in the solar system where life exists. This is because the conditions favourable for the existence of life are available only on the Earth. Conditions present in other planets make life almost impossible.Mercury is the planet closest to the Sun. It is difficult to see it even with a powerful telescope. It does not have any atmosphere. The temperature during the daytime may even go beyond 400°C. The lack of oxygen and the extreme temperature make life on Mercury impossible.
Venus, which comes next in terms of closeness to the Sun, is often described as the sister planet of the Earth. Venus and the Earth are almost identical in size, mass and density. Its diameter is 0.95 times and mass is 0.815 times as compared to the mass and diameter of the Earth. It is surrounded by thick clouds of carbon dioxide (95%). The temperature on its surface is about 95 times more (480°C) than that on the Earth. This makes Venus the hottest planet in the solar system. Under these conditions life is not possible here.
Mars comes after the Earth and is much colder than the Earth. Its average temperature is about – 62°C. At night it may drop to – 101°C. It has an atmosphere much thinner than that of the Earth. It has been found that it contains 1 to 2% argon, 2 to 3% nitrogen, 95% carbon dioxide and 0.3% oxygen. These conditions suggest the possibility of existence of life on it but so far no traces have been detected.
All other planets beyond Mars (Jupiter, Saturn, Uranus, Neptune and Pluto) are farther away from the Sun. The surface temperature of these planets is so low that no living organism can survive there. Besides, their atmospheres contain gases like methane and ammonia which are not favourable to the evolution of living organisms.
Thus our Earth is the only known planet in the solar system where life exists.
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.
Unlike the other planets of our solar system, life developed into many forms on the Earth after its emergence. Liquid water and oxygen for breathing made this possible. Millions of species of plants and animals thrive today on land and in water. Water covers more than two-thirds of the Earth’s surface. Therefore, the Earth appears blue from the moon or from the International Space Station. For this reason, it is also called the ‘blue planet’. Till now, scientists have not discovered life in any other place in the universe, but they believe that one day they will do so.
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.