Geography — World, India and Andhra Pradesh for Group-I · Chapter 1

Earth in the Solar System

What to remember

Earth has two motions. Rotation (about 24 hours, on its axis) gives day and night; revolution (about 365¼ days, around the Sun) with a tilted axis gives the seasons.
Latitude and longitude form the grid. Latitudes measure distance north or south of the Equator; longitudes measure distance east or west of the Prime Meridian. Every 15° of longitude equals 1 hour of time.
India uses one standard time. IST is based on 82°30′E and is 5 hours 30 minutes ahead of GMT.

The Solar System and Earth

The Solar System has the Sun at its centre with eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are rocky (terrestrial) planets; the next four are gas and ice giants. Between Mars and Jupiter lies the asteroid belt. Pluto is a dwarf planet (since 2006).

Earth is the third planet from the Sun. Its shape is an oblate spheroid (geoid): slightly flattened at the poles and bulging at the Equator. Equatorial diameter is a little greater than the polar diameter. Earth is the only known planet with life, because of liquid water, a suitable temperature and an atmosphere. It is sometimes called the "blue planet".

Light from the Sun reaches Earth in about eight minutes. The Moon is Earth's only natural satellite. The Earth moves from west to east.

Rotation

Rotation is the spinning of Earth on its axis, from west to east, in about 24 hours (23 h 56 min with respect to the stars). Effects:

Day and night
Rising and setting of the Sun in the east and west
Difference of time between places on different longitudes
Deflection of winds and ocean currents (Coriolis effect): to the right in the Northern Hemisphere, to the left in the Southern Hemisphere
Daily rise and fall of tides, together with the Moon's pull

The circle of illumination divides the lighted half from the dark half; it is the line along which dawn and dusk occur.

Revolution and seasons

Revolution is Earth's orbit around the Sun in an elliptical path, taking about 365¼ days. The extra quarter day makes a leap year of 366 days every four years (with exceptions for century years not divisible by 400).

The axis is tilted at 23½° from the vertical, or 66½° to the orbital plane, and always points to the same direction (towards the Pole Star). Because of this tilt and revolution, the Sun's vertical rays move between the Tropic of Cancer and the Tropic of Capricorn, giving the seasons.

Date (approx.)	Event	Vertical rays on	Note
21 March	Spring (vernal) equinox	Equator	Equal day and night
21 June	Summer solstice (north)	Tropic of Cancer (23½°N)	Longest day in the north
23 September	Autumn equinox	Equator	Equal day and night
22 December	Winter solstice (north)	Tropic of Capricorn (23½°S)	Shortest day in the north

Perihelion is when Earth is nearest the Sun (early January, about 147 million km). Aphelion is when it is farthest (early July, about 152 million km). Seasons are caused by the tilt, not by distance. In the Southern Hemisphere, seasons are reversed.

Beyond the Arctic and Antarctic Circles (66½° N and S) there is a period of continuous daylight in summer and continuous darkness in winter. At the poles, day and night last about six months each ("midnight sun").

Latitude and longitude

Latitude is the angular distance of a place north or south of the Equator (0°). Lines of latitude are parallels; they run east-west and are all parallel. The poles are at 90°. Important parallels: Equator (0°), Tropic of Cancer (23½°N), Tropic of Capricorn (23½°S), Arctic Circle (66½°N), Antarctic Circle (66½°S).

The Torrid Zone lies between the tropics; the Temperate Zones between the tropic and the polar circle; the Frigid Zones beyond the polar circles. The Tropic of Cancer passes through eight Indian states, including Gujarat and West Bengal. Andhra Pradesh lies south of it, wholly in the Torrid Zone.

Longitude is the angular distance of a place east or west of the Prime Meridian (0°) passing through Greenwich, London. Meridians are half circles running from pole to pole and are not parallel; they meet at the poles. The 180° meridian is the basis of the International Date Line, which has bends to avoid land masses.

Time

Earth rotates 360° in 24 hours, so it turns 15° in one hour and 1° in four minutes. Sun-time is local time; for a place east of Greenwich the time is ahead, for a place west it is behind.

Standard time applies one reference meridian to a whole country. India's standard meridian is 82°30′E, passing near Mirzapur in Uttar Pradesh; it is 5½ hours ahead of GMT (82.5 ÷ 15 = 5.5). Many large countries use more than one time zone.

Worked examples:

1. If it is 12 noon at Greenwich, what is the time at 75°E? 75 ÷ 15 = 5 hours ahead; time is 5 p.m.
2. If it is 10 a.m. at 30°W, time at Greenwich? 30 ÷ 15 = 2 hours behind GMT, so GMT is 12 noon.
3. IST when it is 6 a.m. GMT: 6:00 + 5:30 = 11:30 a.m.

Crossing the International Date Line from west to east, a calendar day is repeated (subtract one day); from east to west, a day is skipped (add one day).

Eclipses and phases (brief): A solar eclipse occurs at new moon when the Moon is between the Sun and Earth. A lunar eclipse occurs at full moon when Earth is between the Sun and Moon.

The Moon, tides and phases

The Moon revolves around Earth in about 27⅓ days and rotates in the same time, so the same face always points to Earth. A full cycle of phases (new moon to new moon) takes about 29½ days. From new moon to full moon the lighted part grows (waxing); from full moon to new moon it shrinks (waning).

Tides are the rise and fall of the sea, caused mainly by the Moon's gravity and, to a smaller degree, the Sun's. Spring tides (very high and very low) occur at new moon and full moon, when the Sun, Moon and Earth are in a line. Neap tides (smaller range) occur at the quarter phases, when the Sun and Moon pull at right angles. Most coasts have two high tides a day.

Other features of Earth's grid and zones

Great circle: any circle that divides the Earth into two equal halves. The Equator is a great circle; the shortest route between two places lies on a great circle.
Heat zones are based on the angle of the Sun's rays. Rays are vertical in the Torrid Zone, slanting in the Temperate Zones and very oblique in the Frigid Zones.
Length of day: at the Equator day and night are always almost equal. The farther from the Equator, the greater the variation between seasons.
Latitude of a place can be found from the height of the Pole Star above the horizon in the Northern Hemisphere.
Longitude and the Date Line: The date changes at the 180° meridian, so a traveller flying west to east across it repeats a calendar day.
Time zones: Greenwich Mean Time is the basis for world time. Some large countries (such as the USA and Russia) use several zones; India uses one, although the east and west of the country differ by about two hours in sunrise time.

Exam traps

1. Rotation vs revolution: rotation gives day and night; revolution gives the year and, with the tilt, the seasons.
2. Perihelion (January) vs aphelion (July): Earth is closest to the Sun in the northern winter.
3. Latitudes are parallel; longitudes are not parallel.
4. Equinox vs solstice: equal day and night vs. longest or shortest day.
5. Tropic of Cancer (23½°N) crosses India; Tropic of Capricorn does not.
6. Standard time uses 82°30′E in India; Greenwich is 0°.
7. Solar eclipse at new moon; lunar eclipse at full moon.
8. Axial tilt is 23½° from the vertical (66½° from the orbital plane).

One-liners

1. Earth rotates from west to east.
2. A leap year has 366 days.
3. The Prime Meridian passes through Greenwich.
4. The Equator is 0° latitude.
5. India's standard meridian is 82°30′E.
6. IST is 5 hours 30 minutes ahead of GMT.
7. Each degree of longitude equals four minutes.
8. The Tropic of Cancer passes through India.
9. The equinoxes fall on about 21 March and 23 September.
10. The northern summer solstice is on about 21 June.
11. The Earth's axis points to the Pole Star.
12. The Earth is third in order from the Sun.

Practice questions

Earth rotates on its axis from
1. East to west
2. South to north
3. West to east
4. North to south
Answer
C. West to east
Rotation is west to east, so the Sun appears to rise in the east.
One rotation of Earth takes about
1. 365 days
2. 30 days
3. 12 hours
4. 24 hours
Answer
D. 24 hours
Rotation causes day and night.
Which motion of Earth causes day and night?
1. Precession
2. Rotation
3. Nutation
4. Revolution
Answer
B. Rotation
The spinning exposes alternate halves to the Sun.
The Earth's axis is tilted from the vertical by
1. 23½°
2. 45°
3. 66½°
4. 90°
Answer
A. 23½°
The tilt (66½° from the orbital plane) is the cause of the seasons.
The point in Earth's orbit nearest to the Sun is called
1. Equinox
2. Perihelion
3. Solstice
4. Aphelion
Answer
B. Perihelion
Perihelion occurs in early January.
The Sun's vertical rays fall on the Tropic of Cancer on about
1. 21 June
2. 22 December
3. 23 September
4. 21 March
Answer
A. 21 June
This is the northern summer solstice.
On the equinoxes, the vertical rays of the Sun fall on
1. The Tropic of Capricorn
2. The Tropic of Cancer
3. The Arctic Circle
4. The Equator
Answer
D. The Equator
Day and night are equal everywhere.
The Prime Meridian passes through
1. Paris
2. Mirzapur
3. Greenwich
4. Washington
Answer
C. Greenwich
It is 0° longitude.
India's standard meridian is
1. 90°E
2. 75°E
3. 82°30′E
4. 80°E
Answer
C. 82°30′E
It passes near Mirzapur in Uttar Pradesh.
Indian Standard Time is ahead of GMT by
1. 6 hours
2. 5 hours 30 minutes
3. 4 hours 30 minutes
4. 5 hours
Answer
B. 5 hours 30 minutes
82.5 degrees divided by 15 gives 5.5 hours.
Which parallel of latitude passes through India?
1. Arctic Circle
2. Antarctic Circle
3. Tropic of Capricorn
4. Tropic of Cancer
Answer
D. Tropic of Cancer
It passes through eight states.
The Solar System planet that is third from the Sun is
1. Earth
2. Mars
3. Mercury
4. Venus
Answer
A. Earth
The order is Mercury, Venus, Earth, Mars.
The shape of the Earth is best described as
1. A perfect sphere
2. A cube
3. An oblate spheroid
4. A cylinder
Answer
C. An oblate spheroid
It bulges at the Equator and is flattened at the poles.
Each degree of longitude corresponds to a time difference of
1. 4 minutes
2. 24 minutes
3. 1 hour
4. 15 minutes
Answer
A. 4 minutes
360 degrees take 24 hours, so 1 degree takes 4 minutes.
When it is 12 noon at Greenwich, the time at 60°E is
1. 8 a.m.
2. 6 p.m.
3. 3 p.m.
4. 4 p.m.
Answer
D. 4 p.m.
60 divided by 15 is 4 hours ahead.
When it is 12 noon at Greenwich, the time at 45°W is
1. 8 a.m.
2. 9 a.m.
3. 10 a.m.
4. 3 p.m.
Answer
B. 9 a.m.
45 divided by 15 is 3 hours behind.
When it is 8 a.m. GMT, the IST is
1. 2:30 p.m.
2. 1 p.m.
3. 12:30 p.m.
4. 1:30 p.m.
Answer
D. 1:30 p.m.
8:00 plus 5:30 is 13:30.
A place has a local time 2 hours behind GMT. Its longitude is
1. 60°W
2. 30°E
3. 30°W
4. 15°W
Answer
C. 30°W
2 hours multiplied by 15 degrees gives 30 degrees; behind means west.
Seasons on Earth are caused mainly by
1. The tilt of the axis with revolution
2. Rotation alone
3. Moon's pull
4. Distance from the Sun
Answer
A. The tilt of the axis with revolution
Distance varies little; the tilt changes the angle of sunlight.
The Coriolis effect deflects winds in the Northern Hemisphere to the
1. Left
2. Right
3. North
4. South
Answer
B. Right
It results from rotation; in the south the deflection is to the left.
Which of the following lines are not parallel to each other?
1. Parallels of latitude
2. Tropic of Cancer and the Equator
3. Arctic and Antarctic Circles
4. Meridians of longitude
Answer
D. Meridians of longitude
Meridians meet at the poles.
Regions beyond the polar circles experience
1. Equal day and night all year
2. No seasons at all
3. Only monsoon rain
4. Periods of continuous daylight or darkness
Answer
D. Periods of continuous daylight or darkness
Because of the axial tilt, the midnight sun occurs there.
The zone between the Tropic of Cancer and the Tropic of Capricorn is called the
1. Torrid Zone
2. Temperate Zone
3. Polar Zone
4. Frigid Zone
Answer
A. Torrid Zone
It receives vertical rays at some time in the year.
A solar eclipse occurs when
1. The Moon is full
2. The Moon lies between the Sun and Earth at new moon
3. The Sun lies between Earth and Moon
4. Earth lies between the Sun and Moon
Answer
B. The Moon lies between the Sun and Earth at new moon
The Moon blocks sunlight.
The International Date Line is based approximately on the meridian of
1. 0°
2. 90°E
3. 180°
4. 90°W
Answer
C. 180°
It bends to avoid land areas.
The planet belt between Mars and Jupiter is
1. The ozone layer
2. The asteroid belt
3. The Kuiper ring
4. The Van Allen belt
Answer
B. The asteroid belt
It holds many rocky bodies.
The extra quarter day of Earth's revolution is accounted for by a
1. Leap year
2. Solstice
3. Aphelion
4. Equinox
Answer
A. Leap year
A year of 366 days occurs every four years.
Why is the time at a place east of Greenwich ahead of GMT?
1. Because of the tilt of the axis
2. Because the Sun moves from east to west faster
3. Earth rotates from west to east so the Sun reaches eastern places first
4. Because the Moon pulls the Earth
Answer
C. Earth rotates from west to east so the Sun reaches eastern places first
Eastward places see the Sun earlier.
Consider the statements. 1. Rotation takes about 24 hours. 2. Revolution around the Sun takes about 365¼ days.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both statements are correct.
Consider the statements on the seasons. 1. Earth is nearest the Sun during the northern summer. 2. Seasons are caused by the tilt of the axis.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Perihelion is in January; only the tilt statement is correct.
Consider the statements on latitudes. 1. The Equator is 0° latitude. 2. Tropic of Capricorn passes through India.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Tropic of Cancer passes through India.
Consider the statements on time. 1. Greenwich is at 82°30′E. 2. IST is based on 82°30′E.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Greenwich is at 0°.
Consider the statements on the International Date Line. 1. Crossing it from east to west, a day is gained. 2. It is a perfectly straight line.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
The line has bends to avoid land.
Consider the statements on eclipses. 1. A solar eclipse occurs at full moon. 2. A lunar eclipse occurs at full moon.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
A solar eclipse happens at new moon.
Consider the statements on the Solar System. 1. Pluto is a dwarf planet. 2. Earth is the largest planet.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Jupiter is the largest planet.
Consider the statements on the circles of latitude. 1. The Antarctic Circle is at 23½°S. 2. The Arctic Circle is at 66½°N.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
The Antarctic Circle is at 66½°S.
Consider the statements on equinoxes. 1. Equal day and night occur on equinox. 2. The summer solstice is on 22 December in the north.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
The northern summer solstice is on 21 June.
Match: (1) 21 March (2) 21 June (3) 23 September (4) 22 December
1. Northern summer solstice, vernal equinox, northern winter solstice, autumn equinox
2. Autumn equinox, northern winter solstice, vernal equinox, northern summer solstice
3. Northern winter solstice, autumn equinox, northern summer solstice, vernal equinox
4. Vernal equinox, northern summer solstice, autumn equinox, northern winter solstice
Answer
D. Vernal equinox, northern summer solstice, autumn equinox, northern winter solstice
Equinoxes in March and September; solstices in June and December.
Match line and value: (1) Equator (2) Tropic of Cancer (3) Arctic Circle (4) Antarctic Circle
1. 0°, 23½°S, 66½°S, 66½°N
2. 23½°N, 0°, 66½°N, 66½°S
3. 0°, 23½°N, 66½°N, 66½°S
4. 66½°N, 0°, 23½°N, 23½°S
Answer
C. 0°, 23½°N, 66½°N, 66½°S
These are the standard parallels.
Match: (1) Perihelion (2) Aphelion (3) Rotation (4) Revolution
1. Year, day and night, farthest from Sun, nearest to Sun
2. Farthest from Sun, nearest to Sun, year, day and night
3. Day and night, year, nearest to Sun, farthest from Sun
4. Nearest to Sun, farthest from Sun, day and night, year
Answer
D. Nearest to Sun, farthest from Sun, day and night, year
Standard definitions.
Consider the statements on Andhra Pradesh and latitude. 1. It lies north of the Tropic of Cancer. 2. Andhra Pradesh lies in the Torrid Zone.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
It lies south of the Tropic of Cancer.
If a place is at 15°E, its local time compared to GMT is
1. 4 minutes ahead
2. 15 minutes behind
3. 1 hour behind
4. 1 hour ahead
Answer
D. 1 hour ahead
15 degrees equals 1 hour; east means ahead.
At the North Pole, day and night each last about
1. Twelve hours
2. Three months
3. Six months
4. One month
Answer
C. Six months
Because of the tilt, there is one long day and one long night.
Which planet is a terrestrial (rocky) planet?
1. Neptune
2. Jupiter
3. Saturn
4. Mars
Answer
D. Mars
The first four planets are rocky; the next four are gas and ice giants.
The Moon is Earth's only
1. Dwarf planet
2. Artificial satellite
3. Natural satellite
4. Planet
Answer
C. Natural satellite
It orbits the Earth.

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