World Geography

medium importance~1 Q in Tier 12 formulas⚡ 12 shortcuts6 subtopics

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Solar system, Earth's motions, latitudes and time

⏱ 7 min read🧩 6 question types🎯 20 practice Q

The idea in one minute

The universe is made of galaxies, and the Sun is our nearest star. Eight planets circle the Sun. Earth spins once a day and goes round the Sun once a year. Spin gives day and night. The tilt of the axis, with the yearly trip, gives the seasons.

The universe and the solar system

The universe is made of galaxies. Ours is the Milky Way (Akash Ganga).

A light year is the distance light covers in one year. It is about 9.46 x 10^12 km. So it measures distance, not time.

The Sun is our nearest star. It is about 150 million km away. Its light takes about 8 minutes 20 seconds to reach Earth. The next star is Proxima Centauri.

The solar system holds the Sun, 8 planets, their moons, asteroids and comets. In 2006 the IAU made Pluto a dwarf planet. That cut the planets from 9 to 8.

The eight planets

Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Memory line: My Very Educated Mother Just Served Us Noodles.

Planet	Exam one-liner
Mercury	Smallest and nearest to the Sun. No atmosphere, no moon. Fastest orbit
Venus	Hottest (thick CO2 blanket) and brightest. Morning/Evening Star. Rotates east to west
Earth	Blue Planet, about 71% water. Only known planet with life
Mars	Red Planet (iron-oxide dust). Olympus Mons. Moons Phobos and Deimos
Jupiter	Largest planet. Great Red Spot. Moon Ganymede is the largest satellite
Saturn	Rings of ice and rock. Least dense. Moon Titan is the second largest satellite
Uranus	Ice giant. Rolls on its side. Found in 1781 by William Herschel
Neptune	Farthest planet. Deepest blue. Strongest winds

Tip: Venus is hotter than Mercury, though it is farther from the Sun. Its thick CO2 traps heat.

The first four are small rocky inner (terrestrial) planets. Then comes the asteroid belt. The last four are large gas outer (Jovian) planets.

Comets, meteors and the Moon

Comets grow tails that always point away from the Sun. Halley's comet returns every 76 years. It was last seen in 1986. It is next due in 2061.
Ceres, the largest asteroid, lies in the asteroid belt.
Meteors ("shooting stars") burn up in the air. Those that reach the ground are meteorites.
The Moon is Earth's only natural satellite. It is about 3.84 lakh km away.
The Moon takes about 27.3 days to spin once. It also takes 27.3 days to circle Earth. So the same face always points at Earth.
It has almost no atmosphere. Neil Armstrong stepped on it on 20 July 1969 (Apollo 11), with Buzz Aldrin.

Shape, rotation and revolution

Earth is an oblate spheroid (geoid). It is flat at the poles and bulges at the equator.

Equatorial diameter: about 12,756 km. Polar diameter: 12,714 km.
Equatorial circumference: about 40,075 km.

Motion	Time taken	Effect
Rotation (west to east)	23 h 56 min 4 s (sidereal day)	Day and night, wind deflection
Revolution	365 days 6 hours 9 minutes	Seasons, with the tilted axis

The spare 6 hours add up to one day. We add it as 29 February in every 4th (leap) year.

The axis is tilted 23.5 degrees from the vertical. It makes 66.5 degrees with the orbit plane. On 3 January Earth is nearest the Sun (perihelion). On 4 July it is farthest (aphelion).

Watch: Rotation gives day and night. Revolution with the tilt gives seasons. Distance from the Sun does not.

Equinoxes and solstices

Date	Event	Effect in the Northern Hemisphere
21 March	Spring equinox	Day equals night everywhere
21 June	Summer solstice	Longest day. Sun overhead at the Tropic of Cancer
23 September	Autumn equinox	Day equals night everywhere
22 December	Winter solstice	Shortest day. Sun overhead at the Tropic of Capricorn

On the equator, day and night stay about equal all year. Beyond the Arctic and Antarctic Circles (66.5 degrees), summer brings months-long days. That is the "land of the midnight sun".

Latitudes, longitudes and time

Latitudes are 180 imaginary circles (90 N + 90 S) counted from the equator.

Line	Value
Equator	0 degrees, the longest parallel
Tropic of Cancer / Capricorn	23.5 N / 23.5 S
Arctic / Antarctic Circle	66.5 N / 66.5 S
Prime Meridian	0 degrees through Greenwich (London)
International Date Line	About 180 degrees, bends around islands

Longitudes are 360 meridians that join the poles. The Prime Meridian was fixed at the 1884 conference.

Earth turns 360 degrees in 24 hours. So 15 degrees = 1 hour and 1 degree = 4 minutes.

Example: India's meridian is 82.5 E. 82.5 x 4 = 330 minutes. So IST = GMT + 5 h 30 min.

Places east of Greenwich are ahead in time. Places west are behind.

Crossing the IDL east to west (towards Asia) you add a day. West to east you lose a day.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common5 practice Q

Planet superlatives and pairs

How to spot it:

'Largest / hottest / brightest planet', 'Earth's twin', 'red planet', 'planet with rings', match-the-column on planets.

Method

Recall the size ladder: Jupiter, Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury.
Fix one anchor fact per planet: Venus hottest, Mars red, Saturn rings.
Give the largest moon (Ganymede) to Jupiter and the second largest (Titan) to Saturn.
Eliminate options that break an anchor.

Why it works:

Each planet has one famous fact, and options are built to mix them.

Try this

Which planet is called 'Earth's twin' for its similar size?

Show solution

Venus is close to Earth in size.
It is also the hottest and brightest planet.

Answer

Venus

Type 2common3 practice Q

The Moon: distance, motions and landings

How to spot it:

'How far is the Moon', 'why the same face', first man on the Moon, atmosphere of the Moon.

Method

Distance is about 3.84 lakh km.
Rotation time equals revolution time, about 27.3 days.
The Moon has almost no atmosphere.
Armstrong landed on 20 July 1969 (Apollo 11).

Why it works:

The same-face fact is the most asked, and it rests on equal rotation and revolution times.

Try this

Why do we always see the same face of the Moon?

Show solution

The Moon spins once in about 27.3 days.
It also circles Earth once in about 27.3 days.

Answer

Its rotation takes the same time as its revolution around Earth.

Type 3very common3 practice Q

Equinoxes, solstices and the overhead Sun

How to spot it:

'Longest / shortest day', 'Sun is overhead the Tropic of Cancer on', 'when are day and night equal'.

Method

Solstices: 21 June (Tropic of Cancer) and 22 December (Tropic of Capricorn).
Equinoxes: 21 March and 23 September.
Match the season to the hemisphere: north summer is south winter.

Why it works:

Four dates and four Sun positions cover almost every question.

Try this

On which date is the Sun overhead at the Tropic of Capricorn?

Show solution

The Sun is over Capricorn at the winter solstice of the north.
That date is 22 December.

Answer

22 December

Type 4very common3 practice Q

Rotation vs revolution and their effects

How to spot it:

'Rotation causes', 'sidereal day', 'why a leap year', perihelion and aphelion dates.

Method

Rotation (23 h 56 min 4 s) gives day and night.
Revolution (365 d 6 h 9 min) with the tilt gives seasons.
The extra 6 hours make a leap year every 4 years.
Perihelion 3 January, aphelion 4 July.

Why it works:

Questions separate 'day and night' from 'seasons', so match each to its motion.

Try this

Why is a leap year seen every fourth year?

Show solution

Earth takes 365 days 6 hours 9 minutes to go round the Sun.
The spare 6 hours add up to one day in four years.

Answer

The extra day is added as 29 February.

Type 5very common3 practice Q

Longitudes, time zones and the IDL

How to spot it:

Time gap between two longitudes, IST and GMT, adding or losing a day at the IDL.

1^{\circ}\text{ of longitude} = 4\text{ minutes of time}

Method

Turn 360 degrees in 24 hours gives 1 degree = 4 minutes.
Multiply the longitude gap by 4.
East is ahead, west is behind.
IDL east to west adds a day.

Why it works:

One constant, 4 minutes per degree, solves every time-gap question.

Try this

Two places are 30 degrees of longitude apart. What is their local time gap?

Show solution

30 x 4 = 120 minutes.
120 minutes = 2 hours.
The eastern place is ahead.

Answer

2 hours

Type 6occasional

Comets, asteroids and meteors

How to spot it:

'Halley's comet returns every', 'largest asteroid', 'shooting stars are', comet-tail direction.

Method

Comets: tails point away from the Sun. Halley's comet returns every 76 years.
Asteroids sit in the belt between Mars and Jupiter. Ceres is the largest.
Meteors burn up in the air. Those that reach the ground are meteorites.

Why it works:

These small bodies are asked as short definitions.

Try this

Which name is given to a piece of space rock that reaches the ground?

Show solution

A rock that burns up in the air is a meteor.
One that reaches the ground is a meteorite.

Answer

Meteorite

Formula sheet

Time difference from longitude

\Delta t = \Delta\lambda \times 4\ \text{minutes}

1° of longitude = 4 min; 15° = 1 hour

Local time

T_{\text{local}} = T_{\text{GMT}} \pm \frac{\lambda}{15}\ \text{h}

+ for east longitudes, − for west

Shortcuts that save time

⚡ Planet superlatives in one line

Mercury smallest and nearest. Venus hottest, brightest, spins backward. Mars red. Jupiter largest, with Ganymede. Saturn rings and lightest. Uranus on its side. Neptune farthest and windiest.

Example

Which is the hottest planet?

Show solution

Mercury is nearest, but it has no atmosphere to trap heat.
Venus has a thick CO2 atmosphere that traps heat.

Answer

Venus

⚡ IST from the meridian

India's meridian is 82.5 E. 82.5 x 4 = 330 minutes, which is 5 h 30 min ahead of GMT.

Example

When it is 12 noon at Greenwich, what is the time at 90 E?

Show solution

90 x 4 = 360 minutes.
360 minutes = 6 hours ahead of Greenwich.

Answer

6:00 p.m.

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Calling Mercury the hottest planet.

Venus is hotter, though it is farther from the Sun.

Mistake 02

Adding time for places west of Greenwich.

West is behind, east is ahead.

Mistake 03

Blaming perihelion (January) for the Northern summer.

Seasons come from the tilt and revolution, not from distance.

Mistake 04

Treating a light year as a unit of time.

It is a unit of distance.

Mistake 05

Saying Pluto is the ninth planet.

Since 2006 Pluto is a dwarf planet, so there are 8 planets.

Quick revision

Read this the night before the exam.

8 planets (Pluto is a dwarf planet since 2006). Order: Mercury, Venus, Earth, Mars, asteroid belt, Jupiter, Saturn, Uranus, Neptune.
Venus hottest and Morning/Evening Star. Jupiter largest with Ganymede. Saturn least dense. Mars red with Olympus Mons.
Light year is a distance unit. The Sun is 8 light-minutes away.
Rotation 23 h 56 m 4 s gives day and night. Revolution 365 d 6 h 9 min gives seasons and leap years.
21 June: Sun over Tropic of Cancer. 22 December: Tropic of Capricorn. 21 March and 23 September: equal day and night.
1 degree of longitude = 4 minutes. IDL east to west: add a day. West to east: lose a day.

Practice: 20 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 4 min · wrong answers go to your mistake notebook automatically.

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