World Geography
🔒 Log in to trackSolar system, Earth's motions, latitudes and time
🔒 Log in to trackSolar system (8 planets since the IAU decision of 2006, when Pluto became a dwarf planet)
Order from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune ('My Very Educated Mother Just Served Us Noodles').
| Planet | Remember it as |
|---|---|
| Mercury | Smallest planet, nearest to the Sun, fastest orbit |
| Venus | Hottest planet (thick CO₂ atmosphere), brightest, 'Earth's twin', morning/evening star, rotates east to west (retrograde) |
| Earth | 'Blue planet'; only known planet with life |
| Mars | 'Red planet' (iron oxide); Olympus Mons; moons Phobos and Deimos |
| Jupiter | Largest planet; Great Red Spot; Ganymede — largest moon of the solar system |
| Saturn | Prominent rings; least dense (lighter than water); largest moon Titan |
| Uranus | Rotates on its side (extreme axial tilt); also retrograde spin |
| Neptune | Farthest planet; strongest winds |
Sunlight takes about 8 minutes 20 seconds to reach the Earth. The Moon is Earth's only natural satellite; the same face always points towards Earth.
Motions of the Earth
| Motion | Period | Effect |
|---|---|---|
| Rotation (west to east) | About 24 hours (sidereal day 23 h 56 m 4 s) | Day and night |
| Revolution | About 365¼ days | Seasons (with the tilt) |
- Axis is tilted 23½° from the perpendicular (makes 66½° with the orbital plane).
- Perihelion (nearest to Sun) about 3 January; aphelion (farthest) about 4 July.
- Summer solstice 21 June — longest day in the Northern Hemisphere, Sun overhead at the Tropic of Cancer. Winter solstice 22 December — Sun over the Tropic of Capricorn.
- Equinoxes — 21 March and 23 September — Sun overhead at the equator; day = night everywhere.
Latitudes, longitudes and time
| Line | Value |
|---|---|
| Equator | 0° (longest parallel, a great circle) |
| Tropic of Cancer / Capricorn | 23½° N / 23½° S |
| Arctic / Antarctic Circle | 66½° N / 66½° S |
| Prime Meridian | 0° through Greenwich (London) |
| International Date Line | Roughly 180°, zig-zags to avoid land |
- Earth turns 360° in 24 hours → 15° = 1 hour, 1° = 4 minutes.
- Places east of Greenwich are ahead in time; places west are behind.
- Crossing the IDL westward (e.g. USA → Japan) you add a day; crossing eastward you repeat a day.
Detailed notes
The universe, the Sun 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 - about 9.46 x 10^12 km - so it measures distance, not time. The Sun, our nearest star, is about 150 million km (8 light-minutes) away; 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, taking the planets from 9 to 8.
The eight planets - the exam one-liners
| Planet | Key facts |
|---|---|
| Mercury | Smallest, nearest to the Sun, no atmosphere or moon; extreme day-night temperatures |
| Venus | Hottest planet (thick CO2 greenhouse); brightest; the Morning/Evening Star; 'Earth's twin' in size; rotates east to west |
| Earth | The Blue Planet - about 71% water; only known planet with life |
| Mars | The Red Planet (iron-oxide dust); Olympus Mons, the solar system's largest volcano; moons Phobos and Deimos |
| Jupiter | Largest planet; Great Red Spot storm; its moon Ganymede is the solar system's largest satellite |
| Saturn | Famous rings of ice and rock; least dense planet; moon Titan is the second largest satellite |
| Uranus | 'Ice giant'; axis so tilted that it seems to roll on its side; discovered in 1781 by William Herschel |
| Neptune | Farthest planet; deepest blue; the strongest winds of the solar system |
Order from the Sun: Mercury, Venus, Earth, Mars - the small rocky inner (terrestrial) planets; then the asteroid belt; then Jupiter, Saturn, Uranus, Neptune - the large gas outer (Jovian) planets. The largest asteroid Ceres lies in this belt. Comets (e.g. Halley's, seen every 76 years - last 1986, next 2061) grow tails that always point away from the Sun. Meteors ('shooting stars') burn up in the air; the ones that reach the ground are meteorites.
The Earth's size, shape and the Moon
Earth is an oblate spheroid (geoid) - flattened at the poles, bulging at the equator: equatorial diameter about 12,756 km, polar 12,714 km, equatorial circumference about 40,075 km. Its only natural satellite, the Moon, is about 3.84 lakh km away; it rotates on its axis in the same time (about 27.3 days) as it takes to revolve around Earth, so we always see the same face. It has practically no atmosphere, so astronauts' footprints stay; Neil Armstrong stepped on it on 20 July 1969 (Apollo 11), with Buzz Aldrin.
Rotation, revolution and the seasons
- Rotation - Earth spins west to east once in 23 h 56 min 4 s (the sidereal day), causing day and night and wind deflection (Coriolis effect).
- Revolution - Earth goes round the Sun in 365 days 6 hours 9 minutes; the extra 6 hours is adjusted by 29 February every 4th (leap) year. Revolution plus the tilted axis (23.5 degrees) causes the seasons.
- On 3 January Earth is nearest the Sun (perihelion); on 4 July farthest (aphelion).
Equinoxes and solstices
| Date | Event | Effect in the Northern Hemisphere |
|---|---|---|
| 21 March | Vernal (spring) equinox | Day = night everywhere |
| 21 June | Summer solstice | Longest day; Sun overhead the Tropic of Cancer |
| 23 September | Autumnal equinox | Day = night everywhere |
| 22 December | Winter solstice | Shortest day; Sun overhead the Tropic of Capricorn |
On the equator, day and night stay about equal all year; beyond the Arctic/Antarctic Circles (66.5 degrees), summer brings months-long days - the 'land of the midnight sun'.
Latitudes, longitudes and time
Latitudes are 180 imaginary circles (90 N + 90 S) measured from the equator; the important ones are the Equator (0), Tropics (23.5 N/S) and Circles (66.5 N/S). Longitudes are 360 meridians joining the poles; 1 degree = 4 minutes of time. The Prime Meridian (0 degrees, Greenwich) was fixed at the 1884 conference; places east are ahead in time, west behind. India keeps IST = GMT + 5 h 30 min (meridian 82.5 E). The International Date Line (IDL) follows roughly the 180th meridian, bending around islands; crossing it east to west (towards Asia) you ADD a day, west to east you LOSE a day.
Quick revision
- 8 planets (Pluto = dwarf planet since 2006); order M-V-E-M (asteroid belt) J-S-U-N.
- Venus hottest + Morning/Evening star; Jupiter largest + Ganymede biggest moon; Saturn least dense; Mars red + Olympus Mons.
- Light year = distance unit; the Sun is 8 light-minutes away.
- Rotation 23 h 56 m 4 s (day-night); revolution 365 d 6 h (seasons, leap year).
- 21 June summer solstice (Sun on Tropic of Cancer); 22 Dec winter solstice; 21 Mar & 23 Sep equinoxes.
- 1 degree longitude = 4 minutes; IDL: east to west = add a day; west to east = lose a day.
Types of questions asked
Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.
Type 1: Planet superlatives and pairsvery common5 practice Q
'Which is the largest/hottest/brightest planet?', 'Earth's twin', 'red planet', 'planet with rings', planet-property match-the-column.
- Size ladder: Jupiter > Saturn > Uranus > Neptune > Earth > Venus > Mars > Mercury.
- Venus - hottest, brightest, Morning/Evening Star, 'Earth's twin', rotates east to west; Mars - red planet with Olympus Mons; Saturn - rings, least dense.
- For match-the-column, fix one anchor per planet and eliminate; 'largest satellite' (Ganymede) belongs to Jupiter, second largest (Titan) to Saturn.
Example: Which planet is known as the 'Earth's twin' because of its similar size and mass?
Venus. It is also the hottest planet (thick CO2) and the brightest, called the Morning/Evening Star.
Type 2: The Moon: distance, motions and landingscommon3 practice Q
'How far is the Moon', 'why does the Moon always show the same face', first man on the Moon, the Moon's atmosphere.
- Distance about 3.84 lakh km; the Moon is Earth's only natural satellite.
- Its rotation period equals its revolution period (about 27.3 days) - hence the same face always faces Earth.
- It has practically no atmosphere; Neil Armstrong stepped on it on 20 July 1969 (Apollo 11).
Example: Why do we always see the same face of the Moon?
Because the Moon's rotation on its axis takes the same time (about 27.3 days) as its revolution around the Earth.
Type 3: Equinoxes, solstices and the overhead Sunvery common3 practice Q
'Longest/shortest day', 'the Sun is overhead the Tropic of Cancer on', 'when is day and night equal', dates with hemisphere effects.
- Solstices: 21 June (longest day in the N. Hemisphere; Sun overhead Tropic of Cancer) and 22 December (shortest day; Sun overhead Tropic of Capricorn).
- Equinoxes: 21 March and 23 September - day and night equal everywhere.
- The hemisphere rule: what is summer in the north is winter in the south; on the equator days stay about 12 hours.
Example: On which date is the Sun vertically overhead the Tropic of Capricorn?
22 December - the winter solstice of the Northern Hemisphere (its shortest day) and the height of summer in the south.
Type 4: Rotation vs revolution and their effectsvery common3 practice Q
'Rotation causes', 'length of the sidereal day', 'why a leap year', perihelion and aphelion dates.
- Rotation (once in 23 h 56 min 4 s) causes day-night, the Sun's apparent eastward rise, tides (partly) and wind deflection.
- Revolution (365 d 6 h 9 min) with the 23.5-degree tilt causes the seasons; the extra 6 h gives a leap year every 4 years.
- Perihelion (nearest Sun) - 3 January; aphelion (farthest) - 4 July. In questions, separate 'day and night' (rotation) from 'seasons' (revolution).
Example: Why is a leap year observed every fourth year?
The Earth takes 365 days 6 hours 9 minutes to revolve; the leftover 6 hours collects into one extra day (29 February) every four years.
Type 5: Longitudes, time zones and the IDLvery common3 practice Q
Time difference between two longitudes, IST-GMT relation, adding/losing a day across the IDL, count of latitudes/meridians.
- The Earth turns 360 degrees in 24 hours, so 1 degree = 4 minutes; 15 degrees = 1 hour.
- East of Greenwich is ahead, west behind; IST is based on 82.5 E = GMT + 5 h 30 min.
- The IDL follows about 180 degrees: crossing east to west (towards Asia) add a day, west to east lose a day.
Example: Two places lie 30 degrees of longitude apart. What is the difference in their local time?
30 x 4 minutes = 120 minutes = 2 hours; the eastern place is ahead.
Formulas
1° of longitude = 4 min; 15° = 1 hour
+ for east longitudes, − for west
Shortcut tricks
⚡ Planet superlatives in one line
Mercury smallest–nearest, Venus hottest–brightest–backward spin, Mars red, Jupiter largest (Ganymede biggest moon), Saturn rings–lightest, Uranus on its side, Neptune farthest–windiest.
Example: Which is the hottest planet?
Venus — not Mercury — because its thick CO₂ atmosphere traps heat.
⚡ IST from the meridian
India's meridian is 82½°E → 82.5 × 4 = 330 minutes = 5 h 30 min ahead of GMT.
Example: When it is 12 noon at Greenwich, what is the time at 90°E?
90 × 4 = 360 min = 6 h ahead → 6:00 p.m.
Where students lose marks
Calling Mercury the hottest planet — Venus is hotter despite being farther.
Adding time for places west of Greenwich — west is behind, east is ahead.
Mixing perihelion (January) with summer in the Northern Hemisphere — seasons come from the tilt, not distance.
Practice sets — 20 questions
Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.
Topic test · 10 questions
Suggested time 4 min · wrong answers go to your mistake notebook automatically.