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State Board · Class 8 · Geography

Lesson 1: Local Time and Standard Time

5 concepts · 40 practice questions with answers · Free to read

This lesson is also in: मराठी

Rotation of the Earth, day and night, shadows

From daily life

When you sit in a moving bus, the trees seem to run backwards. In the same way the Sun seems to move across the sky, but it is the Earth that turns.

Simple definition

Rotation (परिवलन) is the turning of the Earth on its own axis from west to east once in about 24 hours. It causes day and night.

Rule 1: Day and night — One rotation of 360° takes almost 24 hours, giving sunrise, noon, sunset and midnight. The longitude facing the Sun has day; the one in the dark has night. The Sun only appears to move from east to west. At the poles daytime lasts about 6 months.

DayNightrotationW → EsunrisesunsetSunseen from above the North Pole

In the figure : In the figure the Earth rotates from west to east (anticlockwise when seen from above the North Pole): the half facing the sun has day and the other half has night.

  • Easy : Easy: The Earth turns west to east.
  • Medium : Medium: If one turn is 24 h, half a turn is 12 h.
  • Exam-type : Exam: Why do we get day and night?

Rule 2: Shadows and old clocks — Shadows are longest in the early morning and evening and shortest at noon; they point west in the morning and east in the evening. Old time tools: the Ghatikapaatra (a bowl with a tiny hole floating in water), the sand timer and the shadow of a pole.

EWMorninglong shadowEWNoonshort shadowEWEveninglong shadow

In the figure : In the figure the shadow of a pole is long in the morning and evening and shortest at noon; it points west in the morning and east in the evening.

  • Easy : Easy: Shadow is shortest at noon.
  • Medium : Medium: Morning sun is in the east, so the shadow falls west.
  • Exam-type : Exam: Why is the noon shadow shortest?
GhatikapaatraSand timerEWPole shadow

In the figure : In the figure old ways of measuring time: the Ghatikapaatra (a floating bowl with a tiny hole), the sand timer and the shadow of a pole.

  • Easy : Easy: The sand timer measures time.
  • Medium : Medium: A bowl that sinks in 24 minutes can be used 60 ÷ 24 = 2.5 times per hour.
  • Exam-type : Exam: Name three old time tools.
Remember

Rotation: west → east, 24 h. Shadow shortest at noon.

Common mistake

Thinking the Sun moves round the Earth. The Earth turns.

Shortcut

Memory clue: "सकाळी सावली पश्चिमेला, संध्याकाळी पूर्वेला".

Spot the difference

TimeShadow
MorningLong, points west
NoonShortest
EveningLong, points east

Practice questions

Easy
  1. 1What is rotation?
  2. 2In which direction does the Earth rotate?
  3. 3How long is one rotation?
Medium
  1. 4What is the Ghatikapaatra?
  2. 5When is the shadow longest?
  3. 6Which way does the evening shadow point?
Challenging
  1. 7Sunrise is at 6:15 a.m. and sunset at 6:45 p.m. How long is the daytime?
  2. 8Why do poles have a 6-month day?
Show answers (try them yourself first)
  1. 1The Earth turning on its axis.
  2. 2West to east.
  3. 3About 24 hours.
  4. 4A floating bowl with a tiny hole, used to measure time.
  5. 5Early morning and evening.
  6. 6East.
  7. 712 hours 30 minutes.
  8. 8Because of the Earth's tilt, the Sun stays above the horizon for months.

Local time and longitude

From daily life

When the school bell in Mumbai rings at noon, a clock in Kolkata, which is east, already shows afternoon. Time depends on where you stand east or west.

Simple definition

Local time (स्थानिक वेळ) is the time of a place fixed by the Sun's position, with reference to its noon.

Rule 1: Same longitude, same time — All places on one longitude, from pole to pole, have noon at the same moment, so the same local time. Different longitudes have different sunrise, noon and sunset. East is ahead, west is behind.

a.m.one longitudesame timeeverywhere

In the figure : In the figure all places on one longitude, from pole to pole, have noon at the same moment, so they have the same local time.

  • Easy : Easy: Same longitude, same time.
  • Medium : Medium: Delhi and Dehradun (both about 78°E) show nearly the same local time.
  • Exam-type : Exam: Why does Kolkata see noon before Mumbai?

Rule 2: 15° and 4 minutes — In 24 hours the Earth turns 360°, so in 1 hour it turns 360 ÷ 24 = 15°. 60 minutes ÷ 15° = 4 minutes, so each degree of longitude changes time by 4 minutes. With a 1° interval there are 360 longitudes, so up to 360 local times. Midnight to noon is a.m. (Ante Meridiem); noon to midnight is p.m. (Post Meridiem).

noonmidnight6 p.m.6 a.m.0°180°90°E90°W

In the figure : In the figure, seen from above the North Pole, the Earth turns 15° every hour: places to the east of 0° are ahead in time and places to the west are behind.

  • Easy : Easy: 1 hour = 15°.
  • Medium : Medium: 45° → 45 × 4 = 180 minutes = 3 hours.
  • Exam-type : Exam: Find the time difference for 30°.
360° in24 hours15° in1 hour60 minutes÷ 15°4 minutesper 1°

In the figure : In the figure 360° ÷ 24 hours = 15° in 1 hour, and 60 minutes ÷ 15° = 4 minutes for every 1° of longitude.

Live figure

Local time and longitude

In the app you can move this figure with your finger.

Open in the app
  • Easy : Easy: 1° = 4 minutes.
  • Medium : Medium: 15° = 15 × 4 = 60 minutes.
  • Exam-type : Exam: 10° east is how much ahead?
Remember

360° ÷ 24 = 15° per hour; 1° = 4 min. East ahead, west behind.

Common mistake

Mixing latitude with longitude. Time depends on longitude.

Shortcut

Memory clue: "15° = 1 तास, 1° = 4 मिनिटे".

Spot the difference

AngleTime
1°4 minutes
15°1 hour
360°24 hours

Practice questions

Easy
  1. 1What is local time?
  2. 2Which places have the same local time?
  3. 3How many degrees does the Earth turn in 1 hour?
Medium
  1. 4How many minutes per degree?
  2. 5What does a.m. mean?
  3. 6Is east ahead or behind?
Challenging
  1. 7Pune is at 74°E and Kolkata at 88°E. What is the local time difference?
  2. 8Why can there be 360 local times?
Show answers (try them yourself first)
  1. 1Time fixed by the Sun's position at a place.
  2. 2Places on the same longitude.
  3. 315°.
  4. 44 minutes.
  5. 5Ante Meridiem, midnight to noon.
  6. 6Ahead.
  7. 714° × 4 = 56 minutes; Kolkata is ahead.
  8. 8There are 360 longitudes at 1° interval.

Calculating local time

From daily life

To find a friend's time abroad, you need only two things: how many degrees apart you are, and whether they are east or west of you.

Simple definition

Calculating local time: difference of longitudes × 4 minutes, then add for east, subtract for west, with the Prime Meridian (0°) as the base.

Rule 1: Three steps — Step 1: find the difference of longitudes (opposite sides: add; same side: subtract). Step 2: multiply by 4 minutes; divide by 60 for hours. Step 3: east: add; west: subtract; check whether the date changes.

Difference of longitudessame side: subtract, opposite sides: add1× 4 minutesthen ÷ 60 to get hours2East: add, west: subtractcheck whether the date changes3

In the figure : In the figure three steps: find the difference of the longitudes, multiply it by 4 minutes, then add the time for the east and subtract it for the west.

  • Easy : Easy: East → add.
  • Medium : Medium: 0° and 20°W: 20 × 4 = 80 minutes; behind.
  • Exam-type : Exam: State the three steps.

Rule 2: Worked examples — Example 1: Mashad (Iran) at 60° E: 60 × 4 = 240 min = 4 h. When it is 12 noon at Greenwich, it is 4 p.m. there. Example 2: Manaus (Brazil) at 60° W: 240 min = 4 h behind, so 8 a.m. Example 3: Mumbai 73° E and Kolkata 88° E: difference 15° × 4 = 60 min, Kolkata is 1 hour ahead. Example 4: 12 noon at 60° E gives 6 a.m. at 30° W (90° = 6 hours behind).

WEa.m.Manaus60° WnoonGreenwich0°p.m.Mashad60° E

In the figure : In the figure when it is 12 noon at Greenwich, it is 8 a.m. at Manaus (60° W) and 4 p.m. at Mashad (60° E).

  • Easy : Easy: Greenwich noon → Mashad 4 p.m.
  • Medium : Medium: New Delhi 77°E: 77 × 4 = 308 min = 5 h 8 min ahead of Greenwich.
  • Exam-type : Exam: At 12 noon Greenwich find the time at 74° W (New York). 74 × 4 = 296 min = 4 h 56 min, so 7:04 a.m.
Live figure

Calculating local time

In the app you can move this figure with your finger.

Open in the app
  • Easy : Easy: West means subtract.
  • Medium : Medium: 30° E = +2 hours.
  • Exam-type : Exam: Find the time at 45° W if Greenwich is 12 noon.
Remember

Difference × 4 min; east add, west subtract.

Common mistake

Adding degrees when both places are on the same side. Subtract then.

Shortcut

Memory clue: "पूर्व = पुढे = +; पश्चिम = मागे = −".

Spot the difference

PlacesDegrees
Opposite sidesAdd
Same sideSubtract
Time eastAdd
Time westSubtract

Practice questions

Easy
  1. 1What is the base for finding time?
  2. 2What do we multiply by?
  3. 3For the east do we add or subtract?
Medium
  1. 460° E is how many hours ahead?
  2. 560° W is how many hours behind?
  3. 6Difference of 73° E and 88° E?
Challenging
  1. 7It is 12 noon at Greenwich. Find the time at 140° E (Tokyo).
  2. 8Find the difference between 30° E and 45° W.
Show answers (try them yourself first)
  1. 1Prime Meridian (0°).
  2. 24 minutes.
  3. 3Add.
  4. 44 hours.
  5. 54 hours.
  6. 615°.
  7. 7140 × 4 = 560 min = 9 h 20 min, so 9:20 p.m.
  8. 8Opposite sides: 30 + 45 = 75° = 300 min = 5 hours.

Standard time and time zones

From daily life

If every railway station used its own local time, a train timetable would be impossible. So a country picks one common clock.

Simple definition

Standard time (प्रमाणवेळ) is the local time of the longitude through the middle of a country, used for the whole country.

Rule 1: Why standard time — In a country that is wide east to west, local time at every place would cause confusion for trains, offices and business. So the central longitude's local time is the country's standard time. For global business, the world is divided into 24 time zones from the Prime Meridian, each 15° (1 hour).

180°90°W0°90°E180°Prime Meridian24 zones, 15° (1 hour) each

In the figure : In the figure the world is divided into 24 time zones of 15° each (1 hour); the middle zone is the zone of the Prime Meridian.

  • Easy : Easy: 24 zones.
  • Medium : Medium: 24 × 15° = 360°.
  • Exam-type : Exam: Why are time zones needed?

Rule 2: How many standard times — The number depends on the east–west extent, not the north–south extent. If the east–west extent is within one or two hours, one standard time is enough; if more, more than one. Canada has six. China and Sri Lanka use one. A country long north to south can still have just one.

5 standard times1 standard time1 standard timeeast–west

In the figure : In the figure a country that is wide from east to west needs several standard times; a country that is tall from north to south but narrow needs only one.

  • Easy : Easy: Canada has six standard times.
  • Medium : Medium: A country 45° wide differs by 45 × 4 = 180 min = 3 hours.
  • Exam-type : Exam: Does north–south extent matter?
Remember

Standard time = middle longitude's local time. 24 zones. More east–west width → more times.

Common mistake

Thinking tall countries need more standard times. Only width east–west matters.

Shortcut

Memory clue: "रुंदी = वेळा; उंची = एकच".

Spot the difference

CountryStandard times
Canada6
China1
Sri Lanka1

Practice questions

Easy
  1. 1What is standard time?
  2. 2Why is it needed?
  3. 3How many time zones are in the world?
Medium
  1. 4Width of each zone in degrees?
  2. 5How many standard times does Canada have?
  3. 6Name two countries with one standard time.
Challenging
  1. 7A country spans 30° east–west. Time difference between its ends?
  2. 8Why can a north–south long country have one time?
Show answers (try them yourself first)
  1. 1Local time of a country's central longitude.
  2. 2To avoid confusion in timetables and business.
  3. 324.
  4. 415°.
  5. 5Six.
  6. 6China and Sri Lanka.
  7. 730 × 4 = 120 min = 2 hours.
  8. 8Time depends on longitude, not latitude.

Indian Standard Time and GMT

From daily life

When a cricket match in India shows 7:30 p.m., a viewer in London sees 2:00 p.m. on his clock. Everyone's clock is set from one world reference, Greenwich.

Simple definition

IST (भारतीय प्रमाणवेळ) is the local time of 82°30′ E (Mirzapur) and is 5 h 30 min ahead of GMT (ग्रीनिच सरासरी वेळ).

Rule 1: IST — The 82°30′ E longitude passes through Mirzapur (near Allahabad, Uttar Pradesh), the middle of India's east–west extent. When the Sun is overhead on it, it is 12 noon everywhere in India. No place in India differs from IST by more than one hour. 82.5° × 4 = 330 min = 5 h 30 min. Mumbai (73° E) is (82.5 − 73) × 4 = 38 min behind IST; Kolkata (88° E) is (88 − 82.5) × 4 = 22 min ahead.

WEa.m.Mumbai73° Ep.m.IST · Mirzapur 82°30′ Ep.m.Kolkata88° E

In the figure : In the figure IST is the local time of 82°30′ E (Mirzapur): going by the clocks, Mumbai (73° E) is 38 minutes behind it and Kolkata (88° E) is 22 minutes ahead.

  • Easy : Easy: IST is for the whole country.
  • Medium : Medium: 82.5 − 73 = 9.5°; 9.5 × 4 = 38 min.
  • Exam-type : Exam: Why is 82°30′ E chosen?

Rule 2: GMT and the date line — Greenwich (England) time is GMT, the international standard; other countries' standard times are counted from it. IST = GMT + 5:30; 5 p.m. at Greenwich is 10:30 p.m. in India. The date changes at 180° (International Date Line), where a new day starts when it is 12 noon at the Prime Meridian.

12369a.m.12369a.m.+ 5 h 30 minaheadGreenwich (GMT)India (IST)

In the figure : In the figure IST is 5 hours 30 minutes ahead of GMT: when it is 5 a.m. at Greenwich, it is 10:30 a.m. in India.

  • Easy : Easy: IST = GMT + 5:30.
  • Medium : Medium: GMT 2 p.m. → IST 7:30 p.m.
  • Exam-type : Exam: IST is 9 a.m.; find GMT. 9:00 − 5:30 = 3:30 a.m.
0° noon180° date linenew day starts

In the figure : In the figure the date changes at the 180° longitude (International Date Line), the longitude opposite the Prime Meridian.

  • Easy : Easy: Date changes at 180°.
  • Medium : Medium: 0° + 180° = 180°, directly opposite.
  • Exam-type : Exam: What happens at the date line?

Rule 3: Jantar-Mantar and exact clocks — Maharaja Sawai Jai Singh II of Jaipur built five observatories at Ujjain, Varanasi, Jaipur, Delhi and Mathura (Mathura's no longer exists); time can still be read there in seconds from shadows. The most accurate clocks are made by NIST (USA), needing a 1-second correction once in 20 million years. In India the National Physical Laboratory (NPL), New Delhi, provides exact time.

  • Easy : Easy: Jantar-Mantar tells time by shadow.
  • Medium : Medium: Five observatories were built, one is lost, so four remain.
  • Exam-type : Exam: Who gives exact time in India?
Remember

IST = 82°30′ E = GMT + 5:30. Date changes at 180°.

Common mistake

Thinking India has several standard times. India uses only IST.

Shortcut

Memory clue: "IST = GMT + 5:30".

Spot the difference

ItemValue
IST longitude82°30′ E
IST vs GMT+5 h 30 min
Date line180°

Practice questions

Easy
  1. 1What is IST?
  2. 2Which longitude is used?
  3. 3Through which place does it pass?
Medium
  1. 4What is GMT?
  2. 5How far ahead of GMT is IST?
  3. 6Where does the date change?
Challenging
  1. 7It is 12 noon GMT. What is the IST?
  2. 8IST is 8 p.m. Find GMT.
Show answers (try them yourself first)
  1. 1The standard time of India.
  2. 282°30′ E.
  3. 3Mirzapur.
  4. 4The local time of Greenwich, the international standard.
  5. 55 h 30 min.
  6. 6At 180°.
  7. 75:30 p.m.
  8. 88:00 − 5:30 = 2:30 p.m.

Quick revision

  • Rotation: west to east, 24 hours, 360°. 15° = 1 hour. 1° = 4 minutes.
  • East = ahead (add). West = behind (subtract).
  • Local time depends on the longitude. Standard time is the local time of the central longitude of a country.
  • 24 time zones. IST = 82°30′ E = GMT + 5 hours 30 minutes. The date changes at 180°.