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DGCA Air Navigation · 77 questions

Solar System & Time: DGCA Air Navigation Questions and Answers

77 practice questions on Solar System & Time with answers and explanations, plus the key concepts and formulas, from the TrueHeading question bank.

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About this topic

Use this page to revise Solar System & Time for the DGCA Air Navigation paper. Below are the key ideas first, then 12 practice questions with answers and explanations, picked from the 77 questions in the TrueHeading bank for this topic.

In short: The Earth turns 360° in 24 h, so 15° = 1 hour and 1° = 4 minutes.

Open the full set in the student zone to answer every question, track your accuracy and take timed mock tests.

Key concepts: Solar System & Time

6 ideas to know cold.

How do longitude and time relate?

The Earth turns 360° in 24 h, so 15° = 1 hour and 1° = 4 minutes.

LMT=UTC+longitude E15\text{LMT}=\text{UTC}+\frac{\text{longitude E}}{15}
  • East is later (add), west is earlier (subtract)
  • IST = UTC + 5 h 30 min, based on 82.5° E

What are civil, nautical and astronomical twilight?

Defined by how far the centre of the Sun is below the horizon:

  • Civil: 6° below
  • Nautical: 12° below
  • Astronomical: 18° below

Why do we have seasons, and where are the tropics?

The Earth’s axis is tilted about 23.5° to the orbit plane.

  • Tropic of Cancer 23.5° N, Tropic of Capricorn 23.5° S
  • Arctic and Antarctic circles at 66.5°
  • Equinoxes about 21 March and 23 September; solstices about 21 June and 22 December
  • Earth is closest to the Sun (perihelion) in early January

Crossing the International Date Line?

It follows 180° longitude. Going east you repeat a day (date goes back one); going west you skip a day (date goes forward).

Tip: East = lose nothing on the clock but gain a date: subtract a day.

How are standard time and UTC related?

Standard time = UTC + longitude/15 (rounded to the nearest hour, east positive).

LMT=UTC+λE15\text{LMT}=\text{UTC}+\dfrac{\lambda_E}{15}
  • India: UTC + 5:30
  • Local mean time: 4 minutes per degree of longitude
  • Crossing the date line westward: add a day

What causes the seasons?

The tilt of the earth's axis (23.5°) to the plane of its orbit.

  • Sun overhead the Tropic of Cancer on 21 June, the equator on 21 March and 23 September, Capricorn on 21 December
  • Day length varies with latitude and season
  • Civil twilight: sun 6° below the horizon

Practice questions: Solar System & Time

Tap “Show answer” after you have tried each one.

Q1When it is 2300 UTC, the local mean time at longitude 75°E is:

  1. 1800
  2. 0500
  3. 0300
  4. 0400
Show answer

Answer: D. 0400

LMT = UTC + (longitude × 4 min) = 2300 + 5h00min = 0400.

Q2When it is 1030 UTC, the local mean time at longitude 135°E is:

  1. 0130
  2. 1930
  3. 2030
  4. 1830
Show answer

Answer: B. 1930

LMT = UTC + (longitude × 4 min) = 1030 + 9h00min = 1930.

Q3When it is 1500 UTC, the local mean time at longitude 20°E is:

  1. 1720
  2. 1340
  3. 1520
  4. 1620
Show answer

Answer: D. 1620

LMT = UTC + (longitude × 4 min) = 1500 + 1h20min = 1620.

Q4When it is 1300 UTC, the local mean time at longitude 60°W is:

  1. 0900
  2. 1700
  3. 1000
  4. 0800
Show answer

Answer: A. 0900

LMT = UTC − (longitude × 4 min) = 1300 − 4h00min = 0900.

Q5When it is 1230 UTC, the local mean time at longitude 30°W is:

  1. 1130
  2. 1430
  3. 0930
  4. 1030
Show answer

Answer: D. 1030

LMT = UTC − (longitude × 4 min) = 1230 − 2h00min = 1030.

Q6When it is 1400 UTC, the local mean time at longitude 82.5° East is:

  1. 1930 LMT
  2. 2215 LMT
  3. 0830 LMT
  4. 2030 LMT
Show answer

Answer: A. 1930 LMT

LMT = UTC + longitude ÷ 15 hours (east is ahead) = 1400 + 5.50 h = 1930.

Q7When it is 0200 UTC, the local mean time at longitude 15° East is:

  1. 0300 LMT
  2. 0100 LMT
  3. 0330 LMT
  4. 0400 LMT
Show answer

Answer: A. 0300 LMT

LMT = UTC + longitude ÷ 15 hours (east is ahead) = 0200 + 1.00 h = 0300.

Q8When it is 1300 UTC, the local mean time at longitude 15° East is:

  1. 1200 LMT
  2. 1400 LMT
  3. 1430 LMT
  4. 1500 LMT
Show answer

Answer: B. 1400 LMT

LMT = UTC + longitude ÷ 15 hours (east is ahead) = 1300 + 1.00 h = 1400.

Q9It is 2100 local mean time at longitude 45° West. The UTC is:

  1. 1800 UTC
  2. 0130 UTC
  3. 0000 UTC
  4. 2100 UTC
Show answer

Answer: C. 0000 UTC

West longitude: UTC = LMT + longitude ÷ 15 = 2100 + 3 h = 0000.

Q10It is 1200 local mean time at longitude 90° West. The UTC is:

  1. 0600 UTC
  2. 1800 UTC
  3. 2100 UTC
  4. 1200 UTC
Show answer

Answer: B. 1800 UTC

West longitude: UTC = LMT + longitude ÷ 15 = 1200 + 6 h = 1800.

Q11India Standard Time is UTC + 5 h 30 min. When it is 1700 IST the UTC is:

  1. 1100 UTC
  2. 1200 UTC
  3. 2230 UTC
  4. 1130 UTC
Show answer

Answer: D. 1130 UTC

UTC = IST − 5 h 30 min = 1130.

Q12Two places are on longitudes 40° E and 20° E. The difference in local mean time is:

  1. 240 minutes
  2. 80 minutes
  3. 160 minutes
  4. 40 minutes
Show answer

Answer: B. 80 minutes

Difference = 20° ÷ 15 = 1.33 h = 80 min. The place further east is ahead.

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