Consider the following statements regarding the rotation of the Earth : 1. Since the Earth rotates by 15° each hour, time zones normally differ by one hour 2. At the International Date Line, the calendar day changes - advancing a day for eastward travel, dropping back a day for westward travel 3. Daylight saving time advances the clock by one hour 4. The Moon rotates and revolves about the Earth in the same direction that the Earth revolves around the Sun Which of the statements given above is/are correct?

Correct — C, 1, 3 and 4. Statement 1 is arithmetic: the Earth turns 360 degrees in 24 hours, that is 15 degrees an hour, and standard time zones are drawn about 15 degrees wide so that neighbouring zones differ by an hour. Statement 3 is a definition: daylight saving time moves the clock forward by one hour in summer so that working hours match daylight. Statement 4 is correct as well — the Moon orbits the Earth and rotates on its axis in the same anticlockwise sense, seen from above the north pole, in which the Earth orbits the Sun. Statement 2 is the one that fails, because it reverses the rule. Crossing the International Date Line travelling east you repeat a day, that is you go back a day; travelling west you skip forward a day. The statement says the opposite.

• (a)1 only — This keeps only statement 1 and discards two correct statements. Daylight saving does advance the clock by an hour, and the Moon's motions are indeed in the same sense as the Earth's revolution.
• (b)3 and 4 only — This drops statement 1, which is the most secure of the four. Fifteen degrees an hour follows directly from 360 degrees in 24 hours, and it is why time zones are an hour apart.
• (d)2, 3 and 4 — This includes statement 2, the only incorrect one. It also drops statement 1. Crossing the date line eastward takes you back a day, not forward.

The Earth rotates west to east at 15 degrees an hour, which is why places to the east see the Sun earlier and why the world is divided into time zones roughly 15 degrees wide. The International Date Line follows the 180-degree meridian with deviations to keep island groups and countries whole. It exists because travelling east adds an hour per zone and travelling west subtracts one; without a line where the date changes, a traveller circling the globe would return with the wrong date. Daylight saving time is a separate seasonal adjustment used in many temperate countries but not in India.

The date-line direction is the single most confused fact in this whole topic, so it is worth pinning down with a concrete case. Fly east from Tokyo to Honolulu and you land on the previous calendar date; fly west from Honolulu to Tokyo and you lose a day from the calendar. The mnemonic is that going east you gain hours as you cross zones, so the line must take a whole day back from you to keep the calendar consistent. India does not observe daylight saving, and it uses a single time zone based on 82.5 degrees east, which is 5 hours 30 minutes ahead of Greenwich.

• The Earth turns 15 degrees of longitude in an hour, which sets the width of a standard time zone.
• Crossing the International Date Line eastward takes you back a day; crossing it westward takes you forward a day.
• The date line follows the 180-degree meridian but bends to keep countries and island groups on one date.
• Daylight saving time advances clocks by one hour in summer; India does not use it.
• Indian Standard Time is based on 82.5 degrees east and runs 5 hours 30 minutes ahead of Greenwich.

• Reversing the date-line rule, which is what statement 2 is built to catch.
• Assuming India observes daylight saving because many countries do.
• Doubting statement 4 because the Moon's rotation is easy to overlook.

A four-statement code item with one reversal buried in it. Confirming the arithmetic of statement 1 first removes half the option list.

• practice — not a real PYQ

  How many degrees of longitude does the Earth rotate through in four minutes?

  • (a)One degree
  • (b)Four degrees
  • (c)Fifteen degrees
  • (d)Sixty degrees

  Answer(a) One degree — 15 degrees an hour works out at one degree every four minutes.

• practice — not a real PYQ

  Indian Standard Time is calculated from which meridian?

  • (a)75 degrees E
  • (b)82.5 degrees E
  • (c)90 degrees E
  • (d)68.7 degrees E

  Answer(b) 82.5 degrees E — which puts IST 5 hours 30 minutes ahead of Greenwich.