Which of the following statements is/are correct? 1. The time at 180° longitude is exactly 12 hours from the 0° longitude, irrespective of whether one travels westward or eastward from the Prime Meridian. 2. A person moving eastward on Tuesday will count the day as Wednesday once the International Date Line is crossed. Select the answer using the code given below.

Correct — A, 1 only. Statement 1 is sound. Local time changes by one hour for every 15 degrees of longitude, so 180 degrees works out at exactly twelve hours whichever way you travel — twelve hours ahead of Greenwich going east, twelve hours behind going west. The clock reading at 180 degrees is the same either way; only the date differs, which is why the Date Line is drawn there. Statement 2 gets that date change backwards. The side of the line at 179 degrees east is a day ahead of the side at 179 degrees west, so a traveller heading east crosses from the earlier date into the later one and has to repeat a day: Tuesday becomes Monday. It is the westward traveller who skips forward from Tuesday to Wednesday. UPSC's own NDA paper of 2022 keyed this the same way — flying east across the line, you lose a day.

• (b)2 only — Rejects the correct statement and keeps the wrong one. The twelve-hour difference at 180 degrees is arithmetic — 180 divided by 15 — and holds in both directions of travel.
• (c)Both 1 and 2 — The tempting choice, because both sentences sound like textbook lines. Statement 2 reverses the direction of the date change; eastward across the line the day is repeated, not advanced.
• (d)Neither 1 nor 2 — Discards a true statement. The Earth turns through 15 degrees of longitude an hour, so 180 degrees is twelve hours from the Prime Meridian by definition.

The Earth turns 360 degrees in 24 hours, so 15 degrees of longitude corresponds to one hour, and 180 degrees to twelve. Going east clocks run ahead; going west they run behind. Those two twelve-hour offsets meet at the 180th meridian, where a difference of a full day accumulates, and the International Date Line is drawn there — deviating around Siberia, the Aleutians, Kiribati and other island groups so that no inhabited territory is split across two dates.

Fix the rule with a journey rather than a formula. Flying east from Tokyo to Honolulu takes you from the calendar-ahead side of the line to the calendar-behind side, and you land on the same date you left, or even the day before. Flying west from Honolulu to Tokyo skips a day. A memory hook that survives the exam hall: going east you gain hours but lose a day; going west you lose hours but gain a day. The examiner relies on candidates carrying only the hours half of that pair.

• One hour of time corresponds to 15 degrees of longitude, so 180 degrees is exactly twelve hours from the Prime Meridian.
• Time at 180 degrees differs from Greenwich by twelve hours whether you travel east or west; only the date differs.
• Crossing the International Date Line eastward, a traveller repeats a day — Tuesday becomes Monday.
• Crossing it westward, a traveller skips a day — Tuesday becomes Wednesday.
• The line follows the 180th meridian but bends around Siberia, the Aleutians, Kiribati and other island groups.

• Reversing the direction of the date change at the Date Line.
• Assuming the Date Line runs exactly along the 180th meridian everywhere.
• Confusing the twelve-hour clock difference with the one-day date difference.

A two-statement item in which one sentence is textbook-accurate and the other is the same textbook sentence with its direction reversed.

• practice — not a real PYQ

  A traveller flying westward across the International Date Line on Sunday will find the date to be

  • (a)Saturday
  • (b)Sunday
  • (c)Monday
  • (d)Friday

  Answer(c) Monday — a westward crossing skips a day.

• practice — not a real PYQ

  The time difference between two places 45 degrees of longitude apart is

  • (a)1 hour
  • (b)2 hours
  • (c)3 hours
  • (d)4 hours

  Answer(c) 3 hours — 15 degrees of longitude corresponds to one hour.