Why this chapter matters for UPSC: The Earth's two motions explain day and night, the year and the seasons. The solstice and equinox positions fix where the Sun is overhead, which is why the Tropic of Cancer matters so much for India's climate, and rotation produces the Coriolis force that steers winds and cyclones. In the rationalised 2023-24 edition this is still Chapter 3, "Motions of the Earth", with the same text.

Contemporary hook: The December solstice of 2026 falls at 20:50 UTC on 21 December, which is 02:20 IST on 22 December (US Naval Observatory). Most world calendars will mark 21 December, yet NCERT's "22nd December" is the correct date for India this year. Solstice and equinox dates are moments, and the calendar date depends on the time zone.

🧠 First Principles — Read This First

NCERT starts with an experiment: a ball for the Earth, a candle for the Sun and a mark for a town X. Turn the ball and the town passes from darkness into light (sunrise) and back again (sunset). That turning is rotation, the movement of the Earth on its axis. The second motion, revolution, is the movement of the Earth around the Sun in a fixed path, its orbit.

Three ideas carry the chapter:

  • Rotation gives day and night. Only half the globe faces the Sun at a time. The line dividing the lit half from the dark half is the circle of illumination. One rotation takes about 24 hours, the earthday, so rotation is the Earth's daily motion.
  • Revolution gives the year. One orbit takes about 365¼ days. The orbit is an ellipse.
  • The tilt gives the seasons. The axis makes an angle of 66½° with the orbital plane (23½° from the perpendicular) and stays inclined in the same direction all through the orbit. So for part of the year the Northern Hemisphere leans towards the Sun and for part of it the Southern Hemisphere does.

NCERT's summary is exact: there are days and nights, and changes in the seasons, "because of the rotation and revolution of the earth respectively".

PART 1 — Quick Reference

The Two Motions (NCERT)

MotionWhat movesTime takenResult
RotationThe Earth spins on its axisAbout 24 hours (one earthday)Day and night; the Coriolis force
RevolutionThe Earth moves around the Sun in an elliptical orbit365¼ days (one year)The year; with the tilted axis, the seasons
  • Axis: an imaginary line; it makes 66½° with the orbital plane (the plane formed by the orbit).
  • Leap year: the extra six hours each year add up to one day in four years. February gets 29 days and the year has 366 days.

Solstices and Equinoxes: NCERT Dates and the 2026 Times

NCERT gives fixed dates. The real events are moments that shift by a few hours each year, so the calendar date can differ between time zones. The 2026 times below are from the US Naval Observatory (USNO); IST is UTC + 5 h 30 min.

EventNCERT date2026 (UTC)2026 (IST)Sun overhead at
March equinox21 March20 March, 14:4620 March, 20:16Equator
June (summer) solstice21 June21 June, 08:2421 June, 13:54Tropic of Cancer (23½°N)
September equinox23 September23 September, 00:0523 September, 05:35Equator
December (winter) solstice22 December21 December, 20:5022 December, 02:20Tropic of Capricorn (23½°S)

In exams, use NCERT's dates unless a question gives a specific year. "Summer" and "winter" solstice are Northern Hemisphere names: on 21 June it is winter in the Southern Hemisphere.

Perihelion and Aphelion

PositionNCERT Class XIUSNO, 2025–2027
Perihelion (Earth nearest the Sun)3 January, 147 million km4 Jan 2025, 3 Jan 2026, 3 Jan 2027
Aphelion (Earth farthest from the Sun)4 July, 152 million km3 Jul 2025, 6 Jul 2026, 5 Jul 2027

The Earth is nearest the Sun in January, in the northern winter. NCERT Class XI notes that the Earth receives slightly more insolation on 3 January than on 4 July, but the effect is masked by other factors such as the distribution of land and sea. Distance does not cause the seasons; the tilt does.

PART 2 — Concepts & Narrative

Rotation: Day and Night

Key Term

Rotation: the movement of the Earth on its axis. The Earth turns from west to east, which is why the Sun appears to rise in the east and set in the west.

Circle of illumination: the circle on the globe that divides day from night. NCERT points out that it does not coincide with the axis, because the axis is tilted.

Earthday: the time the Earth takes to complete one rotation, about 24 hours. It is the daily motion of the Earth.

What if the Earth did not rotate? NCERT's answer: the half facing the Sun would have continuous day and warmth, and the other half would stay dark and freezing. Life would not have been possible in such extreme conditions.

A note on "about 24 hours". The Earth also moves about 1° along its orbit each day (360° in about 365 days). To bring the Sun back to the same place in the sky, it must turn that extra degree, which takes about 4 minutes (the Earth turns 1° in 4 minutes). So one turn measured against the distant stars is about 4 minutes shorter than the solar day of 24 hours. This is arithmetic from the chapter's own figures, not an NCERT statement.

Note

Aryabhata. NCERT records that the ancient Indian astronomer Aryabhata had stated that "the earth is round and rotates on its own axis".

Finding directions with a shadow stick (NCERT activity). Fix a one-metre stick on level ground on a sunny day and mark the tip of its shadow. The first mark is always towards the west. Mark the tip again after 15 minutes and join the two points: you have an approximate east-west line. Stand with the first mark on your left and the second on your right, and you face north. This works everywhere because the Earth rotates from west to east.

The Coriolis Force: Rotation Steers the Winds

Explainer

NCERT Class XI (Fundamentals of Physical Geography, "Atmospheric Circulation and Weather Systems") explains what rotation does to moving air:

  • The rotation of the Earth about its axis affects the direction of the wind. The force is called the Coriolis force, after the French physicist who described it in 1844.
  • It deflects the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is greater when the wind is faster.
  • It is maximum at the poles and absent at the equator. At the equator the wind blows straight into a low-pressure area and fills it instead of letting it intensify, which is why tropical cyclones do not form near the equator.
Pressure systemNorthern HemisphereSouthern Hemisphere
Cyclone (low pressure at the centre)AnticlockwiseClockwise
Anticyclone (high pressure at the centre)ClockwiseAnticlockwise

Revolution and the Leap Year

Key Term

Revolution: the movement of the Earth around the Sun in its orbit. It takes 365¼ days. We count a year as 365 days and set the six hours aside. Six hours a year make one day in four years, and that day is added to February. A year of 366 days is a leap year.

The orbit is an ellipse, and throughout it the Earth's axis stays inclined in the same direction. NCERT shows how to draw an ellipse with two pins, a loop of thread and a pencil: the closer the pins, the more nearly circular the ellipse.

The full leap-year rule. NCERT's "every fourth year" is the simple version. Under the Gregorian calendar, introduced in 1582 by Pope Gregory XIII, a year divisible by four is a leap year, except a year divisible by 100, which is a leap year only if it is also divisible by 400. So 1700, 1800 and 1900 were not leap years, but 2000 was (USNO). The rule gives 97 leap days in every 400 years, an average year of 365 + 97/400 = 365.2425 days, slightly less than NCERT's 365¼. The four-year rule alone runs a little ahead of the Sun, and dropping three century leap days in every 400 years corrects it.

India's national calendar is the Saka calendar, adopted from 22 March 1957 alongside the Gregorian calendar (Know India, Government of India).

Seasons: Why the Tilt Matters

Explainer

NCERT's activity makes the point: walk around a large ellipse drawn on the ground holding a flag that always points to the same far-off treetop. The flag stays inclined in the same direction wherever you are on the ellipse. In the same way, the Earth's axis keeps its inclination as it revolves. Revolution together with this fixed inclination causes the seasons.

When a hemisphere leans towards the Sun:

  • the Sun's rays fall on it more directly, so each part of the ground receives more heat; near the poles the rays are slanting and the heat is less;
  • a larger share of that hemisphere is lit, so days are longer than nights.

When it leans away, the rays are slanting and the nights are longer. The two hemispheres always have opposite seasons, which is why, as NCERT notes, Christmas in Australia falls in summer.

The Four Positions

Summer solstice, 21 June (NCERT). The Northern Hemisphere is tilted towards the Sun and the Sun's rays fall directly on the Tropic of Cancer. A large part of the Northern Hemisphere gets light, so it is summer north of the equator, with the longest day and the shortest night on 21 June. The conditions are reversed in the Southern Hemisphere, where it is winter.

Winter solstice, 22 December (NCERT). The South Pole tilts towards the Sun and the rays fall vertically on the Tropic of Capricorn (23½°S). It is summer in the Southern Hemisphere, with longer days, and winter in the north.

Equinoxes, 21 March and 23 September (NCERT). The direct rays fall on the equator. Neither pole is tilted towards the Sun, so the whole Earth has equal days and equal nights. On 23 September it is autumn in the Northern Hemisphere and spring in the Southern; on 21 March the opposite.

The Earth's four positions (NCERT dates)A flowchart of the Earth's four positions in its orbit, by NCERT's dates. On 21 March, the equinox, the Sun is overhead at the equator, days and nights are equal everywhere, and it is spring in the north and autumn in the south. On 21 June, the summer solstice, the Sun is overhead at the Tropic of Cancer, the north has its longest day, and it is summer in the north and winter in the south. On 23 September, the second equinox, the Sun is again over the equator, with autumn in the north and spring in the south. On 22 December, the winter solstice, the Sun is overhead at the Tropic of Capricorn, and the south has summer and longer days.The Earth's four positions (NCERT dates)21 March: equinoxSun overhead at the equatorEqual days and nights everywhereSpring in the north, autumn in the south21 June: summer solsticeSun overhead at the Tropic of CancerLongest day in the northSummer in the north, winter in the south23 September: equinoxSun overhead at the equatorEqual days and nights everywhereAutumn in the north, spring in the south22 December: winter solsticeSun overhead at the Tropic of CapricornLonger days in the southSummer in the south, winter in the north
Equinoxes are in black-edged boxes and solstices in orange-edged boxes. The dates are NCERT's; the exact moments shift by hours each year (2026: 20 March, 21 June, 23 September and 21 December, UTC, per the US Naval Observatory). Source: NCERT Class VI, ch. 3.
Note

Two refinements to NCERT's wording

  1. "Continuous daylight for about six months beyond the Arctic Circle." NCERT says this about the June solstice. Only the North Pole has about six months of continuous daylight. At the Arctic Circle (66½°N) the Sun stays above the horizon for about one day around the June solstice; the period of continuous daylight grows longer towards the pole. This follows from the geometry of the 23½° tilt: the Sun is overhead at 23½°N, so its rays just reach 90° − 23½° = 66½° beyond the North Pole and light every point up to the Arctic Circle through the whole rotation. NCERT's own exercise asks why the poles have about six months of day and six months of night. The answer: for half the orbit the North Pole leans towards the Sun and for the other half it leans away.
  2. "Equal days and equal nights" at the equinox. This is the geometric picture. In practice sunrise and sunset are timed when the Sun's upper edge touches the horizon, and the atmosphere bends sunlight so that the Sun appears about 34 arcminutes higher at the horizon (USNO). Both effects make the day a few minutes longer than the night on the equinox at most places. Exam answers follow NCERT.

Seasons in India: Two Schemes

The four astronomical positions above do not describe India's weather well, because India's year is shaped by the monsoon. Two Indian schemes are in use, and they differ slightly:

NCERT Class VI, ch. 8 (and NCERT Class XI)IMD rainfall seasonsIMD share of annual rainfall (1971–2020 normal)
Cold weather season (winter): December to FebruaryWinter: January–February3.4%
Hot weather season (summer): March to MayPre-monsoon: March–May11.3%
South-west monsoon season: June to SeptemberSouth-west monsoon: June–September74.9%
Season of retreating monsoon (autumn): October and NovemberPost-monsoon: October–December10.4%

IMD's 1971–2020 all-India annual normal is 1,160.1 mm. The NCERT scheme is the one used in the textbook chapters; IMD's is the one used in its rainfall statistics.

PART 3 — UPSC Integration

The chapter is the physical basis of much of GS1 geography. The overhead Sun moves between the Tropics through the year, and the Tropic of Cancer runs almost halfway through India (see ch. 7), so at the June solstice the Sun is overhead along a line that crosses the middle of the country. Rotation produces the Coriolis force, which decides how winds turn around pressure systems and why cyclones spin anticlockwise in the Bay of Bengal and the Arabian Sea. The same longitude-and-rotation logic gives local time and Indian Standard Time (ch. 2). The south-west monsoon season brings about three-quarters of India's rainfall (IMD), which is why the seasons in this chapter connect to agriculture, water and the economy in GS3.

Exam Strategy

Prelims traps:

  • Seasons come from the tilt, not the distance. The Earth is nearest the Sun in early January, during the northern winter.
  • NCERT dates are rounded. 21 June, 22 December, 21 March and 23 September are the textbook answers; the actual moment shifts by hours each year (for example 2026: 20 March, 21 June, 23 September, 21 December UTC).
  • Coriolis force is zero at the equator and greatest at the poles; cyclones spin anticlockwise in the Northern Hemisphere.
  • 1900 was not a leap year; 2000 was. Century years must be divisible by 400.
  • Only the poles have six months of daylight. At the Arctic Circle continuous daylight lasts about a day.
  • The axis makes 66½° with the orbital plane, which is the same as a tilt of 23½° from the perpendicular. Read which one a question asks for.

Mains angle: Explain why the two hemispheres have opposite seasons (fixed inclination of the axis plus revolution), and link the overhead Sun and the Coriolis force to the Indian monsoon and cyclones.

Practice Questions

Practice (UPSC-pattern, not past papers).

Prelims:

  1. According to NCERT, the Sun's rays fall vertically on the Tropic of Cancer on:
    (a) 21 June
    (b) 21 March
    (c) 22 December
    (d) 23 September

  2. The angle that the Earth's axis makes with its orbital plane is:
    (a) 23½°
    (b) 66½°
    (c) 90°
    (d) 45°

  3. The Earth's seasons are caused mainly by:
    (a) the changing distance between the Earth and the Sun
    (b) revolution, with the axis inclined in the same direction throughout the orbit
    (c) the rotation of the Earth
    (d) the Moon's gravitational pull

  4. In which month is the Earth nearest the Sun?
    (a) July
    (b) January
    (c) March
    (d) June

  5. Consider the following statements about the Coriolis force:

    1. It deflects winds to the right in the Northern Hemisphere.
    2. It is maximum at the equator.
    3. Its absence near the equator is the reason tropical cyclones do not form there.
      Which of the statements given above are correct?
      (a) 1 and 2 only
      (b) 1 and 3 only
      (c) 2 and 3 only
      (d) 1, 2 and 3

  6. Which of the following years was NOT a leap year?
    (a) 1600
    (b) 1900
    (c) 2000
    (d) 2024

Mains:

  1. Why do the Northern and Southern Hemispheres have opposite seasons at the same time? Explain with reference to the solstices and equinoxes. (150 words)

📦 Revision Capsule

Revision Capsule

Hard Facts

  • Rotation: on the axis, west to east, about 24 hours (earthday) → day and night; the circle of illumination divides them.
  • Revolution: around the Sun in an elliptical orbit, 365¼ days → the year; leap year = 366 days, February 29.
  • Axis: 66½° with the orbital plane, inclined in the same direction throughout the orbit → seasons.
  • NCERT dates: summer solstice 21 June (Tropic of Cancer), winter solstice 22 December (Tropic of Capricorn), equinoxes 21 March and 23 September (equator).
  • Perihelion about 3 January (147 million km); aphelion about 4 July (152 million km) (NCERT Class XI).
  • Leap-year rule: divisible by 4, except century years not divisible by 400 (1900 no, 2000 yes).

Core Concepts

  • Rotation → day and night; revolution plus fixed tilt → seasons.
  • Hemispheres have opposite seasons; distance from the Sun does not cause seasons.
  • Coriolis force: right in the north, left in the south, zero at the equator.

Confused Pairs

  • 66½° (axis with the orbital plane) vs 23½° (axis from the perpendicular; latitude of the Tropics)
  • Solstice (Sun overhead at a Tropic; longest or shortest day) vs equinox (Sun overhead at the equator)
  • Perihelion (January, nearest) vs aphelion (July, farthest)
  • Cyclone (anticlockwise in NH) vs anticyclone (clockwise in NH)

PYQ Pattern

  • No UPSC question in the BharatNotes bank is on this chapter directly. Its ideas (overhead Sun, tilt, Coriolis force) underlie the GS1 questions on climate and the monsoon listed on the Class XI climate pages.

Sources

Sources

  • NCERT, The Earth: Our Habitat (Class VI), ch. 3 "Motions of the Earth" (2020-21 print), and the Rationalised 2023-24 edition, ch. 3 (same text): NCERT's book archives as captured by the Wayback Machine, 2020 copy and 2023-24 copy.
  • NCERT, Fundamentals of Physical Geography (Class XI): ch. 8 "Solar Radiation, Heat Balance and Temperature" (perihelion and aphelion), kegy208; ch. 9 "Atmospheric Circulation and Weather Systems" (Coriolis force, Table 9.2), kegy209.
  • US Naval Observatory, Astronomical Applications: Earth's seasons, 2026 (also queried for 2024, 2025 and 2027); Introduction to Calendars (Gregorian leap-year rule, 1582); Rise, Set, and Twilight Definitions (upper edge of the Sun, 34′ refraction). Read 2-Oct-2026.
  • India Meteorological Department, press release on updated rainfall normals (1971–2020), 14-Apr-2022, Table 1(b): imd.gov.in PDF.
  • Know India (Government of India), "National Calendar": Wayback copy, 18-Apr-2026.