Which one of the following correctly explains the change in seasons on Earth?

Correct — A, Tilt of the Earth's axis. The Earth's axis is tilted about 23½° to the plane of its orbit and, crucially, keeps pointing in the same direction in space all year. So as the planet goes round the Sun, first one hemisphere and then the other leans towards it. The leaning hemisphere receives the Sun's rays closer to the vertical and gets longer days, which is summer; six months later the same hemisphere leans away, the rays strike obliquely and the days shorten, which is winter. Take the tilt away and every latitude would receive the same insolation all year — the Earth would still revolve, but there would be no seasons at all.

• (b)Rotation of the Earth on its own axis — Rotation produces day and night and the apparent daily movement of the Sun across the sky. It repeats every 24 hours, so it cannot explain a cycle that takes a year to complete.
• (c)Revolution of the Moon around the Sun — Wrong on the facts as well as the physics. The Moon revolves around the Earth, and the Earth-Moon system together orbits the Sun. The Moon governs the tides and the lunar month; it has no role in the seasons.
• (d)Interaction of the Earth with other planets — Gravitational tugs from the other planets nudge the Earth's orbit over tens of thousands of years and have nothing to do with an annual cycle. Nor does distance from the Sun explain the seasons — the Earth is actually nearest the Sun in early January, in the middle of the northern winter.

Seasons need two ingredients working together: an axis tilted to the orbital plane, and a year-long revolution during which the tilt keeps its direction fixed. The tilt is the cause the question is after, because it is what makes the two hemispheres take turns. Revolution alone with an upright axis would give no seasons; a tilt with no revolution would give a permanent season.

The strongest way to test any explanation of the seasons is the January check. The Earth reaches perihelion, its closest approach to the Sun, in the first week of January — the depth of winter in the northern hemisphere and high summer in the southern. If distance drove the seasons, both hemispheres would be warm together. They are not, because what matters is the angle at which sunlight strikes the ground and how long it stays above the horizon, and both of those are set by the tilt. The same geometry produces the calendar landmarks: the June solstice around 21 June with the Sun overhead at the Tropic of Cancer, the December solstice around 22 December with it overhead at the Tropic of Capricorn, and the two equinoxes around 21 March and 23 September when the Sun is overhead at the equator and day and night are equal everywhere.

• The Earth's axis is inclined at about 23½° (23°26′) to the perpendicular to its orbital plane and maintains that direction through the year.
• The leaning hemisphere gets more vertical rays and longer days — that is summer; the other gets oblique rays and shorter days.
• Summer solstice in the northern hemisphere is around 21 June, winter solstice around 22 December; equinoxes fall around 21 March and 23 September.
• The Earth is closest to the Sun (perihelion) in early January and farthest (aphelion) in early July, so distance cannot be the cause of the seasons.
• Rotation gives day and night; revolution gives the year; the tilt is what turns the year into seasons.

• Explaining the seasons by the Earth's varying distance from the Sun.
• Attributing the seasons to revolution alone, forgetting that an upright axis would give none.
• Accepting an option that misstates a basic fact, such as the Moon revolving around the Sun.

Asked as a single-cause item where the three wrong options are real motions or interactions that produce something else entirely.

• practice — not a real PYQ

  On which one of the following dates is the Sun vertically overhead at the Tropic of Capricorn?

  • (a)21 March
  • (b)21 June
  • (c)23 September
  • (d)22 December

  Answer(d) 22 December — the December solstice, when the southern hemisphere is tilted most towards the Sun.

• practice — not a real PYQ

  If the Earth's axis were perpendicular to the plane of its orbit, which one of the following would follow?

  • (a)There would be no day and night
  • (b)There would be no seasons
  • (c)The year would be shorter
  • (d)The equator would be permanently cold

  Answer(b) There would be no seasons — every latitude would receive the same insolation all year, though day and night would continue as before.