Which one of the following statements about the solstices, an event that occurs when the Sun appears to reach most northerly or southerly, is correct?

Correct — C, the summer solstice occurs in the northern hemisphere on June 21 and on December 21 in the southern hemisphere. A solstice is a moment in the Earth's orbit, and there are only two of them a year. Around 21 June the northern end of the axis leans towards the Sun, the Sun is overhead at the Tropic of Cancer, and the northern hemisphere has its longest day — its summer solstice. The same instant is the southern hemisphere's winter solstice. Around 21 or 22 December the geometry is reversed and the Sun is overhead at the Tropic of Capricorn, giving the southern hemisphere its summer solstice and the northern its winter one. The date is shared; only the seasonal label flips.

• (a)The winter solstice takes place on June 21 in both the northern and southern hemispheres — June 21 cannot be the winter solstice in both hemispheres. On that date the Sun stands overhead at the Tropic of Cancer, so the northern hemisphere is having its longest day, not its shortest.
• (b)The winter solstice takes place on December 21 in both the northern and southern hemispheres — December 21 is the winter solstice in the northern hemisphere only. In the southern hemisphere the same date brings the longest day, because that half of the Earth is then tilted towards the Sun.
• (d)The summer solstice occurs in northern hemisphere on December 21 and on June 21 in southern hemisphere — This reverses both dates. The northern summer solstice is in June, not December, because the axial tilt points the northern end towards the Sun in the northern midsummer.

The Earth's axis is tilted about 23.5 degrees from the perpendicular to its orbital plane, and it keeps pointing in the same direction in space through the year. That fixed tilt, not any change in distance from the Sun, produces the seasons. Twice a year the Sun's overhead position reaches its limit — the Tropic of Cancer in June and the Tropic of Capricorn in December — and those two moments are the solstices. Midway between them the Sun is overhead at the equator and day and night are equal, which are the equinoxes of about 21 March and 23 September.

The wording of the options is the whole difficulty. Every option pairs a date with a season, and only one gets both hemispheres right at once. Fix a single anchor and the rest follows: on 21 June the Sun is overhead at the Tropic of Cancer, which is in the northern hemisphere. Another useful check is that the Earth is actually nearest the Sun in early January, during the northern winter, which proves the seasons cannot be caused by distance.

• The Earth's axis is tilted about 23.5 degrees, and the direction of the tilt stays fixed through the orbit.
• About 21 June the Sun is overhead at the Tropic of Cancer: the northern summer solstice and the southern winter solstice.
• About 21 or 22 December the Sun is overhead at the Tropic of Capricorn: the southern summer solstice.
• The equinoxes fall about 21 March and 23 September, when the Sun is overhead at the equator.
• The Earth is at perihelion in early January, so distance from the Sun does not cause the seasons.

• Assuming a solstice date carries the same season everywhere on Earth.
• Explaining seasons by the Earth's distance from the Sun.
• Mixing up the Tropic of Cancer and the Tropic of Capricorn when placing the June solstice.

A four-way statement item in which the options differ only in which date goes with which hemisphere. One anchor fact settles all four.

• practice — not a real PYQ

  On 22 December the Sun's rays are vertically overhead at which latitude?

  • (a)Equator
  • (b)Tropic of Cancer
  • (c)Tropic of Capricorn
  • (d)Arctic Circle

  Answer(c) Tropic of Capricorn — 23.5 degrees south, which is why the southern hemisphere has its summer then.

• practice — not a real PYQ

  On which pair of dates are day and night of nearly equal length everywhere on Earth?

  • (a)21 March and 23 September
  • (b)21 June and 22 December
  • (c)1 January and 1 July
  • (d)21 March and 21 June

  Answer(a) 21 March and 23 September — the two equinoxes, when the Sun is overhead at the equator.