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Space Physics: Question 2

Syllabus 6.1.1

Structured Core 6 marks

A student living in Wellington, New Zealand keeps an astronomy journal. She writes to a pen-friend, Lars, who lives in Tromsø, Norway, a city at a similarly high latitude, but in the Northern Hemisphere.

(a) In July, the student in Wellington records short, cold days, while at the very same time Lars in Tromsø records long, warm days. Explain, in terms of the tilt of the Earth's axis and the Earth's orbit around the Sun, why the two friends experience opposite seasons in July. [3]

(b) Over the next four weeks, the student also sketches the changing shape of the illuminated part of the Moon that she can see each night from Wellington. Describe, in terms of the changing relative positions of the Sun, Earth and Moon, why the illuminated shape of the Moon that she observes changes during this time, and state approximately how long one complete cycle of phases takes. [3]

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Worked solution

Part (a): Opposite seasons in July

The Earth’s axis is tilted (at about 23.5∘23.5^\circ to the plane of its orbit), and this tilt stays pointing in the same direction in space throughout the year as the Earth orbits the Sun.

In July, the Earth is at the point in its orbit where:

  • The Southern Hemisphere (which includes Wellington) is tilted away from the Sun. Sunlight strikes this hemisphere at a shallower angle, spreading the same amount of energy over a larger area, and the days are shorter, so Wellington experiences winter.
  • The Northern Hemisphere (which includes Tromsø) is tilted toward the Sun at the same time. Sunlight strikes this hemisphere more directly, and the days are longer, so Tromsø experiences summer.

Because the two cities are on opposite hemispheres, they are always tilted in opposite ways relative to the Sun, so they experience opposite seasons at the same time of year.

Part (b): The changing phases of the Moon

The Moon is not a source of light; it only reflects sunlight. At any moment, exactly one half of the Moon is lit by the Sun, while the other half is in darkness.

As the Moon orbits the Earth (taking about a month to complete one orbit), the relative positions of the Sun, Earth and Moon keep changing. This means the fraction of the Moon’s sunlit half that is visible from Earth also keeps changing. Sometimes we see almost all of the lit half (a full moon), sometimes almost none of it (a new moon), and in between we see a crescent or gibbous shape. This is why the illuminated shape of the Moon appears to change from night to night.

One complete cycle, from one full moon (or new moon) back to the next, takes approximately one month (about 29.5 days).

Final answers

  • (a) Wellington has winter and Tromsø has summer in July because the tilted Earth’s axis keeps pointing the same way in space: the Southern Hemisphere is tilted away from the Sun (winter) while the Northern Hemisphere is tilted toward it (summer) at the same time.
  • (b) The Moon’s illuminated shape changes because the changing relative positions of the Sun, Earth and Moon reveal a different fraction of the Moon’s sunlit half each night; one full cycle of phases takes about one month (≈29.5 days).