Why We Have Seasons, Day and Night
FoundationThe Earth spins once in about twenty-four hours, giving day and night; it orbits the Sun in about a year, and its tilted axis (about twenty-three and a half degrees) gives Hong Kong long hot summer days and short cool winter days — the four seasons.
- The Earth's rotation takes about twenty-four hours and gives day and night.
- One revolution round the Sun takes about three hundred and sixty-five days — a year.
- The axis tilted about twenty-three and a half degrees causes the seasons.
文體小知識：科學知識說明文解釋自然現象的成因，配上模型實驗，抽象的天文知識也能看得見。
§Text Passage
1The Earth spins from west to east without stopping, taking about twenty-four hours for one turn — our day. The side facing the Sun has daytime; the side turned away has night. As the Earth spins, day and night take turns. In a darkened room, shine a torch on a spinning globe to model this: the lit part is day, the dark part is night.
2While spinning, the Earth also travels round the Sun along its orbit — a journey called revolution that takes about three hundred and sixty-five days, our year. The Earth's axis is tilted at about twenty-three and a half degrees, and whichever point of the orbit the Earth reaches, the tilt keeps pointing the same way.
3The seasons come from the tilted axis and the revolution together. When the northern hemisphere leans towards the Sun, sunlight strikes it directly, the days are long and the weather hot — summer in Hong Kong. When the northern hemisphere leans away, sunlight strikes at a slant, the days are short and the weather cool — winter. Spring and autumn are the passages between.
4Try a model experiment: in a dark room use a torch as the Sun and carry the globe round it, keeping the tilt unchanged. Watch Hong Kong's position: sometimes the daylight hours are long and the light strikes straight on, like summer; sometimes the day is short and the light slants, like winter. Day, night and the seasons all come from the Earth's movements.
1Summary 重點摘要
The Earth spins once in about twenty-four hours, giving day and night; it orbits the Sun in about a year, and its tilted axis (about twenty-three and a half degrees) gives Hong Kong long hot summer days and short cool winter days — the four seasons.
2Vocabulary cards Vocabulary
The Earth spinning on its own axis, one turn in about twenty-four hours, giving day and night.
The Earth travelling round the Sun along its orbit, one loop in about three hundred and sixty-five days — a year.
The imaginary line the Earth spins around, tilted about twenty-three and a half degrees.
The alternation of day and night, caused by the Earth's rotation.
Sunlight striking the ground straight on, concentrating the heat and making weather hot.
Sunlight striking at a slant, spreading the heat and making weather cool.
The path the Earth follows as it travels round the Sun.
Half of the Earth: north of the equator is the northern hemisphere, south is the southern; their seasons are opposite.
A tool that stands in for the real thing, such as a torch and globe to show day and night.
3Step by step Steps
- Step 1: Model day and night with a globe and torchDay and night can be modelled with a simple experiment. Turn off the room light and let a torch stand for the Sun, shining on the globe: lit-up China is in daytime while dark America is in night, and both exist at the same moment. The experiment shows that at any time half of the Earth faces the light and half faces away. Keep the distance between the torch and the globe unchanged, moving only the spot of light over the globe — then the model is accurate.
- Step 2: See that rotation makes day and nightThe Earth spins like a top without stopping — this is rotation. Turn the globe slowly from west to east for one full turn, standing for the twenty-four hours one rotation takes. Hong Kong moving from light into dark is sunset and nightfall, and moving back into the light is sunrise. Day and night alternate because the Earth keeps rotating. Watch Hong Kong's position as you turn: facing the torch is daytime, turned to the back is night, taking turns.
- Step 3: Learn revolution and the yearWhile it spins, the Earth also travels round the Sun — this is revolution. One circle takes about three hundred and sixty-five days, making one year. Christmas always falls on the twenty-fifth of December because each year the Earth reaches the same point of its orbit at that time; every completed circle starts the seasons anew. Revolution runs the same way as rotation, from west to east; the year also has leap years, adding one day every four years.
- Step 4: Observe the tilt of the axisLook closely at a globe stand: the axis through the Earth is not upright but tilted about twenty-three and a half degrees. However the globe spins, the tilt keeps pointing the same way all round. This tilt is the key reason the seasons exist, so observe it carefully and the next experiment will be easy to understand. The north end of the axis points roughly towards the North Star, which is why its direction never changes — the key to why seasons arise.
- Step 5: Model the seasonsKeep the axis tilted and carry the globe round the torch; the four positions stand for summer, autumn, winter and spring. At the summer position the northern hemisphere leans towards the Sun, the light strikes directly and the days are long, so the weather is hot; at the winter position it leans away, the light strikes at a slant, the days are short and the weather cool. The hemispheres' seasons are exactly opposite: when the north has summer the south has winter, and Christmas in Australia falls in midsummer.
- Step 6: Link to real-life observationThe experiment can be checked against real life. Compare sunrise and sunset times in Hong Kong: in June the Sun rises near five forty in the morning and sets after seven at night, giving more than thirteen hours of daylight; in December it rises near seven and sets near five forty-five, only about eleven hours. Long summer days and short winter days confirm the tilted axis and revolution. Check last year's weather records, or ask your parents to help record this year's sunrise and sunset times — then the observation stands on solid ground.
4Diagrams Diagrams
5Worked example Example
Example: Dad says, Christmas daylight is shorter than July's, and it is cooler too. Use the tilted axis and revolution to explain why Hong Kong is like this.
- In December the Earth has revolved to the part of its orbit where the northern hemisphere leans away from the Sun.
- Sunlight strikes the northern hemisphere at a slant, so the same patch of ground receives less heat and the weather turns cool.
- The northern hemisphere's days are shorter too, so December in Hong Kong has short days and long nights.
- July is just the opposite: the northern hemisphere leans towards the Sun, sunlight strikes directly, the days are long and the weather hot.
- Answer: Because the axis is tilted, the angle and the length of sunlight on the northern hemisphere change as the Earth revolves — giving winter its short, cool days and summer its long, hot days.
6Common mistakes Mistakes
7Quick check Quick check
Q1How does the alternation of day and night happen?
Q2How long does one revolution of the Earth take?
Q3What is the main cause of the seasons?
Q4Why are Hong Kong's summer days long and nights short?
Q5When it is summer in Hong Kong, what season is it in Australia?
8Recap Recap
- The Earth's rotation takes about twenty-four hours and gives day and night.
- One revolution round the Sun takes about three hundred and sixty-five days — a year.
- The axis tilted about twenty-three and a half degrees causes the seasons.
- Hong Kong summers have direct, long, hot days; winters slanted, short, cool days.
- The hemispheres have opposite seasons.