Space Physics: Question 1
Syllabus 6.1.1
A student living in Nairobi keeps a weather diary. Every day she notes that the Sun appears low in the eastern sky at dawn, climbs to its highest point around midday, and disappears below the western horizon by evening, and the whole pattern repeats after about 24 hours.
Which of the following correctly explains why Nairobi experiences this repeating day-and-night cycle?
Show worked solution Hide worked solution
Worked solution
Step 1: Identify what repeats every 24 hours
The pattern described (the Sun rising, crossing the sky and setting, then repeating) happens with a period of about 24 hours. This is the Earth’s rotation period: the time the Earth takes to spin once on its own axis. It is not the same as the Earth’s orbital period around the Sun, which is about 365 days.
Step 2: Explain the cause of day and night
The Earth spins on its own axis, which is tilted, once approximately every 24 hours. As the Earth rotates, Nairobi is carried from the half of the Earth facing the Sun (where it is day) to the half facing away from the Sun (where it is night), and then back again. This is why the Sun appears to rise, cross the sky, and set. It is really Nairobi’s position that is changing, not the Sun’s.
Why the other options are wrong
| Option | What it describes | Why it’s wrong |
|---|---|---|
| A | Earth orbiting the Sun in 24 hours | The Earth’s orbit around the Sun takes about 365 days, not 24 hours; a 24-hour cycle is caused by the Earth’s rotation, not its orbit |
| C | The Sun moving around a stationary Earth | This reverses the real motion: it is the Earth that rotates, not the Sun that travels around the Earth |
| D | The Moon blocking sunlight every 24 hours | The Moon does not pass between the Earth and Sun once a day, and this describes a solar eclipse (a rare event), not the everyday day/night cycle |
Final answers
- The Earth’s rotation on its tilted axis, once approximately every 24 hours, causes day and night, option B