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๐ฌ Think about it
Before clocks and printed calendars, how did people know when to plant crops or hold festivals? They watched the sky. The Sun gave them the day, the Moon the month, the seasons the year. Let's see how calendars were born.
What is a mean solar day?
- The Sun appears to rise in the east and set in the west each day.
- This is due to the Earth rotating on its axis.
- The time from the Sun's highest point one day to the next is 24 hours.
- This 24-hour period is called the mean solar day.
Example:
A shadow is shortest at noon, when the Sun is highest, marking the middle of a solar day.
In this Activity, we will measure the length of a solar day using a stick's shadow.
Materials needed
A 1 m stick, a flat sunlit area, a clock, chalk or markers.
Procedure
1. Fix a 1 m stick vertically in a flat sunlit area.
2. Start at 11:00 a.m. and mark a dot at the shadow's tip every minute.
3. Keep marking until about 1:10 p.m.
4. Find when the shadow was shortest and note its time in Table 11.2.
5. Repeat on the next few days.
6. Find the day's length from the time difference on consecutive days.
2. Start at 11:00 a.m. and mark a dot at the shadow's tip every minute.
3. Keep marking until about 1:10 p.m.
4. Find when the shadow was shortest and note its time in Table 11.2.
5. Repeat on the next few days.
6. Find the day's length from the time difference on consecutive days.
Table 11.2 — Finding the duration of a solar day
| Date | Time of shortest shadow (hh:mm) | Duration of day (hh:mm) |
|---|---|---|
| 22 March 2025 | 12:20 | —— |
| 23 March 2025 | 12:20 | 24:00 |
| 24 March 2025 | 12:19 | 23:59 |
Observation
The shortest shadow occurs near the same time each day, and the day's duration is close to 24 hours.
Explanation
The average time between the Sun's highest points on consecutive days is about 24 hours, confirming the mean solar day as a natural unit of time.
โ Summary
- Stick fixed upright
- Shadow dots marked
- Shortest one found
- Day length measured
How do the Moon and seasons give us months and years?
- The Moon takes about 29.5 days to cycle through all its phases.
- This cycle is the basis for a month.
- The Earth takes about 365 and a quarter days to orbit the Sun.
- This cycle of seasons defines a solar year.
Natural Cycles Become Time Units
Earth rotates once
Gives a day (about 24 hours).
↓
Moon cycles its phases
Gives a month (about 29.5 days).
↓
Earth orbits the Sun: gives a year (about 365.25 days)
Important Points
- The mean solar day is about 24 hours.
- The Moon's phase cycle (about 29.5 days) gives a month.
- The Earth's orbit (about 365.25 days) gives a year.
Important Definitions
- Mean solar day — the average 24-hour time between the Sun's highest points on consecutive days.
- Month — a time unit based on the Moon's phase cycle of about 29.5 days.
- Solar year — the time the Earth takes to orbit the Sun once, about 365 and a quarter days.
๐ NCERT Question 12 — On which periodic phenomenon
On which periodic phenomenon are the day, the month, and the year based?
View Answer →
