Rotation of the Earth and Day-Night
In the morning the sun rises, at noon it is overhead and in the evening it sets. We see this daily but the sun does not really move; our earth turns around itself.
The earth turns around itself from west to east; this is called rotation. One rotation takes 24 hours and so day and night occur.
Key idea 1 : Direction and time of rotation — The earth rotates from west to east and one turn takes 24 hours. Because of rotation we can measure time in days.
Key idea 2 : Stages in one day — In one day we experience stages like sunrise, midday and sunset, and also daytime and nighttime. The side of the earth facing the sun has day and the opposite side has night.
Key idea 3 : Recalling earlier lessons — The earth completes one round of the sun in a little over 365 days; this is revolution. As the earth’s axis is tilted, the sun’s rays do not fall straight everywhere. This leads to seasons later. India is in the northern hemisphere.
- Easy : How long does rotation take? — 24 hours.
- Medium : In which direction does the earth rotate? — West to east.
- Exam-type : India lies in which hemisphere? — Northern.
In the figure: Day-night due to rotation.
Rotation = around itself = day-night.
The sun does not go around the earth; it only seems so as the earth rotates.
Memory clue: West-East-24.
Spot the difference
| Motion | Meaning | Result |
|---|---|---|
| Rotation | Turning around itself | Day and night |
| Revolution | Going around the sun | Seasons |
Practice questions
- 1What is rotation?
- 2In which direction does the earth rotate?
- 3How long does rotation take?
- 4Which stages do we experience due to rotation?
- 5State the difference between rotation and revolution.
- 6Explain how day and night occur.
- 7Why do we say the sun rises though it does not really move?
- 8In which hemisphere is India?
Show answers (try them yourself first)
- 1The earth turning around itself.
- 2West to east.
- 324 hours.
- 4Sunrise, midday, sunset, day and night.
- 5Rotation = around itself; revolution = around the sun.
- 6The side facing the sun is lit: day; the other side is night.
- 7Because the earth rotates our position changes.
- 8Northern hemisphere.
Sunrise, Sunset and Length of Day
In summer it stays light till late and in winter it gets dark early. So the length of the day changes.
The time between sunrise and sunset is the length of the day and the rest is the night. This length changes through the year.
Key idea 1 : How to record — Sunrise and sunset times of our place can be obtained by direct observation, calendar, newspaper or the internet. The length of the day is sunset time minus sunrise time. Subtract the day from 24 hours to get the night.
Key idea 2 : The June record — Records from 19 to 28 June are made and one finds the longest day, how the night changes daily and its likely reason. In the northern hemisphere the time around 21 June has the longest days.
Key idea 3 : September to December — From September to December, records of the 19th to 28th are made to see the day-night lengths. Around September the day and night are almost equal, about 12 hours. The question is whether this change occurs everywhere on earth; the answer comes in Lesson 8.
- Easy : Sunrise 6:00 and sunset 18:30 — day? — 12 h 30 min.
- Medium : Night for the above day? — 11 h 30 min.
- Exam-type : Longest day in the northern hemisphere? — Around 21 June.
In the figure: The periods of records.
Day = sunset − sunrise; night = 24 − day.
The day is not always 12 hours; it changes through the year.
Memory clue: Sunset − sunrise = day.
Spot the difference
| Period | State of day | Find reason |
|---|---|---|
| June | Long day | See Lesson 8 |
| September | Day-night almost equal | See Lesson 8 |
Practice questions
- 1What is the length of the day?
- 2How is the length of the night found?
- 3Where do we get sunrise and sunset times?
- 4If sunrise is 6:15 and sunset 18:45, how long is the day?
- 5How do you think the night changes daily in June?
- 6When are day and night equal?
- 7Why do you think the length of the day changes?
- 8Find how long today’s day is at your place.
Show answers (try them yourself first)
- 1Time between sunrise and sunset.
- 2By subtracting the day from 24 hours.
- 3Observation, calendar, newspaper, internet.
- 412 h 30 min.
- 5It changes slightly each day.
- 6Around September (and March).
- 7Revolution and axial tilt could be the reason.
- 8Student should check a calendar or internet.
The Shadow Experiment
If you hold a pencil in front of a torch and move it, the shadow on the wall moves too. The same happens with the sun and a stick.
The shadow experiment shows that the shadow’s position changes with the relative position of the light source and object. This explains why the place of sunrise changes.
Key idea 1 : Activity 1: torch and candle — Paste a white paper on a table. Keep a torch fixed. Stand a rod or candle between the paper and the torch. Direct the light so the shadow falls on the paper and mark its position. Then slowly move the table with the rod/candle from one side to another and mark the new position of the shadow.
Key idea 2 : Conclusion — When the position of the table changes the position of the shadow also changes. So when the relative position of the light source and object changes the shadow shifts. If we watch the place of sunrise and sunset on the horizon through the year we notice it keeps changing.
Key idea 3 : Activity 2: the year-long stick — Fix a stick 1.5 to 1.7 m long near a wall that gets sunlight at sunrise or sunset. It should stay at the same spot for about a year. Once a week at that time mark the shadow on the wall; if its position changes measure the distance between the earlier and new positions. The activity is started within eight days of school opening and continues till end of December.
- Easy : If the table is moved what happens to the shadow? — It moves too.
- Medium : Stick length needed? — 1.5 to 1.7 m.
- Exam-type : How often to record? — Once a week.
In the figure: How the shadow forms.
Object moves → shadow moves.
The shadow does not move by itself; it moves as the relative position changes.
Memory clue: Torch = sun; stick = observer.
Spot the difference
| Item | In the activity | In reality |
|---|---|---|
| Torch | Light source | The sun |
| Stick/candle | Object | Observer’s place |
Practice questions
- 1What is the shadow experiment for?
- 2If the torch is not moved what moves the shadow?
- 3Why should the stick stay in the same place for a year?
- 4When should the shadow be recorded?
- 5How to measure the difference in shadow position?
- 6State the similarity between torch and sun.
- 7How is the shadow experiment linked to the place of sunrise?
- 8What other shadow experiment will you do at home?
Show answers (try them yourself first)
- 1To understand why the shadow’s position changes.
- 2The change in relative position of object and light.
- 3So comparison is possible.
- 4At sunrise or sunset.
- 5By measuring the distance between earlier and new positions.
- 6Both are sources of light.
- 7As shadow position changes so does the sunrise position.
- 8E.g. moving a toy in front of a lamp.
Changing Position of Sunrise
From our window the sun appears to rise at different spots in June and in December. The reason is revealed in the next lesson.
The shadow and the sun are in opposite directions. So if the shadow shifts north the place of sunrise has shifted south.
Key idea 1 : Shadow and sun on opposite sides — The shadow falls on the side opposite to the sun. So if the shadow moves north the place of sunrise has moved south. From the shadow’s position we can tell where the sun rose on the horizon.
Key idea 2 : September observation — In September see the direction of the stick’s shadow and on which day day and night were equal. Lesson 8 is taken around 22 December, when all records are used to explain the reason.
Key idea 3 : The next question — From the June, September and December records we see the longest day, shortest day and days of equal day and night. The reason behind these changes is the earth’s revolution and axial tilt, which we learn in Lesson 8.
- Easy : Shadow north → sunrise where? — South.
- Medium : Shadow south → sunrise where? — North.
- Exam-type : When is Lesson 8 taken? — Around 22 December.
In the figure: Shadow and sun in opposite directions.
In the textbook this lesson is based on a year-long activity; the September and December records are used later in Lesson 8. (as per the textbook)
Shadow and sun opposite; shadow N → sunrise S.
Shadow and sun are never on the same side; they are opposite.
Memory clue: Shadow is opposite the sun.
Spot the difference
| Shadow | Sunrise | Conclusion |
|---|---|---|
| Northward | Southward | Opposite |
| Southward | Northward | Opposite |
Practice questions
- 1If the shadow moves north in which direction does sunrise shift?
- 2On which sides are shadow and sun?
- 3On which September day were day and night equal?
- 4Does the place of sunrise on the horizon change?
- 5Why is Lesson 8 taken around 22 December?
- 6What do the June, September, December records tell?
- 7What do you think are the reasons for these changes?
- 8If the shadow moves south where will sunrise be?
Show answers (try them yourself first)
- 1South.
- 2Opposite to each other.
- 3Around September (23 September).
- 4Yes, it keeps changing through the year.
- 5Because all records are ready by then.
- 6The longest day, shortest day and equal day-night days.
- 7Revolution and axial tilt.
- 8North.