Probe and Ponder
Q1. Have you ever seen the Moon during the day? Why do you think it is sometimes visible when the Sun is up?
Answer:
Yes, the Moon is often visible during the day. It is visible because the Moon reflects sunlight, and when its position in the sky is far enough from the Sun, the brightness of the sky does not completely overwhelm the Moon's reflected light.
Q2. Imagine you lived on the Moon instead of Earth. What would you mean by a day, a month or a year?
Answer:
On the Moon, a "day" would be the time it takes for the Moon to rotate once on its axis, which is about 29.5 Earth days (a lunar day). A "month" would be the time it takes for the Moon to complete one orbit around the Earth, which is the same as a lunar day because the Moon is tidally locked. A "year" would be the time it takes for the Moon to orbit the Sun, which is one Earth year.
Q3. What would happen if Earth had two moons instead of one? How would that change the night sky?
Answer:
If Earth had two moons, the night sky would be more dynamic. We would see two moons with different phases, orbital periods, and possibly different sizes and brightness. Tides on Earth would be more complex, and calendars would need to account for multiple lunar cycles.
Q4. If we didn't have clocks or calendars, how else could we measure time?
Answer:
Without clocks or calendars, we could measure time using natural periodic events such as:
- The cycle of day and night (rotation of Earth).
- The phases of the Moon.
- The changing seasons.
- The position of the Sun and shadows.
- The movement of stars (e.g., the rising of specific constellations).
Activity 11.1
Q1. Spot the Moon at sunrise in the western direction on the first day after the full Moon. Make a table similar to Table 11.1 in your notebook. Document the following: Date, When you saw the Moon (at sunrise or sunset)? Shade the corresponding Circle with pencil to show the bright portion of the Moon as shown in Fig. 11.1. From the second day onwards also document the following: Is the size of bright portion of the Moon increasing or decreasing from the previous day. Whether the Moon appears closer to or farther from the Sun in the sky than the day before. After about 15 days, you may not be able to see the Moon at sunrise or sunset. For the next 15 days, carry out this activity at sunset. Analyse the data recorded by you in Table 11.1. Did the Moon appear different each day? Was the Moon visible on all days? Did the Moon appear at the same position in the sky as on the previous day? 
Answer:
The Moon appears different each day as its illuminated portion changes. It is not visible on new Moon day. The Moon appears in a different position in the sky each day, moving eastward relative to the Sun.
Activity 11.2
Q1. Take a small soft ball and insert a stick into it (Fig. 11.4a). This represents the Moon. Go to a dark open place (at night), and ask a teacher or guardian to shine a torchlight towards you from about 3 m to represent light coming from the Sun or stand near an electric lamp. Your head represents the Earth. Now hold the ball at arm's length in one hand such that it is slightly above your head as shown in Fig. 11.4b. Keep the ball at position E towards the direction of the lamp. Does the portion of the ball facing you appear to be illuminated or not? Turn around slowly, in the anti-clockwise direction, with your arm outstretched as shown in Fig. 11.4b and keep looking at the ball. Does the shape of the illuminated portion change? Is the line separating the illuminated and non-illuminated portions of the ball curved? Was your observation similar to the changing shape of the illuminated portion of ball shown in Fig. 11.4c? The shape of the illuminated portion of the ball, as seen by you, changes depending on where the ball is with respect to the lamp. 
Answer:
When the ball is held towards the lamp (position E), the illuminated portion faces the lamp, so the student sees the dark side (new Moon). As the student turns, the illuminated portion becomes visible gradually, showing phases similar to the Moon. The line separating the illuminated and dark portions appears curved, matching the crescent and gibbous phases.
Activity 11.3
Q1. Find a small flat area in a ground which receives sunlight during the day. Fix a 1 m stick vertically in it as shown in Fig. 11.7. Start observing at 11:00 a.m. Every minute, mark a dot on the ground at the tip of the stick's shadow. Keep marking dots until around 1:10 p.m. Identify when the shadow was shortest and find out its time by counting the number of dots. Record this time in Table 11.2. Repeat this exercise for the next few days. Find the duration of the solar day by finding a difference in time on two consecutive days as shown in Table 11.2. Find the average duration of the day. Is it nearly equal to 24 hours? 
Answer:
When the Sun is at its highest point (noon), the shadow is shortest. The time between the shortest shadows on two consecutive days gives the length of a solar day, which is approximately 24 hours.
Activity 11.4
Q1. Spotting an artificial satellite is a night sky watching activity like we have done previously. Just before sunrise or after sunset, go to a location, accompanied by an adult, that has a clear view of the sky, without any obstruction of trees or tall buildings. To identify satellites in the sky, look for any moving object in the sky that appears as a point of light with steady or flickering brightness and is moving very fast across the sky. You can see them with the naked eye or with binoculars. You may use mobile apps or websites that provide details of satellites visible in your location and when they will be passing above you in the sky.
Answer:
Artificial satellites can be spotted as fast-moving points of light in the sky, often appearing just after sunset or before sunrise when sunlight still reflects off them. Mobile apps like "Heavens-Above" can help identify satellites and their pass times.
Questions and Activities
Q1. State whether the following statements are True or False. (i) We can only see that part of the Moon which reflects sunlight towards us. (ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases. (iii) Calendars are based on various astronomical cycles which repeat in a predictable manner. (iv) The Moon can only be seen at night.
Answer:
(i) True - We see only the illuminated part of the Moon facing us.
(ii) False - Moon phases are caused by the relative positions of the Sun, Earth, and Moon, not by Earth's shadow.
(iii) True - Calendars are based on predictable astronomical cycles like the day, month, and year.
(iv) False - The Moon can also be seen during the daytime.
Q2. Amol was born on 6th of May on a full Moon day. Does his birthday fall on the full Moon day every year? Explain your answer.
Answer:
No, his birthday does not fall on a full Moon day every year because the lunar cycle is about 29.5 days, which does not align perfectly with the solar calendar. The full Moon occurs on different dates each year.
Q3. Name two things that are incorrect in Fig. 11.10. 
Answer:
Possible incorrect things:
- The Moon phases are not caused by Earth's shadow.
- The size or relative positions of the Sun, Earth, and Moon may be drawn incorrectly.
- Only the new Moon phase can occur when the Moon is between the Earth and Sun; the full Moon occurs when the Earth is between the Moon and Sun.
Q4. Look at the pictures of the Moon in Fig. 11.11, and answer the following questions. 
(i) Write the correct panel number corresponding to the phases of the Moon shown in the pictures above.
| Picture label (e.g. A, B, C, etc.) | Phase of Moon |
|---|---|
| Three days after New Moon | |
| Full Moon | |
| Three days after Full Moon | |
| A week after Full Moon | |
| Day of New Moon |
Answer:
(i) Based on the pictures:
- Three days after New Moon: Crescent (small illuminated sliver) → Picture B.
- Full Moon: Fully illuminated circle → Picture D.
- Three days after Full Moon: Waning gibbous → Picture C.
- A week after Full Moon: Last Quarter (half) → Picture E.
- Day of New Moon: No visible illumination → Picture A.
(ii) Phases never seen from Earth would be those where the Moon's dark side is facing Earth, but the only phase where it is completely invisible is the new Moon. However, all phases shown are seen from Earth at different times.
Q5. Malini saw the Moon overhead in the sky at sunset. (i) Draw the phase of the Moon that Malini saw. (ii) Is the Moon in the waxing or the waning phase?
Answer:
If the Moon is overhead at sunset, it is at the first quarter phase (half Moon). The Moon is in the waxing phase (Shukla Paksha) because the illuminated portion is increasing towards a full Moon.
Q6. Ravi said, "I saw a crescent Moon, and it was rising in the East, when the Sun was setting." Kaushalya said, "Once I saw the gibbous Moon during the afternoon in the East." Who out of the two is telling the truth?
Answer:
Kaushalya is telling the truth. A crescent Moon near the Sun sets soon after sunset; it cannot be rising in the East at sunset. A gibbous Moon can be seen in the afternoon in the East, as it rises before sunset.
Q7. Scientific studies show that the Moon is getting farther away from the Earth and slower in its revolution. Will luni-solar calendars need an intercalary month more often or less often?
Answer:
As the Moon moves farther and revolves slower, the lunar month will become longer. This means the difference between the lunar and solar years will decrease over time, so luni-solar calendars would need intercalary months less often.
Q8. A total of 37 full Moons happen during 3 years in a solar calendar. Show that at least two of the 37 full moons must happen during the same month of the solar calendar.
Answer:
There are 36 months in 3 years. Since there are 37 full moons and only 36 months, by the pigeonhole principle, at least one month must have two full moons. (A "blue moon" is the second full moon in a month.)
Q9. On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?
Answer:
If the Moon is visible from sunset to sunrise, it must be opposite the Sun in the sky. This occurs during the full Moon phase.
Q10. If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?
Answer:
Without leap years, the calendar would shift by about one day every four years (0.25 days per year). Independence Day (August 15) would shift from summer to winter in about 90 years (360 days / 4).
Q11. What is the purpose of launching artificial satellites?
Answer:
Artificial satellites are launched for various purposes, including:
- Communication (telephone, TV, internet).
- Navigation (GPS).
- Weather monitoring and forecasting.
- Disaster management.
- Scientific research (studying space, climate, Earth).
- Military and surveillance purposes.
Q12. On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?
Answer:
(i) A day is based on the rotation of the Earth on its axis.
(ii) A month is based on the cycle of the Moon's phases (lunar month).
(iii) A year is based on the Earth's revolution around the Sun (solar year).
Discover, Design, and Debate
Q1. The Moon's crescent always faces towards the Sun (Fig. 11.12). On days when you see the crescent Moon, point your finger towards the Sun, and slowly move it across the sky towards the Moon taking as short a path as you can. Note how your finger always crosses the illuminated part of the Moon first and clearly shows us that we see sunlight reflected off the Moon. The line joining the tips of the crescent would correspond to the diameter of the Moon. Most of the dates in the Indian National Calendar always map to the same dates in the Gregorian calendar. Can you find out which ones may differ for certain years? Different states in India celebrate the New Year according to their local cultures. Find out the names of the New Year festival celebrated in any 10 states of India. Also find out whether it is based on the lunar calendar or the solar calendar or the luni-solar calendar. Collect Gregorian calendars (the regular calendar you use every day) for the last five years with the help of your family members or teachers or the internet. For each year, look for the dates on which the festivals Eid-ul-Fitr and Diwali were celebrated and list them year wise in a tabular form. Do you notice that the date of Eid-ul-Fitr moves earlier each year — by about 11 days? If you have a corresponding lunar calendar at home or on the internet, check that the month and the day for Eid-ul-Fitr according to the lunar calendar remains the same. Does Diwali follow the same steady pattern, or are there some sudden jumps? Based on your chart, try to guess which year might have included an intercalary month (Adhikamaasa). Obtain a luni-solar calendar and confirm if there is an intercalary month between Diwali in the previous year and that year. Every morning on your way to school, notice the direction in which the Sun rises. Decide on a spot and look towards east, with trees, poles, or buildings acting as markers. Sketch the eastern horizon in your notebook. For the next one year, at the start of each month, stand at the same spot and mark the Sun's position on your sketch. Label it with the name of the month. At the end of the year analyse your sketch. Do you find that the positions of sunrise shift in particular direction? Can you identify it with the Uttarayaan and Dakshinayaan that our ancestors noticed? (Refer the 'A step further' box on page 181). 
Answer:
Indian National Calendar: Most dates in the Indian National Calendar map to the same dates in the Gregorian calendar. However, in leap years, the date shifts by one day (e.g., Chaitra 1 becomes March 21 instead of March 22).
New Year festivals in 10 states:
- Andhra Pradesh: Ugadi (Luni-solar)
- Assam: Bihu (Solar)
- Bengal: Poila Baisakh (Solar)
- Gujarat: Bestu Varas (Lunar)
- Karnataka: Yugadi (Luni-solar)
- Kerala: Vishu (Solar)
- Maharashtra: Gudi Padwa (Luni-solar)
- Punjab: Vaisakhi (Solar)
- Tamil Nadu: Puthandu (Solar)
- Telangana: Ugadi (Luni-solar)
Eid and Diwali dates: Eid-ul-Fitr shifts earlier by about 10-11 days each year because it follows a purely lunar calendar. Diwali follows a luni-solar calendar, so its dates may have sudden jumps when an intercalary month (Adhikamaasa) is added.
Sunrise observation: Over the year, the Sun rises at different points on the eastern horizon. From June to December, it moves southward (Dakshinayan), and from December to June, it moves northward (Uttarayan).