Keeping Time with the Skies — Class 8 Science
Curiosity · Chapter 11
CBSE Class 8 community
View all
6- Mathematics
- Science
- + 3 more
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Phases of the Moon
- The changing shapes of the bright portion of the Moon seen from Earth as it revolves around the Earth.
- Waxing and waning
- The waxing period is when the bright part of the Moon increases; the waning period is when it decreases.
- Mean solar day
- The average time of 24 hours the Sun takes to return to its highest position in the sky on successive days.
- Lunar calendar
- A calendar based on the cycle of the Moons phases, with a year of about 354 days.
- Leap year
- A year with an extra day added to a solar calendar to keep it synchronised with the seasons.
- Artificial satellite
- A human-made object launched to orbit the Earth for communication, navigation, weather monitoring, or research.
Extra questions & answers
Why does the Moon shine?
The Moon does not produce its own light; it shines by reflecting sunlight that falls on it.
What causes the phases of the Moon?
As the Moon revolves around the Earth, we see different fractions of its sunlit half, which changes its apparent shape.
What is the difference between the waxing and waning periods?
During the waxing period the bright portion increases towards full Moon, and during the waning period it decreases towards new Moon.
Why does the Moon rise about fifty minutes later each day?
The Moon moves ahead in its orbit while the Earth rotates, so the Earth must rotate a little more for the Moon to reach the same position.
Which astronomical cycle defines a day, a month, and a year?
The Earths rotation defines the day, the cycle of the Moons phases defines the month, and the Earths revolution around the Sun defines the year.
How does a lunar calendar differ from a solar calendar?
A lunar calendar follows the Moons phases (about 354 days a year), while a solar calendar follows the seasons (about 365 days a year).
Why are leap years added to solar calendars?
Because the Earth takes about an extra quarter day each year, an extra day is added every four years to keep the calendar matched with the seasons.
What is a luni-solar calendar and the role of an intercalary month?
It uses the Moons phases for months but adds an extra month (Adhika Maasa) every few years to stay in step with the seasons.
When does the Indian National Calendar begin and what type is it?
It is a solar calendar that begins on 22 March, the day after the spring equinox.
Why do we launch artificial satellites?
They help with communication, navigation, weather monitoring, disaster management, and scientific research.
Probe and ponder
Have you ever seen the Moon during the day? Why do you think it is sometimes visible when the Sun is up?
Yes, the Moon is often visible during the day, especially during its waxing and waning gibbous and quarter phases. It is visible because it shines by reflecting sunlight, not by its own light. When the Moon is above the horizon in the daytime sky and the Sun illuminates it, we can see the bright part even though the sky is bright. It is not only present at night; it orbits the Earth continuously and can be above the horizon for about 12 hours out of every 24 hours, sometimes by day and sometimes by night.
Imagine you lived on the Moon instead of Earth. What would you mean by a day, a month or a year?
Living on the Moon, a day would be defined by the Moon's rotation on its axis relative to the Sun, which takes about
What would happen if Earth had two moons instead of one? How would that change the night sky?
If Earth had two moons, there would be two objects with phases that depend on their positions relative to the Sun and Earth, possibly with different periods if they were at different distances. The night sky would be brighter on many nights, tides would be higher and more complex due to the combined gravity, and we would see two moons of different phases and apparent sizes moving across the sky. Eclipses could be more frequent.
If we didn’t have clocks or calendars, how else could we measure time?
We could use natural periodic events in the sky: the position and shortest shadow of the Sun to know midday and to count days, the changing phases of the Moon to count months of about
Share your questions ?
Some further questions could be: Why does the same side of the Moon always face Earth? Why is the crescent shape always curved and its horns point away from the Sun? How did ancient people know a year is about
Keep the curiosity alive
1. State whether the following statements are True or False. (i) We can only see that part of the Moon which reflects sunlight towards us.
True. The Moon has no light of its own. We see only the portion of its sunlit half that is facing Earth and reflects sunlight toward us.
1. State whether the following statements are True or False. (ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.
False. Phases are caused by the changing view of the illuminated half of the Moon as it revolves around Earth. The Earth’s shadow on the Moon causes a lunar eclipse, which happens only occasionally on a full Moon day, not the regular phases.
1. State whether the following statements are True or False. (iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.
True. Days are based on the Earth’s rotation relative to the Sun, months on the Moon’s revolution and phases, and years on the Earth’s revolution and the cycle of seasons.
1. State whether the following statements are True or False. (iv) The Moon can only be seen at night.
False. The Moon is above the horizon for roughly 12 hours each day and is often visible in the daylight, especially when it is in its gibbous or quarter phases. Moonrise occurs about 50 minutes later each day and can be in the afternoon.
2. 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.
No. Amol’s birthday on 6th May is fixed by the solar Gregorian calendar of
3. Name two things that are incorrect in Fig. 11.10.
Fig. 11.10 shows the phases incorrectly as being caused by the Earth’s shadow falling on the Moon. In reality phases are due to our view of the sunlit half of the Moon. It also shows incorrect illumination geometry, for example the crescent or gibbous shapes oriented away from the Sun or the new Moon shown illuminated, whereas the illuminated side must always face the Sun and the line joining the horns of a crescent is perpendicular to the direction to the Sun.
4. 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
Matching the illuminated shapes in Fig. 11.11: Day of New Moon = B (fully dark); Three days after New Moon = F (thin waxing crescent); Full Moon = E (fully bright disc); Three days after Full Moon = A (waning gibbous, almost full); A week after Full Moon = C (half-illuminated, quarter Moon).
4. Look at the pictures of the Moon in Fig. 11.11, and answer the following questions. (ii) List the picture labels of the phases of the Moon that are never seen from Earth. Hint: You can use your observations from Activity 11.1 or Fig. 11.2 as reference.
A Moon phase can be seen from Earth only if its lit portion faces the Sun and forms a smooth crescent/gibbous whose bright side is turned toward the Sun. Any panel whose illumination pattern cannot correspond to a real Sun–Earth–Moon geometry (for example, a crescent lit on the side away from the Sun, or an oddly-shaped lit region) is never seen from Earth. In Fig. 11.11, panel D shows such an inconsistent illumination, so it represents a phase that is never actually seen from Earth.
5. Malini saw the Moon overhead in the sky at sunset. (i) Draw the phase of the Moon that Malini saw.
At sunset the Sun is on the western horizon. A Moon that is overhead at that time is
5. Malini saw the Moon overhead in the sky at sunset. (ii) Is the Moon in the waxing or the waning phase?
Waxing phase, specifically Shukla Paksha. The Moon is growing from new Moon toward full Moon. The overhead at sunset half Moon occurs about a week after new Moon, during the waxing cycle.
6. 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?
Kaushalya is telling the truth. A waxing crescent Moon is in the west near the Sun at sunset and sets after the Sun, it does not rise in the east at sunset. A waning crescent does rise in the east but around sunrise, not sunset. A gibbous Moon, especially a waxing gibbous, rises in the east in the mid to late afternoon before sunset and can be seen in the eastern sky during daylight, and a waning gibbous can remain visible in the west in the morning. So Kaushalya’s afternoon gibbous in the east is possible, Ravi’s is not.
7. 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?
Less often. If the Moon moves farther, its orbital period becomes longer, so the lunar month becomes longer than
8. 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.
Three solar years contain
9. On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?
Full Moon or Purnima. The full Moon rises approximately at sunset in the east, stays up all night, is overhead near midnight, and sets at sunrise in the west, opposite the Sun, so it is visible from sunset to sunrise.
10. If we stopped having leap years, in approximately how many years would the Indian Independence day happen in winter?
Without leap years the calendar loses about
11. What is the purpose of launching artificial satellites?
Artificial satellites are human-made objects that orbit Earth at about
12. On which periodic phenomenon are the following measures of time based: (i) day
Day is based on the periodic apparent motion of the Sun due to the rotation of Earth about its axis, measured as the mean solar day of
12. On which periodic phenomenon are the following measures of time based: (ii) month
Month is based on the revolution of the Moon around Earth and the cycle of its phases, which takes about
12. On which periodic phenomenon are the following measures of time based: (iii) year
Year is based on the revolution of Earth around the Sun and the resulting cycle of seasons, the tropical year of about
Discover, design, and debate
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.
Observation task: On a clear evening when a thin waxing crescent is visible in the west after sunset, locate the Sun just below the horizon in the west and the Moon nearby. Draw an imaginary short arc from the Sun to the Moon. You will cross the bright, sunward edge first, proving the bright side faces the Sun. The dark side faces away. Joining the two horns of the crescent gives an approximate diameter line that is perpendicular to the Sun-Moon direction, consistent with reflected sunlight.
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?
In the Indian National Calendar the year starts on 22 March, or 21 March in a Gregorian leap year, because Chaitra 1 is the day after the spring equinox. Regular years have months 1 to 6 with 31 or 30 days as described. In a leap year an extra day is added to Chaitra, so from Chaitra 1 up to about Phalguna the mapping shifts by one day. Therefore dates in the early part of the year, especially in Chaitra, can be one day different in leap years, while later months align again after the extra day is absorbed.
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.
Examples: Maharashtra - Gudi Padwa, luni-solar; Karnataka and Andhra Pradesh - Ugadi, luni-solar; Tamil Nadu - Puthandu, solar sidereal; Kerala - Vishu, solar sidereal; Punjab - Vaisakhi, solar sidereal; Assam - Bohag Bihu, solar sidereal; West Bengal - Poila Baisakh, solar sidereal; Odisha - Pana Sankranti or Maha Vishuba Sankranti, solar; Gujarat - Bestu Varas or Diwali linked, luni-solar; Manipur - Sajibu Nongma Panba, luni-solar. Festivals tied to sidereal solar calendars fall on almost the same Gregorian date each year, while luni-solar ones shift within about a month.
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.
Task outcome: Eid-ul-Fitr is fixed to 1 Shawwal in the purely lunar Islamic calendar of
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).
Observation task: Over a year the sunrise point moves northward from late December to late June and southward from late June to late December. Around 21 June solstice it rises north of due east, around 21 December solstice it rises south of due east, and on the equinoxes around 21 March and 23 September it rises due east. This annual oscillation is Uttarayan, the northward progress for six months, and Dakshinayan, the southward progress for six months, described in texts like the Taittiriya Samhita and Surya Siddhanta. Marking monthly positions on the same eastern horizon sketch reveals this slow swing, which repeats every year and is linked to the seasons.