EN हिं

NCERT Solutions Curiosity Chapter 11 Science • Society • Interdisciplinary Projects — Discover, design, and debate

Book page 188 Updated on2026-09-05

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.
Answer

How to do it. Just after sunset (for a waxing crescent) stand where you can see both the glow of the Sun near the western horizon and the crescent. Point one finger at the Sun, then sweep it along the shortest path in the sky to the Moon. Never look straight at the Sun — point at it, do not stare at it.

What you will find. Every single time, the finger reaches the bright edge of the crescent before it reaches the dark part. The bulging outer edge of the crescent is always turned Sunward.

Why it happens: This is direct evidence that the Moon shines by reflected sunlight rather than by its own. The half of the Moon lit up is the half facing the Sun, so the bright side of the disc we see must be the side nearer the Sun in the sky. If the Moon glowed by itself there would be no reason for its bright part to keep pointing at the Sun.

The line joining the tips. The two tips of a crescent are the points where the light–dark boundary meets the Moon's rim. That boundary is a great circle running round the Moon through its poles, so its two visible ends are always at opposite points of the rim — and the line joining opposite points of a circle is a diameter. Measuring across the horns therefore measures the full width of the Moon, however thin the crescent is.

Try this: Hold a straight stick or a taut thread across the two tips and see how it lines up at right angles to your finger's Sun-to-Moon path. The crescent always bends away from the Sun, never towards it.
Q2.
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?
Answer

The dates that can differ are the ones that fall between 29 February and 20 April — that is, the last three weeks of Phalguna and the whole of Chaitra. Everything from 1 Vaisakha to 9 Phalguna maps to the same Gregorian date every single year.

Month of the Indian National CalendarDaysGregorian dates in a common year
Chaitra30 (31 in a leap year)22 March – 20 April (21 March – 20 April in a leap year)
Vaisakha3121 April – 21 May
Jyaistha3122 May – 21 June
Asadha3122 June – 22 July
Sravana3123 July – 22 August
Bhadra3123 August – 22 September
Asvina3023 September – 22 October
Kartika3023 October – 21 November
Agrahayana3022 November – 21 December
Pausa3022 December – 20 January
Magha3021 January – 19 February
Phalguna3020 February – 21 March (20 February – 20 March when February has 29 days)
Why it happens: The Indian National Calendar has a fixed length of 365 days, so as long as the Gregorian year is also 365 days the two run in perfect step. The only disturbance is the Gregorian leap day. When 29 February exists it becomes 10 Phalguna, so from 1 March onwards every Gregorian date carries a Saka day-number one higher than usual, right up to 20 April. Then the calendar puts things right by giving Chaitra 31 days instead of 30, so the new year begins on 21 March and 1 Vaisakha is back on 21 April as always.
Check it yourself: Independence Day is 24 Sravana and Republic Day is 6 Magha — in every year, leap or not, because both fall inside the fixed stretch of the year. Verify it in Fig. 11.9 style: Sravana begins on 23 July, so 15 August is its 24th day.
Q3.
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.
Answer

How to do it. Ask family members and classmates from different states, then check each festival's date over three or four years on a Gregorian calendar. That test alone tells you the calendar type: a date that hardly moves means a solar festival; a date that wanders by up to a month means a luni-solar one; a date that drifts steadily earlier through all the seasons means a purely lunar one.

Sample answer:

StateNew Year festivalCalendar it followsWhen it falls
MaharashtraGudi PadwaLuni-solarFirst day after the new Moon of Chaitra (March–April)
Karnataka, Andhra Pradesh, TelanganaUgadiLuni-solarThe same day as Gudi Padwa
GujaratBestu VarasLuni-solarThe day after Diwali, in Kartika (October–November)
Jammu and KashmirNavrehLuni-solarFirst day of Chaitra (March–April)
ManipurCheiraoba (Sajibu Nongma Panba)Luni-solarFirst day of the month of Sajibu (April)
PunjabVaisakhiSolar (sidereal)13 or 14 April almost every year
West BengalPoila BaisakhSolar (sidereal)14 or 15 April
AssamBohag Bihu (Rongali Bihu)Solar (sidereal)14 or 15 April
Tamil NaduPuthanduSolar (sidereal)14 April
KeralaVishuSolar (sidereal)14 or 15 April
OdishaPana Sankranti (Maha Vishuva Sankranti)Solar (sidereal)14 April
Why it happens: The pattern in the table is not an accident. All the mid-April New Years mark the same astronomical event — the Sun entering the star pattern of Mesha (Aries) on the sidereal solar calendar — which is why they cluster on 14 April everywhere from Punjab to Kerala. The luni-solar New Years are tied instead to a phase of the Moon in a named month, so their Gregorian dates move about, but only within a month, because of the Adhika Maasa correction.
Q4.
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.
Answer

Sample answer (fill your own table from the calendars you collect and compare — dates for a festival can differ by a day between regions):

YearEid-ul-FitrShift from previous yearDiwali (Lakshmi Puja)Shift from previous year
202114 May—4 November—
20223 May11 days earlier24 October11 days earlier
202322 April11 days earlier12 November19 days later — a jump
202411 April11 days earlier1 November11 days earlier
202531 March11 days earlier20 October12 days earlier

Eid-ul-Fitr moves back by almost exactly 11 days every year, without a single exception. On the lunar calendar itself, though, the date does not move at all — it is always 1 Shawwal, the day after the crescent Moon is sighted at the end of Ramazan.

Diwali mostly moves back by about 11 days too, but between 2022 and 2023 it jumped forward by 19 days. That jump is the signature of an Adhika Maasa: an extra lunar month was inserted in 2023 (an extra Shravana), which pushed every later festival of that year about a month ahead.

Expected shift without a correction = 11 days earlier
Actual shift 2022 → 2023 = 19 days later
Difference = 19 + 11 = 30 days ≈ one extra lunar month
Why it happens: Eid-ul-Fitr follows a purely lunar calendar, which makes no attempt to keep up with the seasons — so it slips 11 days every year and can occur in any Gregorian month. Diwali follows a luni-solar calendar, which does the same slipping but cancels it every two or three years with an intercalary month. That is why Diwali never leaves autumn while Eid-ul-Fitr travels right through the year.
Tip: A shift of about −11 days is normal; a shift of about +19 days marks the Adhika Maasa year. Add a column of your own for Holi or Dussehra and you will find the very same jump in the same year.
Q5.
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?
Answer

How to do it well. Everything depends on standing in exactly the same place each time, so choose a fixed mark on the ground — a doorstep, a particular paving stone — and sketch the eastern horizon once, carefully, with its trees, poles and rooftops. After that you only add a small circle for the Sun and the month's name.

What you will find. The sunrise point does not stay put; it swings steadily along the horizon and turns round twice a year.

Time of yearWhere the Sun risesName given to the movement
About 21 December (winter solstice)Farthest south of eastTurning point — Uttarayan begins
January to MarchCreeping northward month by monthUttarayan
About 21 March (spring equinox)Exactly east — and the Sun sets exactly westUttarayan continues
About 21 June (summer solstice)Farthest north of eastTurning point — Dakshinayan begins
July to SeptemberCreeping southwardDakshinayan
About 23 September (autumn equinox)Exactly east againDakshinayan continues

Yes — this is exactly the Uttarayan and Dakshinayan of the 'Our scientific heritage' box. The Taittirīya Saṁhitā records the same pattern: "Thus the Sun moves southwards for six months and northwards for six months."

Why it happens: The Earth's axis is tilted, so as the Earth goes round the Sun the point where the Sun's rays meet the horizon at dawn shifts north and south through the year. Across India the swing is roughly 24° either side of due east near the southern tip and about 27° near Delhi — a wide enough gap on the horizon that ancient observers could see it against fixed landmarks and use it as a calendar, without any instrument at all.
Did you know? Because the sidereal year is 20 minutes longer than the tropical year, festivals fixed to the sidereal calendar slowly slide away from the solstice they once marked. Makar Sankranti now moves ahead by one day every 71 years — which is why it falls in mid-January today although it was once tied to the December solstice.
Was this helpful? Report an error