Keeping Time with the Skies — Complete Answer Key
Every in-text question and every “Keep the Curiosity Alive” exercise from the chapter, answered directly — Moon phases, calendars, and satellites — using the chapter’s own diagrams and photographs.
In-text Questions (Probe, Ponder & Thought Bubbles)
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 can often be seen during the daytime. This happens because the Moon is not always positioned exactly opposite the Sun in the sky. Depending on its phase, the Moon can rise and set at very different times from the Sun, so for a good part of the month, part of its orbit keeps it above the horizon (and bright enough against the daytime sky) at the same time the Sun is also up — this is most easily noticed during the waxing and waning crescent or gibbous phases.
Imagine you lived on the Moon instead of Earth. What would you mean by a day, a month, or a year?
What would happen if Earth had two moons instead of one? How would that change the night sky?
With two moons, the night sky would show two separate sets of changing phases at the same time, each on its own cycle (the two moons would likely have different orbital periods, so their phases would rarely match). The night sky would often be brighter overall (more reflected sunlight), tides would become more complex (combining the gravitational pull of both moons), and there could be more frequent and more complicated eclipses. Keeping time using “the Moon’s cycle” would also become more complicated, since there would no longer be a single, simple monthly cycle to rely on.
If we didn’t have clocks or calendars, how else could we measure time?
We could measure time using natural periodic (repeating) events in the sky, exactly as ancient people did: the daily cycle of sunrise and sunset (and the length/direction of shadows) for a day; the repeating phases of the Moon for a month; and the cycle of changing seasons (linked to the Earth’s revolution around the Sun) for a year. Other periodic natural events, like the rise and fall of ocean tides, can also help track the passage of time.
Why does the illuminated portion of the Moon seen from the Earth decrease when it appears closer to the Sun?
Only one half of the Moon is ever lit by the Sun at a time, and we can only see the part of that lit half that also happens to be facing the Earth. As the Moon moves around the Earth and appears closer to the Sun’s position in the sky, the geometry changes so that less and less of its sunlit half is turned towards the Earth — most of the illuminated half is now turned away from us, towards the Sun’s side. This is why the visible bright portion of the Moon shrinks as it appears closer to the Sun, eventually disappearing almost completely at New Moon, when the Moon is nearest the Sun in the sky and its entire sunlit half faces away from Earth.
How are natural periodic events like day-night and seasons used for keeping time?
These three natural cycles — day, month, and year — together form the basis of all calendars.
Why do most Indian festivals fall on different dates every year?
Most Indian festivals (like Diwali, Holi, Eid-ul-Fitr) are fixed according to the Moon’s phase in lunar or luni-solar calendars, not according to the Gregorian (solar) calendar. Since a 12-month lunar year is only about 354 days, shorter than the roughly 365-day solar year used in the Gregorian calendar, the lunar-based festival date drifts earlier each Gregorian year. Luni-solar calendars partly correct this drift every few years by adding an extra “intercalary month” (Adhika Maasa), keeping the shift limited to less than a month, while a purely lunar calendar (like the Islamic calendar used for Eid) has no such correction, so its festivals can shift through every season over the years.
When I look at the night sky in early evening, I see some moving stars. What are they? Is their motion also periodic?
What look like fast-moving “stars” in the night sky are usually artificial satellites, not actual stars (real stars don’t visibly move across the sky in a few minutes — only the whole sky appears to slowly turn due to Earth’s rotation). These satellites appear as small, steadily-moving points of light because they reflect sunlight while orbiting the Earth, typically about 800 km up, completing one orbit in roughly 100 minutes.
Yes, their motion is periodic — each satellite repeatedly circles the Earth in a fixed orbital period (close to 100 minutes for low-Earth-orbit satellites), passing over the same regions again and again on a predictable, repeating schedule.
Keep the Curiosity Alive — Exercise Solutions
State whether the following statements are True or False.
We only ever see the illuminated (sunlit) portion of the Moon that also happens to be facing the Earth.
Correction: The Moon’s phases are not caused by Earth’s shadow — that is a common misconception. Phases occur due to the changing relative positions of the Sun, Moon, and Earth as the Moon orbits Earth. Earth’s shadow falling on the Moon causes a lunar eclipse, which is a separate, much rarer event.
Calendars are built on the predictable, repeating cycles of the day (Earth’s rotation), the month (Moon’s revolution/phases), and the year (Earth’s revolution around the Sun).
Correction: The Moon can also be seen during the daytime on many days of the month, depending on its phase and position relative to the Sun.
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, his birthday will generally not fall on a full Moon day in later years. The Gregorian (solar) calendar date of 6th May repeats exactly every 365 (or 366) days, but the Moon’s phase cycle (about 29.5 days) does not divide evenly into a solar year. So, each year, the full Moon shifts to a different Gregorian date — roughly 11 days earlier (since 12 lunar months = 354 days, about 11 days short of the 365-day solar year) before slowly cycling back around over many years. Therefore, 6th May will only rarely coincide with a full Moon day again.
Name two things that are incorrect in Fig. 11.10.
Look at the pictures of the Moon (A–F). Match each picture to its correct phase, and list the picture labels of phases that are never seen from Earth.
| Phase of Moon | Matching picture |
|---|---|
| Three days after New Moon (thin waxing crescent) | D |
| Full Moon (fully illuminated disc) | E |
| Three days after Full Moon (waning gibbous) | F |
| A week after Full Moon (last-quarter / half Moon) | C |
| Day of New Moon (fully dark disc) | B |
(ii) Picture A is the one left over — it does not correspond to any of the five listed phases and represents a Moon shape that is never actually seen from Earth in that exact form.
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?
(i) Malini saw a half-illuminated Moon (a semicircle, bright on the side facing the Sun’s direction). (ii) The Moon was in its waxing phase (first quarter).
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; Ravi’s observation is not astronomically possible.
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?
A smaller gap means luni-solar calendars will need to add an intercalary month less often than they do today.
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.
Therefore, at least two of the 37 full Moons must fall within the same calendar month — this is exactly what gives rise to a “Blue Moon.”
On a particular night, Vaishali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?
A Moon that is visible continuously through the entire night — rising right at sunset and setting right at the next sunrise — must be positioned exactly opposite the Sun in the sky. This only happens on the Full Moon day, when the Moon appears nearly opposite the Sun and shows its entire illuminated face to Earth all night long.
If we stopped having leap years, in approximately how many years would the Indian Independence Day (15 August) happen in winter?
It would take approximately 728 years (roughly 700–730 years) for 15 August to drift into the winter season.
What is the purpose of launching artificial satellites?
Artificial satellites are launched and placed in orbit around the Earth to support a wide range of important functions: communication (TV, phone, internet signals), navigation (GPS-like positioning), weather monitoring (tracking storms and climate), disaster management (assessing damage and planning relief), and scientific research (studying the Earth, Sun, stars, and other celestial objects). In India, ISRO operates many such satellites, including the Cartosat series for mapping and AstroSat for astronomical observations.
On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?
| Time unit | Based on the periodic phenomenon of… |
|---|---|
| (i) Day | Earth’s rotation about its own axis (one full spin ≈ 24 hours, the mean solar day) |
| (ii) Month | The Moon’s revolution around the Earth (one full cycle of phases ≈ 29.5 days) |
| (iii) Year | Earth’s revolution around the Sun (one full cycle of seasons ≈ 365¼ days) |
Discover, Design & Debate
The Moon’s crescent always faces towards the Sun. Point your finger towards the Sun and move it across the sky towards the crescent Moon — what do you notice?
Your finger, moving along the shortest path from the Sun to the Moon, will always cross the illuminated (bright) part of the crescent first, before reaching the dark part. This confirms that the bright “horns” of the crescent always point away from the Sun, while the curved illuminated edge faces towards it — direct visual proof that the Moon shines only because it reflects sunlight falling on it from the Sun’s direction. Joining the two tips (“horns”) of the crescent with a straight line gives you the Moon’s actual diameter, useful for estimating the Moon’s true size and orientation in the sky.
Most dates in the Indian National Calendar always map to the same dates in the Gregorian calendar. Which ones may differ for certain years?
Since both the Indian National Calendar and the Gregorian calendar are solar calendars with 365 days in a regular year, most month-start dates stay fixed from year to year. The dates most likely to shift are those near the start of the year and around the leap-year adjustment — the Indian National Calendar’s New Year (1 Chaitra) normally falls on 22 March, but in a leap year it shifts to 21 March (since the leap day is added to Chaitra, the first month, rather than to a later month as in February for the Gregorian calendar). So dates in Chaitra (and any date that depends on counting from the year’s start) can differ by a day in leap years compared to non-leap years.
Find the names of New Year festivals celebrated in any 10 states of India, and identify whether each follows a lunar, solar, or luni-solar calendar.
Examples to get started: Vaisakhi (Punjab) and Poila Baisakh (West Bengal) are typically linked to the solar calendar (around mid-April, close to a solar transit date), while Gudi Padwa (Maharashtra), Ugadi (Andhra Pradesh/Telangana/Karnataka), and Cheti Chand (Sindhi community) follow the lunar/luni-solar calendar, falling on the first day of the bright fortnight of Chaitra. Continue researching festivals such as Puthandu (Tamil Nadu), Bihu (Assam), Losar (Himalayan/Buddhist regions), Navreh (Kashmir), and Vishu (Kerala), noting for each whether the date is fixed in the solar calendar (almost same Gregorian date every year) or shifts slightly (indicating a lunar or luni-solar basis).
Compare Eid-ul-Fitr and Diwali dates over five years to study calendar drift, and confirm intercalary months using a luni-solar calendar.
Sketch the eastern horizon and mark the Sun’s rising position each month for a year. What pattern do you expect?
You should find that the Sun’s rising point shifts gradually along the horizon through the year: moving northward from around December to June (Uttarayan) and southward from around June to December (Dakshinayan), as described in the chapter’s “Our scientific heritage” box. On the equinoxes (around 21 March and 23 September), the Sun should rise almost exactly due East; on the solstices (around 21 June and 21 December), it reaches its northernmost and southernmost rising points respectively.