NCERT Solutions for Class 8 Science Chapter 11 Keeping Time with the Skies (PDF) – 2027

Yes. The Moon is often seen in the daytime sky (Meera saw it at the kite festival). It shines by reflecting sunlight, and this light is bright enough to show against the blue sky. The Moon rises about 50 minutes later each day, so on many days it is above the horizon while the Sun is also up — for example, moonrise on 7 April 2025 was at 14:23. It is not seen near new Moon, when it is too close to the Sun.

Every unit of time comes from a natural cycle, so on the Moon:

• Day — sunrise to sunrise would take about 29.5 Earth days (about two weeks of daylight, then two weeks of night), because the same half of the Moon always faces the Earth.
• Month — seen from the Moon, the Earth would show phases in the same 29.5 days, so the month and the day would be about the same length.
• Year — the Moon goes round the Sun along with the Earth, so a year would still be about 365¼ days.

The two moons would have different orbits and periods, so the night sky would change in several ways:

• Two different phases could be seen on the same night; both would rarely be full together.
• Fully dark nights would become rare.
• A “month” would no longer be one clear unit, so calendars would be harder to make.
• Eclipses would happen more often, and tides would be less regular.

By counting natural periodic events — events that repeat with a fixed period. That is exactly how every calendar in history began, and the chapter uses three of them.

Yes. The shape of the bright part is a little different every single day, and the change goes in one direction for a whole fortnight before it reverses.

Starting at sunrise the day after full Moon, this is what your Table 11.1 should show:

No. For a day or two around new Moon (Amavasya) the Moon cannot be seen: its non-illuminated half faces the Earth, and it is in nearly the same direction as the Sun, so it is lost in the Sun’s glare.

No. At the same time (say, sunrise) on successive days the Moon is seen in a different part of the sky. During the waning fortnight it appears closer to the Sun each day (and during the waxing fortnight, farther from it). This is because the Moon moves ahead in its orbit each day, so the Earth must turn about 50 minutes more for the Moon to come back to nearly the same place in the sky (Fig. 11.6).

Yes. Seen at sunrise each day, the Moon appears in a different part of the sky — and closer to the Sun each day as its bright part decreases:

Sample observation. Moonrise gets later by about 50 minutes each day. The chapter’s own two readings show it:

Different days give slightly different values — usually between about 30 and 70 minutes — but the average over a month is close to 50 minutes.

The Sun always lights exactly one half of the Moon — the half facing the Sun. We see only the part of this lit half that faces the Earth. When the Moon appears far from the Sun (full Moon), the whole lit half faces us. As the Moon appears closer to the Sun, more of its lit half is turned away from the Earth and we see it more edge-on, so the bright part we see gets smaller — gibbous, half, then crescent. At new Moon, nearest the Sun, only the dark half faces us.

At position E — the ball held out towards the lamp — the side facing you is not illuminated. You see a dark ball.

Yes. As you turn anticlockwise, the shape of the illuminated portion changes smoothly, in a fixed order:

Yes. At the gibbous and crescent positions (B, D, F and H) the line separating the illuminated and non-illuminated portions looks curved. At the two half-lit positions (C and G) it looks straight, and at A (fully lit) and E (dark) no such line is seen.

Yes. The sequence you see matches Fig. 11.4c position by position, and each position matches a real phase of the Moon in Fig. 11.5:

Yes. In Activity 11.1, as the bright part of the Moon decreased (waning), the Moon appeared closer to the Sun each day, and on new Moon day it could not be seen at all. As the bright part increased (waxing), it appeared farther from the Sun, until at full Moon it was opposite the Sun. Fig. 11.5a shows the same thing: at E (new Moon) the Moon is in the direction of the Sun, and at A (full Moon) it is on the far side of the Earth.

By counting them. Any event that repeats after a fixed interval can be used as a unit, and measuring time is simply counting how many of those intervals have gone by. The three cycles in this chapter give the three units we still use.

The shortest shadow of the day marks the moment the Sun is highest in the sky, so the gap between two such moments on consecutive days is one solar day. Using the readings printed in Table 11.2:

Yes, it is almost exactly 24 hours.

Because most Indian festivals are fixed by the phase of the Moon, while the dates we quote come from the Gregorian solar calendar — and a lunar year and a solar year are not the same length.

So a festival fixed by the Moon comes about 11 days earlier on the Gregorian calendar each year. In luni-solar calendars an extra month (Adhika Maasa) added every 2–3 years moves it about a month later again, so its date shifts by less than a month. Festivals on a purely lunar calendar, such as Eid-ul-Fitr, can fall in any Gregorian month.

They are not stars — they are artificial satellites orbiting the Earth. They look like points of light with steady or flickering brightness that move fast across the sky, and are best seen just after sunset or just before sunrise, when they are still lit by the Sun.

Yes, their motion is periodic. Most orbit about 800 km above the Earth’s surface and take roughly 100 minutes for one orbit — about 14 orbits a day.

Sample observation: About an hour after sunset, a steady point of light, like a faint star, was seen moving smoothly from the west towards the north-east. It crossed a large part of the sky in about 4–5 minutes and then faded away without setting. It did not blink and had no coloured lights, so it was a satellite, not an aeroplane.

No. His birthday returns on 6 May every year, but the full Moon does not.

So in one year the Moon completes 12 cycles and then moves on about 11 days more. On his next birthday the Moon is about 11 days past full — a waning crescent, not a full Moon.

1. Stars are seen through the dark part of the Moon. The Moon is a solid, opaque body; it hides every star behind it, so no star can ever appear inside the Moon’s disc — not even in its dark part.
2. The Moon is drawn in front of a cloud (the cloud on the left is partly hidden behind the Moon). Clouds are only a few kilometres above us, while the Moon is about 3,84,000 km away, so a cloud can hide the Moon, but the Moon can never hide a cloud.

(i)

(ii) F. In F the dark part is a small “bite” at the top-left: the line between the bright and dark parts ends at two points on the Moon’s edge that are not opposite each other. Sunlight always lights exactly half the Moon, so this line must always join two opposite points of the edge. Hence F is never seen from the Earth.

(i) She saw a half Moon — exactly half the disc bright, with a straight dividing line, and the bright half turned towards the setting Sun in the west.

(ii) It is in the waxing phase (Shukla Paksha).

Kaushalya is telling the truth. Ravi cannot be right.

• Ravi: a Moon rising in the east just as the Sun sets in the west is opposite the Sun (180° away). At that position the whole lit half faces us, so it must be a full Moon, not a crescent. A crescent is always fairly close to the Sun in the sky.
• Kaushalya: a waxing gibbous Moon rises in the east in the afternoon (moonrise around 2:00–4:00 p.m.), so it can be seen in daylight.

Less often. A slower Moon takes longer to go through its phases, so each lunar month becomes longer and 12 lunar months add up to more than 354 days. The lunar year then falls short of the solar year by less than 11 days, so it takes more years for the shortfall to add up to a whole month. The intercalary month will therefore be needed less often.

Count the months available and compare them with the number of full Moons.

Suppose, on the contrary, that no two full Moons shared a month. Then each of the 37 full Moons would need a month of its own, so we would need at least 37 different months. But only 36 months exist in those three years. That is impossible, so the supposition is wrong: at least one calendar month must contain two full Moons.

She would have seen the full Moon (Purnima).

To be in the sky from sunset to sunrise, the Moon must rise when the Sun sets and set when the Sun rises. This happens only when the Moon is opposite the Sun, and then the whole illuminated half faces the Earth.

About 700 to 750 years — roughly seven centuries.

Artificial satellites are launched to do jobs that are difficult or impossible from the ground:

Observation: Your finger always reaches the bright edge of the crescent first — the bulging bright side of the crescent always faces the Sun. This shows that the Moon’s light is sunlight reflected by the half of the Moon that faces the Sun. The two tips of the crescent lie at opposite ends of the Moon’s edge, so the line joining them is a diameter of the Moon.

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.

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

Eid-ul-Fitr comes about 11 days earlier every year (its lunar date, 1 Shawwal, stays the same). Diwali also moves about 11 days earlier, but from 2022 to 2023 it jumped 19 days later — so the year 2023 had an intercalary month (an extra Shravana, the Adhika Maasa).

Yes. The sunrise point moves along the horizon. From about 21 December to 21 June it shifts northward month by month (Uttarayan); from about 21 June to 21 December it shifts southward (Dakshinayan). The Sun rises exactly in the east only around the equinoxes (about 21 March and 23 September). This is the Uttarayan and Dakshinayan that our ancestors noticed.