Solutions-NCERT-Class-8-Science-Curiosity-Chapter-11-Keeping Time with the Skies-CBSE

Yes, the Moon can often be seen during the day.

• This happens because the Moon does not produce its own light; it reflects sunlight. When the Moon is above the horizon during daytime and its illuminated part faces the Earth, it can be seen even though the Sun is shining.
• The Moon’s position changes as it revolves around the Earth. Except on a new moon day, the Moon is usually at some angle away from the Sun in the sky. Because of this separation, sunlight reflects from the Moon and reaches our eyes, making the Moon visible during the day.

If we lived on the Moon, the meanings of day, month, and year would be different from those on Earth.

• Day on the Moon: A day would mean the time the Moon takes to rotate once on its axis. This is about 29½ Earth days. So one lunar day would include about two weeks of daylight followed by two weeks of night.
• Month on the Moon: A month would be the time the Moon takes to complete one revolution around the Earth. This is also about 29½ Earth days, so on the Moon a day and a month would be almost the same length.
• Year on the Moon: A year would be the time the Moon (along with the Earth) takes to go once around the Sun. This would be the same as an Earth year, about 365 days.

So, on the Moon, days and months would be very long, while the year would remain nearly the same as on Earth.

If Earth had two moons, the night sky and life on Earth would look quite different.

Changes in the night sky:

• We would often see two bright objects in the sky at night instead of one.
• Depending on their positions, the moons might appear close together, on opposite sides of the sky, or even seem to chase each other across the sky.
• Nights would be brighter, especially when both moons were full.
• We might see more frequent and complex eclipses, with moons eclipsing the Sun or each other.

Other effects on Earth:

• Tides would become stronger and more irregular, as both moons would pull on Earth’s oceans.
• The phases of the moons would be more complicated, creating many different combinations of shapes in the sky.
• Calendars and timekeeping based on the Moon would be more complex.

Overall, having two moons would make the night sky more fascinating and dynamic, with noticeable effects on Earth’s oceans and natural rhythms.

If we didn’t have clocks or calendars, we could still measure time by observing natural events and regular patterns in nature.

• Day and night: The rising and setting of the Sun can be used to measure a day. The position of the Sun in the sky (morning, noon, evening) helps divide the day into parts.
• Shadow movement: Shadows change their length and direction as the Sun moves. This idea is used in a sundial to estimate time during the day.
• Phases of the Moon: The changing shapes of the Moon can be used to measure months, from one new moon to the next.
• Seasons: Changes in weather, length of days, plant growth, and animal behavior help measure years and seasons (summer, rainy season, winter).
• Stars and constellations: The positions of stars in the night sky change gradually over the year. Ancient people used this to track long periods of time.
• Natural cycles: Events like tides, flowering of plants, migration of birds, or even the heartbeat and breathing rhythm can help measure shorter or longer intervals.

So, by carefully observing nature, people can measure time even without clocks or calendars.

True: We can only see the part of the Moon that reflects sunlight towards Earth.

False: The Earth's shadow causes lunar eclipses, not the regular phases of the Moon.

True: Calendars are based on repeating astronomical events like day-night, Moon phases, and seasons.

False: The Moon can also be seen during the daytime, depending on its phase and position in the sky.

No, Amol's birthday does not fall on a full Moon Day every year. This is because

the Moon's phases follow a lunar cycle of about 29.5 days, while the calendar year

follows the solar cycle of about 365 days. So, the date of the full Moon changes each year in the Gregorian calendar.

Incorrect things are:

(i) Stars are shown near the Moon during the daytime, which is incorrect because stars are not visible in the daytime sky.

(ii) The Moon's dark part is shaded incorrectly to show a phase. The shadow in the figure suggests it's caused by Earth's shadow, which is not true for regular Moon phases - they are caused by the Moon's position relative to the Earth and Sun, not a shadow.

(iii) Clouds are shown behind the Moon : Clouds are present in Earth’s atmosphere, while the Moon is very far away from the Earth, beyond the atmosphere. So clouds cannot appear behind or around the Moon.

(i)

(ii) Picture B (New Moon phase) is never seen from Earth because the illuminated side of the Moon is facing away from us.

(i) At sunset, the Moon is overhead only during the first quarter (a week after New Moon), when the right half is illuminated. So, we need to draw a half Moon (right half bright, left half dark).

(ii) Waxing phase (because it occurs after New Moon and the bright part is increasing).

Kaushalya is telling the truth.

• A crescent Moon near sunset is a waxing crescent, which appears low in the western sky and sets shortly after the Sun—it wouldn’t be rising in the east at that time.
• A gibbous Moon (waxing) typically rises in the early afternoon in the east and is visible during the afternoon and evening, so seeing it in the eastern sky in the afternoon is perfectly consistent.

Luni-solar calendars will need an intercalary month more often as the Moon moves farther and slower and it takes longer to complete a cycle. So, a lunar year becomes even shorter compared to the solar year.

Yes, at least two full Moons must happen in the same solar month.

A solar calendar has 12 months x 3 years = 36 months.

37 full Moons in 36 months

If we place 37 full Moons into 36 months, then at least one month must contain two full Moons, because there are more full Moons than months.

As the Moon is visible all night long only on a Full Moon, hence it is a Full Moon.

One leap year adds ~ 1 day every 4 years.

Without leap years, the calendar shifts by 1 day every 4 years.

• August 15 is in mid-monsoon (summer).
• To reach mid-December (winter in India), we need a drift of about 4 months (≈ 120 days).
• At 1 day per 4 years, that’s: 120 x 4 = 480 years

So, in about 480 years, Independence Day would fall in mid-December, i.e., winter in India.

Artificial satellites are man-made objects placed in orbit around the Earth for a variety of purposes, including:

• Communication
• Navigation
• Weather monitoring and forecasting
• Disaster management
• Scientific research
• Earth observation and mapping

• Day: Based on Earth's rotation on its axis (approximately 24 hours from one solar noon to the next).
• Month: Based on the cycle of the Moon's phases (a lunar month is ~29.5 days).
• Year: Based on Earth's revolution around the Sun (a solar year is ~365 ¼ days).