NCERT MCQ Solutions for Class 8 Science Curiosity Chapter 11 Keeping Time with the Skies

The Moon’s changing phases result from its revolution around the Earth. As it moves, different portions of its illuminated half are visible from Earth. The Earth’s shadow causes eclipses, not phases. Only the sunlit part of the Moon facing Earth is visible on any given day.

The Moon takes around 29.5 days to complete a full cycle from one full Moon to the next. This cycle includes all visible phases like crescent, half and gibbous, forming the basis of the month in lunar calendars used across many cultures.

A day is the shortest unit of time in calendars, defined by Earth’s rotation. One complete rotation brings the Sun back to its highest point in the sky. This cycle lasts 24 hours and is called a mean solar day, forming the foundation of timekeeping.

The apparent motion of the Sun rising in the east and setting in the west is due to the Earth rotating west to east on its axis. This daily cycle is responsible for day and night and determines the concept of a solar day.

A lunar calendar defines a month based on the Moon’s revolution around the Earth. This cycle lasts approximately 29.5 days. The phases of the Moon repeat in this period, making it a natural way to count months in various ancient and modern cultures.

A solar year is about 365.25 days while a lunar year is about 354 days. This difference creates a mismatch in months and seasons, requiring adjustments like adding an intercalary month in luni-solar calendars to synchronize both cycles.

Luni-solar calendars add an extra or intercalary month called Adhika Maasa every 2–3 years to match the lunar cycle with the solar year. This keeps the festivals and seasons aligned, maintaining cultural and agricultural relevance across generations.

A mean solar day is the average time between two successive noons, when the Sun is at its highest point in the sky. This duration is fixed at 24 hours and forms the standard unit of time measurement in daily life.

At noon, the Sun is at its highest point in the sky, casting the shortest shadow. As a way to observe and record solar time by tracking the movement of shadows during the day.

On a New Moon day, the side of the Moon that faces the Earth is not illuminated by the Sun. Therefore, it is not visible to observers on Earth. The Moon phases depend on light reflection and viewing angle.

The gravitational pull of the Moon affects the water on Earth, causing high and low tides.
These tides follow a regular pattern, shifting about 50 minutes daily. This cycle is closely linked to the Moon’s orbit and position relative to Earth.

The Moon rises approximately 50 minutes later each day. This occurs because the Moon moves forward in its orbit while the Earth rotates.
Observing this shift is a key part of understanding Moon phases and timekeeping.

Solar calendars use the Earth’s revolution around the Sun to define a year.
These calendars, like the Gregorian calendar, align months with the seasons and adjust for the quarter-day by adding a leap year every four years.

The Indian National Calendar begins the year on 22 March, the day after the spring equinox.
This date aligns with the seasonal cycle and was established for official purposes. Leap years are adjusted similarly to the Gregorian calendar.

Meghnad Saha, a pioneering astrophysicist, chaired the Calendar Reform Committee. His scientific work led to the formulation of the Indian National Calendar.
He also contributed significantly to astrophysics and timekeeping systems in India.

A shadow clock or sundial uses the position of the Sun in the sky to measure time. The shadow cast by a vertical object changes direction and length throughout the day. This observation helps in tracking the passage of time without mechanical devices.

A gnomon is a stick or pole that casts a shadow used to measure time in ancient sundials. How the gnomon’s shadow moves as the Sun moves across the sky, helping early observers divide the day into parts.

A clepsydra is a device that measures time using the flow of water. Ancient people used this method at night or during cloudy days when sunlight and shadows were unavailable.
It is one of the earliest forms of non-solar timekeeping.

A solar year is approximately 365.25 days and represents the time Earth takes to complete one full revolution around the Sun.
This measurement is the basis of solar calendars and determines the sequence of seasons.

In Indian calendars, the day when the entire face of the Moon visible from Earth is illuminated is called Purnima.
It marks the full Moon phase and holds religious and cultural significance across various Indian traditions.

Amavasya refers to the New Moon day when the Moon is not visible from Earth. The point in the lunar cycle where the Sun-lit side of the Moon faces away from Earth, resulting in no moonlight.

A tithi is the lunar day in Indian calendars. It is based on the angular distance between the Sun and the Moon. Tithi helps determine festival dates and is different from the fixed dates in solar calendars.

A year is about 365.25 days long. To adjust for the extra 0.25 days each year, one full day is added every four years as 29 February. This keeps the calendar aligned with Earth’s position around the Sun and the seasonal cycle.

India uses both the Indian National Calendar and the Gregorian calendar for official purposes.
The Indian National Calendar begins on 22 March and was designed to align with Indian festivals and scientific principles, while the Gregorian is used globally.

The clepsydra, or water clock, was used in ancient times to measure time at night or during cloudy days when shadows from the Sun were not available.
It worked by measuring the steady flow of water from one vessel to another.

The Moon’s gravitational pull not only causes the phases of the Moon to appear from Earth but also controls ocean tides.
The timing and height of tides correspond closely with the Moon’s orbit and position.

A day is defined by one full rotation of the Earth on its axis. This rotation causes the cycle of day and night and sets the basic unit for timekeeping.
This natural astronomical event as the origin of a solar day.

During a lunar eclipse, the Moon passes into the Earth’s shadow, making it partially or fully disappear. This is different from a New Moon, where the unlit side faces Earth. 

A luni-solar calendar incorporates both the phases of the Moon and the position of the Sun. Indian traditional calendars follow this system and add an intercalary month when needed to synchronize lunar months with solar seasons.

Humans have long relied on observing the skies—such as the Sun, Moon and stars—to track time.
These observations helped create calendars, understand seasons and celebrate festivals at appropriate times across civilizations.