CBSE Class 8 Science Revision Notes Chapter 11 Keeping Time with the Skies

The changing phases of the Moon and the repeated movements of Earth provide natural ways to measure days, months and years. These astronomical cycles also form the basis of calendars, festivals and modern satellite observations.

The sky contains several repeating patterns that humans have used to measure time. The apparent daily movement of the Sun, the phases of the Moon and Earth’s revolution around the Sun help define a day, month and year.

These CBSE Class 8 Science Revision Notes Chapter 11 explain Moon phases, Moonrise, calendars, festivals and artificial satellites. The notes follow the current 2026–27 chapter for quick and organised revision.

Key Takeaways

• Moon phase cycle: One complete cycle of Moon phases takes about 29.5 days.
• Natural time units: Earth’s rotation defines a day, Moon phases define a month and Earth’s revolution defines a year.
• Calendar types: Calendars may be lunar, solar or luni-solar.
• Artificial satellites: They support communication, navigation, weather monitoring and scientific research.

Access Class 8 Science Chapter 11 Keeping Time with the Skies Notes in 30 Minutes

Revise the chapter in three parts:

First 10 minutes: Moon phases, waxing, waning and Moonrise
Next 10 minutes: Day, month, year and different calendar systems
Final 10 minutes: Indian festivals, National Calendar and artificial satellites

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Moon Phases in Class 8 Science Chapter 11 Notes

The Moon is spherical and does not produce its own light. It appears bright because it reflects sunlight.

The visible bright portion changes as the Moon revolves around Earth. These changing appearances are called phases of the Moon.

What Are the Phases of the Moon?

The changing shapes of the illuminated part of the Moon seen from Earth are called Moon phases.

The Moon itself does not change shape. We see different portions of its sunlit half from different positions on Earth.

Full Moon

A full Moon appears as a complete bright circle.

It occurs when the entire illuminated side visible from Earth faces us. A full Moon day is also called Purnima.

On a full Moon day, the Moon is nearly opposite the Sun in the sky. It usually rises near sunset and sets near sunrise.

New Moon

On a new Moon day, the illuminated part is not visible from Earth.

The non-illuminated side faces us, while the Moon appears close to the Sun in the sky. A new Moon day is also called Amavasya.

Crescent Phase

A crescent Moon appears when less than half of its illuminated portion is visible.

A crescent may appear before or after the new Moon, depending on whether the Moon is waxing or waning.

Half Moon Phase

A half Moon appears when half of the illuminated portion visible from Earth can be seen.

It usually occurs about one week after the new Moon and about one week after the full Moon.

Gibbous Phase

A gibbous Moon appears when more than half, but less than the whole, illuminated portion is visible.

It occurs between the half Moon and full Moon stages.

Waxing and Waning Moon in Class 8 Science Notes

The illuminated portion of the Moon increases and decreases in a regular cycle.

Waxing Period

The waxing period begins after the new Moon.

During this period:

• The visible bright portion increases.
• The Moon changes from crescent to half and then gibbous.
• It finally becomes a full Moon.
• It is called Shukla Paksha in India.

A waxing Moon is usually easier to observe around sunset.

Waning Period

The waning period begins after the full Moon.

During this period:

• The visible bright portion decreases.
• The Moon changes from gibbous to half and then crescent.
• It finally reaches the new Moon stage.
• It is called Krishna Paksha in India.

A waning Moon is usually easier to observe around sunrise.

Difference Between Waxing and Waning

Why Do Moon Phases Occur?

The Moon revolves around Earth. At all times, the half facing the Sun is illuminated.

However, the illuminated half does not always face Earth completely. We see only the part of the illuminated surface directed towards us.

As the Moon changes position around Earth, the visible fraction of its illuminated surface changes. This causes the phases.

Full Moon Position

During a full Moon, Earth is approximately between the Sun and Moon.

The illuminated side of the Moon faces Earth, so it appears as a bright circle.

New Moon Position

During a new Moon, the Moon is approximately between Earth and the Sun.

The illuminated side faces away from Earth. Therefore, the bright portion cannot be seen.

Are Moon Phases Caused by Earth’s Shadow?

No. Regular Moon phases are not caused by Earth’s shadow.

They occur because of the changing relative positions of the Sun, Earth and Moon.

Earth’s shadow falling on the Moon causes a lunar eclipse. A lunar eclipse is different from the normal monthly phase cycle.

Why Do Eclipses Not Happen Every Month?

A lunar eclipse can occur only on a full Moon day. A solar eclipse can occur only on a new Moon day.

However, the Moon’s orbit is slightly tilted compared with Earth’s orbit around the Sun. Therefore, the Sun, Earth and Moon do not align perfectly every month.

Moonrise and Moon Position in CBSE Class 8 Science Chapter 11 Notes

The Moon does not rise at the same time every day. Its position changes because it moves ahead in its orbit while Earth rotates.

Why Is the Moon Visible During the Day?

The Moon reflects sunlight and may be above the horizon during daytime.

Depending on its phase and position, it may rise in the afternoon or remain visible after sunrise.

The bright daytime sky can make the Moon harder to notice, but it is often still visible.

Where Is the Moon on a Full Moon Day?

On a full Moon day, the Moon is nearly opposite the Sun.

When the Sun rises in the east, the full Moon may be setting in the west. At sunset, the full Moon may rise in the east.

Where Is a Waxing Moon Seen?

A waxing Moon appears farther from the Sun each day.

It is generally easier to observe after sunset.

Where Is a Waning Moon Seen?

A waning Moon moves closer to the Sun in the sky each day.

It is generally easier to observe around sunrise.

Why Does the Moon Rise Later Each Day?

Earth completes one rotation in about 24 hours. During the same period, the Moon moves forward in its orbit around Earth.

Earth must rotate a little more for the Moon to return to nearly the same position in the sky.

Therefore, the Moon generally rises about 50 minutes later each day.

Natural Cycles and Time in Class 8 Science Revision Notes Chapter 11

Before clocks and modern calendars, people used repeating natural events to measure time.

The main natural cycles are Earth’s rotation, Moon phases and Earth’s revolution around the Sun.

Day

Earth rotates around its axis.

The apparent movement of the Sun from east to west and its return the next day provides the natural basis of a day.

Mean Solar Day

The average time taken by the Sun to move from its highest position in the sky on one day to its highest position the next day is called a mean solar day.

A mean solar day is about 24 hours.

The Sun’s highest position can be identified by observing shadows. A vertical object casts its shortest shadow when the Sun is highest in the sky.

Month

The Moon takes about 29.5 days to pass through a complete cycle of phases.

This cycle forms the natural basis of a month.

Year

Earth takes nearly 365 and one-quarter days to complete one revolution around the Sun.

During this time, Earth experiences one complete cycle of seasons. This cycle forms the basis of a solar year.

Natural Time Cycles

Calendars in Keeping Time with the Skies Notes

Calendars organise days, months and years. Different calendars use different astronomical cycles.

The three main types are lunar, solar and luni-solar calendars.

Lunar Calendar Class 8 Notes

A lunar calendar is based mainly on the phases of the Moon.

One lunar month is about 29.5 days. Twelve lunar months form a lunar year of about 354 days.

Main Features of a Lunar Calendar

• Based on Moon phases
• Contains about 12 lunar months
• Lunar year is about 354 days
• Shorter than the solar year by about 11 days
• Months shift through different seasons over time

Because the lunar year is shorter, the same lunar month does not remain linked with the same season every year.

Solar Calendar Class 8 Notes

A solar calendar is based on Earth’s revolution around the Sun and the cycle of seasons.

The Gregorian calendar used widely today is a solar calendar.

Main Features of a Solar Calendar

• Based on a year of about 365 days
• Remains connected with seasons
• Months are adjusted to total 365 days
• Uses leap years to correct the extra quarter day

Why Do We Have Leap Years?

Earth takes about 365 and one-quarter days to complete one revolution.

The extra quarter day adds up to about one full day in four years. Therefore, one additional day is added in a leap year.

In the Gregorian calendar, February has 29 days during a leap year.

Complete Gregorian Leap Year Rule

A year divisible by 4 is generally a leap year.

However:

• Century years are not leap years unless divisible by 400.
• Therefore, 1700, 1800 and 1900 were not leap years.
• The years 1600 and 2000 were leap years.

This rule keeps the calendar closely aligned with seasons.

Luni-Solar Calendar Class 8 Notes

A luni-solar calendar uses Moon phases for months but adjusts itself to remain linked with the solar year and seasons.

Twelve lunar months total about 354 days. This is about 11 days shorter than the solar year.

What Is Adhika Maasa?

The yearly difference accumulates over time.

Every two or three years, an extra month called Adhika Maasa or an intercalary month is added.

This keeps lunar months and seasons approximately aligned.

Features of Luni-Solar Calendars

• Months follow Moon phases.
• Years remain connected with seasons.
• An extra month is added every few years.
• Several traditional Indian calendars are luni-solar.

Amant and Purnimant Calendars

In an Amant calendar, a new month begins after the new Moon and ends on the next new Moon.

In a Purnimant calendar, a new month begins after the full Moon and ends on the next full Moon.

Lunar, Solar and Luni-Solar Calendars

Indian National Calendar in Class 8 Chapter 11 Science Notes

The Indian National Calendar is used by the Government of India along with the Gregorian calendar for official purposes.

It is a solar calendar containing 365 days in a normal year.

Main Features

• It follows the Shaka Era.
• The year normally begins on 22 March.
• In a leap year, it begins on 21 March.
• Chaitra is the first month.
• Months contain either 30 or 31 days.
• In a leap year, one extra day is added to Chaitra.

Months of the Indian National Calendar

The months include:

• Chaitra
• Vaisakha
• Jyestha
• Ashadha
• Shravana
• Bhadrapada
• Ashwin
• Kartika
• Margashirsha
• Pausha
• Magha
• Phalguna

Calendar Reform Committee

The Government of India formed the Calendar Reform Committee in 1952.

The committee recommended a unified National Calendar. It came into official use in March 1956.

Indian astrophysicist Meghnad Saha served as the chairperson of the committee.

Festivals and Astronomical Cycles

Many Indian festivals follow lunar or luni-solar calendars. Their dates therefore change in the Gregorian calendar.

Festivals Linked with Moon Phases

Festivals following a purely lunar calendar may move earlier by about 11 days in the Gregorian calendar each year.

Luni-solar calendars add an extra month every few years. This keeps festival dates within a limited seasonal range.

Festivals Linked with Solar Calendars

Some festivals follow solar or sidereal cycles.

Examples include:

• Makar Sankranti
• Pongal
• Bihu
• Vaisakhi
• Poila Baisakh
• Puthandu

These festivals usually occur on nearly the same Gregorian dates each year.

Uttarayan and Dakshinayan

The Sun appears to rise northward of east during part of the year and southward during another part.

Its apparent northward movement from December to June is called Uttarayan.

Its apparent southward movement from June to December is called Dakshinayan.

Artificial Satellites in Class 8 Science Chapter 11 Notes

The Moon is Earth’s natural satellite. Human-made objects sent into orbit around Earth are called artificial satellites.

Many artificial satellites move hundreds of kilometres above Earth and complete an orbit in a short period.

Uses of Artificial Satellites

Artificial satellites support:

• Communication
• Television and internet services
• Navigation
• Weather monitoring
• Disaster management
• Mapping
• Scientific research
• Study of stars and space

Indian Artificial Satellites and Missions

ISRO has launched many satellites and space missions.

Vikram Sarabhai

Vikram Ambalal Sarabhai is known as the Father of the Indian Space Programme.

He played an important role in the development of India’s satellite and launch programmes.

What Is Space Debris?

Old satellites, rocket parts and other discarded objects in orbit form space debris.

This debris may:

• Collide with working satellites
• Damage spacecraft
• Crowd useful orbits
• Fall towards Earth

Small debris generally burns up in the atmosphere. Larger pieces may reach the ground.

Countries are working on methods to track and remove dangerous space debris.

Important Terms in Class 8 Science Chapter 11

Quick Revision of Keeping Time with the Skies

• The Moon shines by reflecting sunlight.
• Moon phases result from changing Sun-Earth-Moon positions.
• Earth’s shadow does not cause regular Moon phases.
• Waxing means the bright part increases.
• Waning means the bright part decreases.
• One Moon phase cycle takes about 29.5 days.
• Moonrise usually occurs about 50 minutes later each day.
• Earth’s rotation defines a day.
• Moon phases define a month.
• Earth’s revolution defines a year.
• Lunar calendars contain about 354 days.
• Solar calendars use leap years.
• Luni-solar calendars use an extra month.
• The Indian National Calendar is a solar calendar.
• Artificial satellites support communication, mapping and scientific research.

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