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Keeping Time with the SkiesClass 8 Science Notes

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Section 1 of 4· 1 of 4 complete

How Does the Moon's Appearance Change and Why?

Let's explore how the Moon's appearance changes over a month. The best time to start observing is after a full Moon day, when it's easiest to spot the Moon in the sky.

Example

Think of observing the Moon like keeping a diary. Each day, you note down what it looks like and where it is in the sky. This helps you understand the patterns of the Moon's changes.

Activity 11.1: Let us explore

Here's how to track the Moon's changes:

Day 1: Spot the Moon at sunrise in the western direction after a full Moon.
Create a table to document your observations, including:
- Date
- Time you saw the Moon (sunrise or sunset)
- Shade a circle to show the bright portion of the Moon (like in Fig. 11.1)
- From the second day onwards, also note:
  - Whether the bright portion of the Moon is increasing or decreasing compared to the previous day.
  - Whether the Moon appears closer to or farther from the Sun than the day before.
For about 15 days, observe at sunrise. After that, you may need to switch to sunset observations for the next 15 days.

Analyzing your data, consider these questions:

Did the Moon look different each day?
Was the Moon visible every day?
Did the Moon appear in the same position each day?

Phases of the Moon

The phases of the Moon are the different shapes of the bright portion of the Moon as seen from Earth.

The bright portion of the Moon decreases from a full circle to a half circle in about a week. This is part of the waning period of the Moon.
The bright portion continues to shrink for another week until it's no longer visible.
The day when the Moon appears as a full bright circle is called the full Moon day (or Purnima).
The day when the Moon is not visible is called the new Moon day (or Amavasya).
After the new Moon, the bright side grows to a half circle in about a week and to a full circle in another week. This is the waxing period.
In India, the waning period is called the Krishna Paksha, and the waxing period is called the Shukla Paksha.
The Moon goes through a waning period followed by a waxing period in a cyclical manner.
The cycle from one full Moon to the next takes about a month.

Note

The phases of the Moon follow a predictable cycle, which is why they're useful for measuring time.

Locating the Moon

The Moon's position in the sky changes daily.

On a full Moon day, the Moon is nearly opposite the Sun. When the Sun rises in the East, the Moon is almost setting in the West.
As the bright part decreases, the Moon appears to move closer to the Sun.
When the bright part is a half circle, the Moon is overhead at sunrise.
A few days later, the crescent Moon appears even closer to the Sun.
A waxing Moon is easiest to spot at sunset, and a waning Moon at sunrise.
The Moon always rises and sets at different times than the Sun.
The Moon rises about 50 minutes later each day. Sometimes, it rises in the afternoon, so you can spot it during daylight.

Example

Knowing the phase of the Moon can help you predict when and where to look for it. If you know it's a waxing crescent, look for it in the western sky after sunset.

Making sense of our observations

The Moon shines because it reflects sunlight.

The half of the Moon facing the Sun receives sunlight and becomes illuminated.
The other half facing away from the Sun doesn't receive sunlight and remains non-illuminated.
The Moon revolves around the Earth.
Only one half of the Moon always faces the Earth.
The portion of the Moon facing the Earth is not always its illuminated part.
We can only see the illuminated portion of the Moon from Earth.
Sometimes, the entire illuminated portion faces the Earth, and sometimes only a part of it.
On New Moon day, we don't see the illuminated portion at all, as only the non-illuminated portion faces the Earth.

Note

The shape of the Moon itself doesn't change. What changes is how much of the illuminated portion we can see from Earth.

Activity 11.2: Let us explore

This activity helps you understand why we see phases of the Moon.

Take a small soft ball and insert a stick into it. This represents the Moon.
Go to a dark place and ask someone to shine a torchlight on you from about 3 meters away. The torch represents the Sun, and your head represents the Earth.
Hold the ball at arm's length, slightly above your head. Keep the ball at position E towards the lamp. Is the portion of the ball facing you illuminated?
Turn around slowly, keeping your arm outstretched and looking at the ball. Does the shape of the illuminated portion change? Is the line separating the illuminated and non-illuminated portions curved?

When the ball is opposite the lamp (at A), you see the entire illuminated portion, like a full Moon. When the ball is towards the lamp (at E), you see the non-illuminated portion, like a new Moon. In other positions, the line separating the illuminated and non-illuminated portions appears curved, just like the phases of the Moon.

Example

Imagine holding a ball in a dark room with a lamp. As you turn, the light on the ball changes, showing different amounts of the illuminated surface. This is similar to how the phases of the Moon appear.

Phases of the Moon Explained

The Moon revolves around the Earth in about one month.
The side of the Moon that faces the Sun is illuminated.
The portion of the Moon that faces the Earth is what we see.
At positions B and H, more than half of the illuminated portion, called the gibbous phase, is visible.
At positions D and F, less than half of the illuminated portion, called the crescent phase, is visible.
The change in the fraction of the illuminated portion seen from Earth causes the phases of the Moon.
From A to C to E, we see the waning phase.
From E to G and back to A, we see the waxing phase.
On the New Moon day, the Moon appears closest to the Sun.
On the Full Moon day, the Moon appears farthest from the Sun.

The Moon moves ahead in its orbit while the Earth completes one rotation. This is why the Moon takes about 50 minutes longer to appear in the same spot in the sky each day.

Note

The phases of the Moon are not caused by the Earth's shadow. They are due to the changing angles at which we view the illuminated portion of the Moon as it orbits Earth.

How Did Calendars Come into Existence?

The apparent motion of the Sun, rising in the east and setting in the west, is due to the Earth's rotation. This cycle is the foundation of the day, a basic unit of time.

The average time it takes for the Sun to go from its highest position in the sky one day to its highest position the next day is 24 hours. This is called the mean solar day.
You can find the Sun's highest position by measuring the length of shadows. The shadow is shortest when the Sun is at its highest point.

Activity 11.3: Let us measure a day!

This activity helps you measure the length of a day:

Find a flat area that receives sunlight.
Fix a 1-meter stick vertically in the ground.
Start observing at 11:00 a.m. Every minute, mark the tip of the stick's shadow with a dot.
Continue marking dots until around 1:10 p.m.
Identify when the shadow was shortest and record the time.
Repeat this for a few days.
Find the duration of the solar day by finding the difference in time on consecutive days.

Example

Using a stick to measure time might seem old-fashioned, but it's how people kept track of days for thousands of years! It shows the direct connection between the Sun's position and our concept of time.

The phases of the Moon give us another natural cycle longer than a day.

The Moon takes about 29.5 days (nearly a month) to cycle through all its phases.
This cycle is the basis for the month, another unit to measure time.

The next larger unit is related to the cycle of seasons.

The Earth revolves around the Sun and takes nearly 365 and a quarter days to complete one revolution.
During this time, the Earth undergoes one cycle of seasons.
This cycle defines a solar year.

Note

Days, months, and years are all based on natural cycles: the Earth's rotation, the Moon's phases, and the Earth's revolution around the Sun.

Lunar calendars

Ancient people noticed that about 12 cycles of the Moon's phases fit into one cycle of seasons.

This led to the creation of lunar calendars.
The day is the shortest unit, the month is nearly 29.5 days, and the lunar year consists of 12 lunar months.

Example

Imagine early farmers needing to know when to plant crops. They might have used a lunar calendar to track the months, knowing that certain phases of the Moon corresponded to the best times for planting.

However, in a lunar calendar, the seasons don't stay synchronized with the same lunar months each year. The seasons repeat in approximately 365 days, while the lunar year is 354 days long.

Solar calendars

The need for a year to synchronize with seasons led to the creation of solar calendars.

The Gregorian calendar, widely used today, is a solar calendar.
Months in solar calendars are adjusted to add up to 365 days.
Some months have 30 days, others 31, and February has only 28 days.

The Earth takes nearly an extra quarter of a day to complete one revolution.

These extra hours add up to approximately one day every four years.
To adjust for this, solar calendars add an extra day every four years, creating a leap year.
In a leap year, February has 29 days.

Note

Leap years keep the calendar synchronized with the seasons by accounting for the extra fraction of a day it takes for the Earth to orbit the Sun.

The Earth takes slightly less time than 365 and a quarter days to go from one spring equinox to the next.

Adding a day every four years adds a little too much over time.
To fix this, leap years are skipped every 100 years (like in 1700, 1800, and 1900).
However, skipping all of them would make the calendar lag slightly behind.
So, every 400 years, a leap year is added back (like in 1600 and 2000).

Seasons are caused by the Earth's revolution around the Sun and its movement from the spring equinox to winter equinox and back.

The time between successive spring equinoxes is called the tropical year.
The Gregorian calendar is based upon the tropical year.

The stars that rise at sunset change throughout the year due to the Earth's revolution.

The time for the same stars to rise again at sunset is called the sidereal year.
The sidereal year can also be used to define a solar calendar.
The sidereal year is longer than the tropical year by about 20 minutes.
Astronomers use the sidereal year to track the Earth's position in its orbit.

Example

Think about how the stars look different in summer versus winter. The sidereal year is based on when those same stars reappear at sunset each year.

Our scientific heritage

People in ancient times observed the sky and developed calendars.

They noticed patterns and cycles in natural events and determined that the year was approximately 365 days.
They observed that the Sun doesn't always rise exactly in the East. In summer, it rises a little north of East, and in winter, a little south of East.
These extremes happen on the solstices, around June 21 and December 21.
The Sun's apparent northward movement from December to June is called Uttarayan, and its southward movement from June to December is Dakshinayan.

Note

Ancient skywatchers didn't have telescopes, but they were keen observers. They built calendars by watching the Sun's path and the stars' movements.

In the past, the equinoxes and solstices were tracked by identifying the stars that rose at sunset.

Ancient Indian texts noted that the pattern of stars, Capricorn (called Makar), would be in the background of the Sun around the winter solstice.

Over the years, different types of calendars have evolved based on specific needs.

Luni-solar calendars

Luni-solar calendars use the Moon's phases for counting days and months but also make adjustments to stay in sync with the cycle of seasons.

The 12 lunar months add up to 354 days, which is about 11 days short of the solar year.
Every 2-3 years, the accumulated difference becomes close to a full month.
Therefore, every few years, an extra month (called Adhika Maasa or intercalary month) is added to the year.

Example

Think of luni-solar calendars as a blend of lunar and solar calendars. They use the Moon to track months but add an extra month every few years to stay aligned with the seasons.

You may have heard of the names of the months in Indian luni-solar calendars: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha (or Agrahayan), Pausha, Magha, and Phalguna.

In some communities, the new month starts on the first day after the new Moon and ends on the day of the new Moon (Amant).
In others, the start of the new month corresponds to the day after the full Moon, and the month ends on the full Moon (Purnimant).

The Indian National Calendar

The Government of India uses a national calendar along with the Gregorian calendar for official purposes.

It is a solar calendar consisting of 365 days in a year.
The year begins on March 22, the day after the spring equinox.
Months have either 30 or 31 days.
The names of the months were taken from traditional Indian calendars.
In a regular year, the second to sixth months have 31 days, and the rest have 30 days.
Leap years are matched to the Gregorian calendar by adding a day to Chaitra, the first month of the year.
In such years, the new year begins on March 21 of the Gregorian calendar.

Note

The Indian National Calendar is a solar calendar that aims to provide a uniform system for official purposes while drawing from traditional Indian calendars.

In 1952, the Government of India set up a Calendar Reform Committee (CRC) to examine existing calendars and recommend a uniform calendar. The CRC's 'Unified National Calendar' was adopted for use with effect from 21 March 1956 CE, that is, 1 Chaitra 1878 Saka. The Indian National Calendar follows the general principles as that of the Surya Siddhanta.

Example

Meghnad Saha was a pioneering astrophysicist of India, known for the Saha equation. He was also the chairperson of the Calendar Reform Committee.

Many Indian festivals are tied to the phases of the Moon and are based on lunar or luni-solar calendars.

Diwali falls on the new Moon of Kartika.
Holi is on the full Moon of Phalguna.
Buddha Purnima is on the full Moon of Vaisakha.
Eid-ul-Fitr is celebrated after sighting the crescent Moon at the end of Ramazan.
Dussehra is celebrated on the tenth day in Ashwina.

Note

The reason that many Indian festivals fall on different dates every year is that they are based on the lunar cycle, which doesn't perfectly align with the solar year.

For festivals based on luni-solar calendars, the Gregorian calendar dates can shift, but typically by less than a month. This is because luni-solar calendars add an intercalary month every few years. In contrast, purely lunar calendars do not account for this difference, and festivals can occur in different months of the Gregorian calendar year after year.

A few festivals in India, like Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh, and Puthandu, follow a solar sidereal calendar.

These festivals happen on almost the same date every year in the Gregorian calendar.
A long time ago, these festivals were tied to either a solstice or an equinox.
Due to the small difference in the sidereal and tropical years, the dates of these festivals slowly shift away from the solstices/equinoxes.
This shift is due to the slow wobble of the Earth's axis.
For example, Makar Sankranti moves ahead by one day every 71 years.

The dates of many Indian festivals are based on the exact lunar phase at sunrise.

Sunrise occurs earlier in Eastern India and later in Western India, so these dates can shift by a day between these regions.
The Positional Astronomy Center publishes the Rashtriya panchang, a detailed calculation of the positions of celestial objects, to maintain uniformity throughout the country.

Example

Think about how different regions of India celebrate the same festival on slightly different days. This often depends on the precise timing of astronomical events like sunrise or the lunar phase.

The Moon and moonlight have inspired ragas in Indian classical music.

Chandrakauns, Chandranandan, and Shubhapantuvarali are a few ragas that display the moon's imagery.
Mudras (hand gestures), for example, Chandrakala and Ardhachandran relating to the Moon, can be found in Indian classical dance Bharatanatyam.
Traditional painting styles like Madhubani and Warli invoke depictions of the Moon and the Sun.

Why Do We Launch Artificial Satellites in Space?

The Moon is Earth's natural satellite, orbiting our planet. Man-made satellites sent by various countries also orbit the Earth.

These artificial satellites appear as tiny specks moving in the night sky.
Most orbit about 800 km above Earth's surface and take roughly 100 minutes to complete one orbit.

Example

If you've ever seen a "shooting star" that moves too slowly, it might have been an artificial satellite! These satellites are constantly circling our planet, performing important tasks.

These satellites help us in many ways:

Communication
Navigation
Weather monitoring
Disaster management
Scientific research

The Indian Space Research Organisation (ISRO) has launched many satellites that support these activities.

Note

Artificial satellites have become essential to modern life, enabling everything from global communication to accurate weather forecasts.

The Cartosat series of satellites, launched by ISRO, capture high-quality images of the Earth.

These images improve maps, plan cities, and handle natural disasters.
Bhuvan, a mapping platform, uses these images to show terrain, soil, land use, vegetation, and more.

AstroSat, another ISRO mission, makes scientific observations of stars and other celestial objects. India's other space missions include Chandrayaan 1, 2, and 3 to the Moon; Aditya L1 to study the Sun; and Mangalyaan to Mars. ISRO also lets Indian students build and launch small satellites, such as AzaadiSat, InspireSat-1, and Jugnu.

Activity 11.4: Let us identify

This activity will help you spot artificial satellites:

Just before sunrise or after sunset, go to a location with a clear view of the sky.
Look for any moving object in the sky that appears as a point of light with steady or flickering brightness.
Satellites move very fast across the sky.
You can see them with the naked eye or with binoculars.
You may use mobile apps or websites that provide details of satellites visible in your location.

After their useful life, many satellites and their rocket parts become space junk or space debris.

This debris crowds space and could collide with working satellites.
Small debris burns up in the atmosphere.
Larger pieces can crash on the ground.
Countries are working together to remove this dangerous debris.

Example

Space junk is a growing problem. It's like litter in space, and it can damage or destroy working satellites.

Vikram Sarabhai, a researcher in space science and nuclear physics, is known as the Father of the Indian Space programme. He pioneered the effort to launch the first artificial satellites. The Vikram Sarabhai Space Centre (VSSC), located in Thiruvananthapuram, is named after him.

Keeping Time with the SkiesClass 8 Science Notes

How Does the Moon's Appearance Change and Why?

Activity 11.1: Let us explore

Phases of the Moon

Locating the Moon

Making sense of our observations

Activity 11.2: Let us explore

Phases of the Moon Explained

How Did Calendars Come into Existence?

Activity 11.3: Let us measure a day!

Lunar calendars

Solar calendars

Our scientific heritage

Luni-solar calendars

The Indian National Calendar

Are Festivals Related to Astronomical Phenomena?

Why Do We Launch Artificial Satellites in Space?

Activity 11.4: Let us identify