Keeping Time with the Skies | CBSE Class 8 Science Notes

This note covers the Moon’s phases and changing position, observations of sunlight and shadows, natural cycles of time, lunar and solar calendars, leap years, the Indian National Calendar, festival dates, artificial satellites, and space debris.

What changes as the Moon passes through its phases?

The phases of the Moon are the changing shapes of its bright portion as seen from Earth on successive days. The Moon itself remains spherical. It shines by reflecting sunlight, rather than producing its own light.

A full Moon appears as a complete bright circle. The full Moon day is also called Purnima. A new Moon is the phase when the illuminated portion is not visible from Earth. The new Moon day is also called Amavasya.

How do waxing and waning differ?

Waning means that the visible bright portion decreases. Starting from a full circle, it becomes a half circle in about a week. It continues shrinking for another week until it is no longer visible. In India, this period is generally called Krishna Paksha.

Waxing means that the visible bright portion increases. After the new Moon, the bright portion grows to a half circle in about a week and then becomes a full circle in another week. This period is called Shukla Paksha.

A gibbous phase shows more than half of the illuminated portion; a crescent phase shows less than half. Both shapes occur during waxing and waning. Knowing the shape alone is therefore different from knowing whether the bright portion is increasing or decreasing.

Moon images form a circle between a full Moon on the left and a new Moon on the right. The intermediate images include gibbous, half-lit and crescent phases. Four intervals are each labelled about one week.

See Fig. 11.2 in your NCERT textbook

The sequence repeats: waning is followed by waxing, and another full Moon begins the next cycle. The interval from one full Moon to the next is about a month. These repeated changes provide a natural way to track time.

How can observations reveal the Moon’s changing appearance?

A useful Moon record connects its appearance with the observation time and its position relative to the Sun. Begin at sunrise on the first day after a full Moon, looking westward. This starting point makes the Moon easiest to locate for the activity.

What should an observation record contain?

1. Write the date and record whether the observation was made at sunrise or sunset.
2. Draw a circle and shade its dark portion so that the unshaded area represents the Moon’s bright portion.
3. From the second day onwards, record whether the bright portion has increased or decreased compared with the previous day.
4. Record whether the Moon appears closer to or farther from the Sun in the sky than on the previous day.
5. After about 15 days, when the Moon may not be visible at sunrise or sunset, continue the activity at sunset for the next 15 days.

Compare successive records rather than treating one drawing as a complete description of the cycle. Ask whether the Moon was visible every day, whether its bright portion changed, and whether it appeared in the same position at the same observation time.

The record distinguishes two observations: the size of the bright portion and the Moon’s separation from the Sun in the sky. A smaller visible bright portion does not mean that the Moon itself has become smaller. Its appearance and location change together as it travels around Earth.

Repeated observations also challenge the idea that the Moon belongs exclusively to the night sky. It can sometimes be seen during daylight. A record of when it is visible is therefore as important as a drawing of its shape.

Why do we see different illuminated portions of the Moon?

Illuminated means lit by light. Sunlight lights the half of the Moon facing the Sun; the other half faces away and remains non-illuminated. The Moon’s movement around Earth is called its revolution. During this movement, the fraction of its illuminated portion visible from Earth changes.

One half of the Moon always faces Earth, but that Earth-facing half is not always the illuminated half. Sometimes the whole illuminated portion faces us. At other times, only part of it does. At new Moon, the non-illuminated portion faces Earth.

Parallel sunrays enter from the left. The Moon’s left half is labelled illuminated and the right half non-illuminated. The illustration separates the half receiving sunlight from the half facing away.

See Fig. 11.3 in your NCERT textbook

How does a ball model explain the phases?

1. Insert a stick into a small soft ball to represent the Moon. The observer’s head represents Earth.
2. In a dark open place, use an electric lamp or have a guardian shine a torch from about 3 metres away. The light represents sunlight.
3. Hold the ball at arm’s length, slightly above the head, towards the lamp. The side facing the observer is non-illuminated, modelling new Moon.
4. Turn slowly anticlockwise with the arm outstretched, continuing to look at the ball. Different fractions of the bright portion become visible.
5. When the ball is opposite the lamp’s direction, its entire illuminated portion faces the observer, modelling full Moon.

The model changes the position of the ball relative to the lamp and the observer without changing the ball’s shape. It therefore explains how a spherical Moon can appear crescent, half-lit, gibbous or full. The changing view of the illuminated portion produces the phases.

Earth is central, with eight Moon positions labelled A to H and sunlight arriving from the right. A is full Moon and E is new Moon. Orange dashed markings identify Earth-facing portions. A lower row shows the corresponding phases. Sizes and distances are not to scale.

See Fig. 11.5 in your NCERT textbook

When should we look for the Moon, and why are phases not eclipses?

On full Moon day, the Moon is nearly opposite the Sun in the sky. As the Sun rises in the east, the Moon is almost setting in the west. During subsequent mornings, its bright portion decreases and it appears closer to the Sun.

When the waning bright portion becomes a half circle, the Moon is overhead at sunrise. A few days later, its crescent appears still closer to the Sun. A waxing Moon is easiest to spot at sunset; a waning Moon is easiest to spot at sunrise.

Why does moonrise become later?

Rotation is Earth’s turning about its own axis, the line about which it spins. As Earth completes a rotation in 24 hours, the Moon also moves ahead along its orbit, its path around Earth. Earth must rotate some more to bring it to nearly the same apparent position.

The Moon rises about 50 minutes later each day. Sometimes it rises in the afternoon, around 2:00 to 4:00 p.m. It may take about 30 minutes after the listed moonrise time to become high enough to see.

People in different parts of Earth see nearly the same phase on a given day. Earth’s one-day rotation takes much less time than the Moon’s revolution, which lasts nearly a month. This does not mean that everyone observes the Moon at the same local time.

What distinguishes phases from eclipses?

Note: Earth’s shadow falling on the Moon produces a lunar eclipse, not the regular Moon phases. Phases result from the changing arrangement of the Sun, Moon and Earth and our changing view of the illuminated portion.

A solar eclipse involves the Moon blocking the Sun from view. Lunar eclipses can occur only at full Moon and solar eclipses only at new Moon. They do not happen every month because the Moon’s orbit is slightly tilted relative to Earth’s orbit around the Sun.

How do natural cycles define a day, a month and a year?

A periodic event repeats in a regular cycle. The apparent daily movement of the Sun, the Moon’s changing phases and the cycle of seasons provide different time scales. Observing these cycles allowed people to develop calendars before modern instruments were available.

The Sun appears to rise eastward and set westward, returning again the next day. This apparent periodic motion is primarily due to Earth’s rotation. It supplies the basis for the day as a unit of time.

Definition: A mean solar day is the average interval between the Sun’s highest position in the sky on one day and its highest position on the following day. Its duration is 24 hours.

A lunar month is based on the Moon completing its phase cycle. A solar year follows the seasonal cycle. A month is therefore longer than a day, while a year covers a much longer cycle containing nearly 12 lunar months.

The Moon’s phase cycle lasts about 29.5 days, so “about a month” is an approximation, not a statement that every calendar month has this duration. Similarly, a year’s natural cycle is not exactly 365 days. Calendar rules must deal with these differences.

Keeping these cycles distinct helps explain why different calendars can measure the passage of time accurately while assigning different month names or festival dates to the same day.

How can a stick’s shadow help measure a solar day?

A shadow changes as the Sun’s apparent position changes during the day. When the Sun reaches its highest position, the shadow of a fixed object is shortest. Finding this moment on successive days provides a way to measure a solar day.

What is the observation procedure?

1. Choose a small, flat patch of ground that receives sunlight. Fix a stick 1 metre long vertically in the ground.
2. Begin at 11:00 a.m. Mark a dot at the tip of the shadow every minute until around 1:10 p.m.
3. Identify the shortest shadow and find its time by counting the dots. Record that time and repeat the observations over the next few days.
4. Find the interval between the shortest-shadow times on two consecutive dates. Compare these intervals with the mean solar day.

A vertical stick stands on the ground beneath several illustrated Sun positions. Dotted lines connect the positions to the stick, while ground lines show shadows of different lengths and directions.

See Fig. 11.7 in your NCERT textbook

In the observed example, the shortest shadow occurs at 12:20 on 22 March 2025 and again at 12:20 on 23 March 2025. The interval is $24\,\text{hours} + 0\,\text{minutes} = 24\,\text{hours}$.

On 24 March 2025 the shortest shadow occurs at 12:19, one minute earlier by the clock. The interval from the previous day's shortest shadow is $24\,\text{hours} - 1\,\text{minute} = 23\,\text{hours}\,59\,\text{minutes}$.

The second interval is nearly, but not exactly, 24 hours. This is why the definition uses an average. The activity measures the interval from one highest-Sun position to the next, rather than treating the duration of daylight as the solar day.

Both the date and the time matter when calculating an interval across consecutive days. Two identical clock readings on successive dates represent a complete day, not a zero interval.

How do lunar and solar calendars differ?

A lunar calendar organises time using the Moon’s phase cycle. It has 12 lunar months in a lunar year. A solar calendar instead keeps its year aligned with the cycle of seasons. Agricultural activities made anticipating the seasons particularly important.

Twelve lunar months total 354 days. Using the approximate phase-cycle duration of 29.5 days, $12 \times 29.5\,\text{days} = 354\,\text{days}$. Seasons repeat in approximately 365 days, so a lunar year does not keep the same months aligned with the same seasons in successive years.

This mismatch concerns the seasonal alignment, not the regularity of the Moon’s phases.

Why does the Gregorian calendar need leap years?

The Gregorian calendar is a widely used solar calendar. February ordinarily has 28 days. A leap year adds a day to February, giving it 29 days: $28\,\text{days} + 1\,\text{day} = 29\,\text{days}$.

Earth takes nearly an extra quarter of a day beyond 365 days to go once round the Sun, and these extra hours accumulate to approximately one day every four years. Using a quarter-day per year gives $4 \times \frac{1}{4}\,\text{day} = 1\,\text{day}$.

The basic rule makes years divisible by four leap years, but century corrections are also needed. A year divisible by 100 is skipped unless it is divisible by 400. Thus 1700, 1800 and 1900 were skipped, while 1600 and 2000 were leap years.

The interval between successive spring equinoxes is slightly less than 365 and a quarter days. Adding one day every four years therefore adds slightly too much over time. The century corrections keep the calendar closely matched to the seasons.

How do observations of the Sun and stars distinguish types of year?

An equinox marks a seasonal position when the Sun rises exactly in the east and sets exactly in the west. The spring equinox is the one associated with spring. A tropical year is the interval between successive spring equinoxes and is the basis of the Gregorian calendar.

A sidereal year uses the stars as its reference: it is the time taken for the same stars to rise again at sunset. The stars rising at sunset change through the year as Earth revolves around the Sun.

The sidereal year exceeds the tropical year by 20 minutes: $T_{\mathrm{sidereal}} - T_{\mathrm{tropical}} = 20\,\text{minutes}$, where the two symbols denote the respective year durations expressed in minutes. This small difference takes a long time to produce noticeable differences between calendars.

Astronomers use the sidereal year to track Earth’s position in its orbit.

What are Uttarayan and Dakshinayan?

The Sun does not always rise exactly east. In summer its rising position is a little north of east, and in winter a little south of east. The extreme positions occur at the solstices, around 21 June and 21 December.

Uttarayan is the Sun’s apparent northward movement from December to June. Dakshinayan is its apparent southward movement from June to December. This annually repeating pattern is closely connected with the seasons.

Careful observers could recognise such cycles without knowing that Earth revolves around the Sun or having modern instruments. They established a year of approximately 365 days by watching recurring patterns. Stars rising at sunset also helped people track equinoxes and solstices.

These are different reference patterns for following a year. The Sun’s changing rising position, the repeating seasons and the changing stars visible at sunset all connect the calendar to observations of the sky.

How does a luni-solar calendar keep lunar months aligned with seasons?

A luni-solar calendar combines lunar months with adjustments that keep the year aligned with the seasons. It primarily uses Moon phases for counting days and months. Its extra adjustment addresses the difference between 12 lunar months and a solar year.

The 354 days in 12 lunar months fall short of a solar year by nearly 11 days. Using 365 days for the rounded solar year, $365\,\text{days} - 354\,\text{days} = 11\,\text{days}$.

Over two to three years, the accumulated difference becomes close to a month. Some calendars therefore insert an extra month every few years.

This inserted month is called Adhika Maasa or an intercalary month. It brings the lunar cycle and solar year back into step. This adjustment differs from adding a leap day: an intercalary month adjusts a lunar-month calendar, whereas a Gregorian leap day corrects the solar year’s fractional day.

How can lunar months begin at different phases?

In an Amant calendar, a month begins on the day after new Moon and ends on new Moon. In a Purnimant calendar, it begins on the day after full Moon and ends on full Moon. Both conventions connect the month boundary to a particular phase.

Traditional month names include Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha or Agrahayan, Pausha, Magha and Phalguna. Similar-sounding names occur across different Indian luni-solar calendars.

How is the Indian National Calendar organised?

The Indian National Calendar is a solar calendar used by the Government of India alongside the Gregorian calendar for multiple official purposes. An ordinary year has 365 days. Its months have either 30 or 31 days, with names drawn from traditional Indian calendars.

The year ordinarily begins on 22 March. Chaitra is its first month. In a regular year, the second through sixth months have 31 days each, while the other months have 30 days each. Its leap years match those of the Gregorian calendar.

The five months of 31 days and seven months of 30 days give the regular-year total: $5 \times 31\,\text{days} + 7 \times 30\,\text{days} = 365\,\text{days}$.

The extra day belongs to Chaitra, not to February in this calendar. February receives the extra day in the Gregorian system. Matching leap years allows the two calendars to remain connected while retaining different month names and starting dates.

Why was calendar reform undertaken?

In 1952, the Government of India appointed the Calendar Reform Committee to examine the calendars then in use and recommend an accurate, uniform national calendar. Its work addressed the practical need for a common calendar across the country.

The committee’s chairperson was Meghnad Saha, an Indian astrophysicist, meaning a scientist studying the physical nature of stars and other celestial objects. He studied stars and their temperatures and developed the Saha equation. The Saha Institute of Nuclear Physics in Kolkata bears his name.

Sharing traditional month names does not make the National Calendar a lunar calendar. Its year follows the solar cycle, and its leap-year adjustment is linked to the Gregorian calendar.

Why do festival dates shift between Gregorian years?

Many Indian festivals follow Moon phases through lunar or luni-solar calendars. Their Gregorian dates can consequently change between years. A fixed date in a lunar calendar is not automatically a fixed date in a solar calendar.

For festivals following luni-solar calendars, the Gregorian shift is typically less than a month. The intercalary month added every few years corrects the difference between lunar and solar years. A purely lunar calendar makes no such seasonal adjustment.

Eid-ul-Fitr can therefore occur in different Gregorian months over the years. Its Gregorian date moves earlier by about 11 days each year. Luni-solar festival dates can instead show jumps associated with an inserted month.

Why are some dates almost fixed?

Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh and Puthandu follow a solar sidereal calendar, a solar calendar based on the sidereal year. They occur on almost the same Gregorian date each year, rather than remaining permanently fixed.

A slow wobble of Earth’s axis causes sidereal-calendar festival dates to shift relative to the tropical calendar. Makar Sankranti moves ahead by one day every 71 years. The small difference between the two kinds of year accumulates over time.

Many festival dates depend on the exact lunar phase at sunrise. Since sunrise occurs earlier in eastern India than western India, dates can differ by a day between these regions. The government’s Positional Astronomy Center publishes the Rashtriya panchang, calculations of celestial positions used to give advance festival dates for holiday declarations.

Why are artificial satellites useful, and what is space debris?

A satellite is an object that orbits a larger body, such as a planet. The Moon is Earth’s natural satellite. Artificial satellites are human-made objects placed in orbit around Earth or another celestial body. They may look like tiny points moving across the night sky.

Most orbit about 800 kilometres above Earth’s surface and take roughly 100 minutes per orbit. These are qualified descriptions, not exact values for every satellite. Satellites support communication, navigation (finding position or direction for travel), weather monitoring, disaster management and scientific research.

What work do Indian satellites perform?

The Indian Space Research Organisation (ISRO) launches satellites for these purposes. Cartosat satellites supply high-quality Earth images for improving maps, planning cities and managing natural disasters.

Bhuvan is a mapping platform using these images to display terrain, soil, land use and vegetation. AstroSat observes stars and other celestial objects scientifically. ISRO also enables Indian students to build and launch small satellites such as AzaadiSat, InspireSat-1 and Jugnu.

Other Indian missions include Chandrayaan 1, 2 and 3 to the Moon, Aditya L1 to study the Sun, and Mangalyaan to Mars. These examples connect space exploration with both scientific observation and practical services on Earth.

How can satellites be observed, and what happens after use?

With an adult, choose an unobstructed sky view just before sunrise or after sunset. Look for a point of steady or flickering light moving very fast across the sky. Satellites can be seen with the naked eye or binoculars.

Space debris, also called space junk, includes many satellites and rocket parts left after their useful lives. It crowds space and could collide with working satellites. Small debris burns up in the atmosphere while falling; larger pieces can crash on the ground.

Vikram Sarabhai, known as the Father of the Indian Space programme, pioneered efforts to launch the first artificial satellites. The Vikram Sarabhai Space Centre in Thiruvananthapuram develops rockets and launch vehicle technology and is named after him.

Glossary

• Moon phases — Changing shapes of the Moon’s bright portion as seen from Earth on successive days.
• Waxing — The period during which the visible bright portion of the Moon increases towards full Moon.
• Waning — The period during which the visible bright portion decreases from full Moon towards new Moon.
• Crescent — A phase in which less than half of the Moon’s illuminated portion is visible from Earth.
• Gibbous — A phase in which more than half of the Moon’s illuminated portion is visible from Earth.
• Mean solar day — Average interval between successive highest positions of the Sun in the sky, lasting 24 hours.
• Lunar month — A time unit based on one Moon-phase cycle, lasting about 29.5 days.
• Tropical year — The interval between successive spring equinoxes, used as the basis of the Gregorian calendar.
• Sidereal year — Time required for the same stars to rise again at sunset, exceeding the tropical year by 20 minutes.
• Luni-solar calendar — A calendar using lunar months while adjusting its year to remain aligned with the seasons.
• Intercalary month — An extra month inserted every few years in some calendars to align lunar cycles with the solar year.
• Leap year — A year with an additional calendar day, placed in February in the Gregorian calendar.
• Artificial satellite — A human-made object placed in orbit around Earth or another celestial body.
• Space debris — Satellites and rocket parts left after useful service that crowd space and could threaten working satellites.

Common errors and misconceptions

• Misconception: The Moon physically changes shape. Correct: It remains spherical; the fraction of its illuminated portion visible from Earth changes.
• Misconception: Earth’s shadow causes the monthly phases. Correct: Earth’s shadow causes a lunar eclipse. Phases follow changing positions and views of the illuminated Moon.
• Misconception: The Moon is visible only at night. Correct: It can sometimes be visible in daylight, including afternoons when it rises before sunset.
• Misconception: People in different countries see very different phases of the Moon on the same day. Correct: People in different parts of Earth see nearly the same phase on a given day.
• Misconception: Every year divisible by four is a Gregorian leap year. Correct: Century years are skipped unless divisible by 400.
• Misconception: An intercalary month and a leap day are the same adjustment. Correct: One adds a month to align lunar and solar cycles; the other adds a day.
• Misconception: The Indian National Calendar is lunar because it uses traditional month names. Correct: It is solar, with leap years matched to the Gregorian calendar.
• Misconception: Every artificial satellite has an exact 100-minute orbit. Correct: Most orbit about 800 kilometres above Earth and take roughly 100 minutes per orbit.

Exam-style questions with model answers

Q1. Distinguish waxing from waning using the change in the Moon’s visible bright portion. [2 marks]

1. During waxing, the visible bright portion increases from new Moon towards full Moon.
2. During waning, the visible bright portion decreases from full Moon towards new Moon.

Q2. A spherical ball is lit by a lamp. An observer turns with the ball held at arm’s length, slightly above the head. Explain how this models new Moon, full Moon and the cause of phases. [3 marks]

1. When the ball is towards the lamp, its non-illuminated side faces the observer. This models new Moon, when the illuminated portion is not visible from Earth.
2. When the ball is opposite the lamp, the observer sees its whole illuminated portion. This models full Moon.
3. As the observer turns, the visible fraction of the illuminated portion changes although the ball remains spherical. The same changing view explains the Moon’s phases.

Q3. A stick’s shortest-shadow times are 12:20 on 22 March 2025, 12:20 on 23 March 2025 and 12:19 on 24 March 2025. Find the two successive solar-day intervals and explain why shortest shadows and an average are used. [4 marks]

1. From 12:20 on 22 March to 12:20 on 23 March, the interval is 24 hours.
2. From 12:20 on 23 March to 12:19 on 24 March, the interval is 23 hours 59 minutes.
3. The shadow is shortest when the Sun reaches its highest position, so successive shortest shadows identify the endpoints of a solar day.
4. The intervals are nearly, but not exactly, identical. A mean solar day is the average interval of 24 hours. The average of these two intervals is 23 hours 59 minutes 30 seconds, which is nearly 24 hours.

Q4. A lunar month lasts about 29.5 days, and 12 lunar months total 354 days. A solar year is nearly 365 and a quarter days; use 365 days for a rounded comparison. Explain the mismatch and the luni-solar correction, distinguishing it from a Gregorian leap day. [5 marks]

1. Twelve lunar months are shorter than the approximately 365-day solar year. Using the supplied rounded durations, the difference is approximately 11 days.
2. Without a seasonal correction, the lunar months do not remain aligned with the same seasons in successive years, even though the lunar phase cycle repeats.
3. The difference accumulates over successive years. In about two to three years it becomes close to the length of a full lunar month.
4. Some luni-solar calendars insert an intercalary month, also called Adhika Maasa, every few years to bring lunar months and the seasonal cycle into step.
5. A Gregorian leap day is a different correction: it adds one day to February to account for the solar year extending beyond 365 days.

Q5. Gregorian years divisible by four are leap years, except century years must be divisible by 400. February ordinarily has 28 days and gains one day in a leap year. Apply the rule to 1700 and 2000, then state the February change. [3 marks]

1. The year 1700 is a century year but is not divisible by 400. It is therefore excluded from leap years despite being divisible by four.
2. The year 2000 is a century year divisible by 400. It remains a leap year under the century correction.
3. In a Gregorian leap year, February receives an extra day and has 29 days instead of its ordinary 28 days.

Q6. Compare the Indian National Calendar’s regular-year start, month lengths and leap adjustment with the Gregorian calendar. Give four points. [4 marks]

1. Both calendars are solar. The Indian National Calendar’s regular year begins on 22 March, whereas the Gregorian year begins on 1 January.
2. In a regular Indian National Calendar year, months two to six have 31 days and the others have 30. Gregorian months have 30 or 31 days except February, which has 28 days in an ordinary year.
3. Its leap years match Gregorian leap years, but its additional day is placed in Chaitra, the first month.
4. In a leap year it begins on 21 March. The Gregorian calendar instead puts its additional day in February.

Q7. What is space debris? State one possible danger it poses to a working satellite. [2 marks]

1. Space debris includes many satellites and rocket parts remaining in space after their useful lives.
2. It crowds space and could collide with a working satellite.

Q8. Explain five uses of artificial satellites: communication, navigation, weather monitoring, disaster management and scientific research. Include Cartosat or AstroSat where relevant. [5 marks]

1. Communication satellites relay signals between places on Earth, helping transmit telephone calls, television programmes and data over long distances.
2. Navigation is another use. This involves finding position or direction for travel: signals from satellites let a ship, vehicle or traveller work out where it is and which way to go.
3. Weather monitoring uses satellites to observe clouds and storms from space. Such images help in forecasting the weather and in warning people of severe weather, and satellites launched by the Indian Space Research Organisation support this activity.
4. Disaster management benefits from satellite information. Cartosat provides high-quality Earth images that help handle natural disasters, as well as improve maps and assist city planning.
5. Scientific research uses observations from space. AstroSat, for example, makes scientific observations of stars and other celestial objects.

Key takeaways

• The Moon stays spherical; its phases arise because different fractions of its illuminated portion are visible from Earth.
• Waxing increases the visible bright portion, while waning decreases it; the complete phase cycle takes about 29.5 days.
• A waxing Moon is easiest to locate at sunset, while a waning Moon is easiest to locate at sunrise.
• A mean solar day averages 24 hours between successive highest positions of the Sun in the sky.
• Lunar years contain 354 days, so their months do not stay aligned with the same seasons in successive years.
• Solar calendars use leap-day adjustments, while some luni-solar calendars add an intercalary month to align lunar months with seasons.
• The Indian National Calendar is solar, ordinarily begins on 22 March, and adds its leap day to Chaitra.
• Artificial satellites support practical services and scientific research; space debris could collide with working satellites.

Test yourself

What are Purnima and Amavasya?

Purnima is the full Moon day, while Amavasya is the new Moon day.

Does a crescent mean that the Moon must be waxing?

No. Crescent phases occur during both waxing and waning; the difference is whether the bright portion increases or decreases.

Why must Earth rotate some more to see the Moon in nearly the same position?

The Moon advances along its orbit while Earth completes a rotation, so Earth must turn further.

Why do eclipses not occur every month?

The Moon’s orbit is slightly tilted relative to Earth’s orbit around the Sun.

What is Adhika Maasa?

It is an extra month inserted in some calendars to keep lunar cycles and the solar year in step.

How do Amant and Purnimant month endings differ?

An Amant month ends at new Moon; a Purnimant month ends at full Moon.

Why is the Indian National Calendar classified as solar?

Its year follows the solar cycle, with leap years matched to the Gregorian calendar.

How do Cartosat and AstroSat differ in purpose?

Cartosat supplies Earth images for mapping, planning and disaster management; AstroSat scientifically observes stars and other celestial objects.