Keeping Time with the Skies
Chapter Notes | Science Class 8
1. Introduction
The Moon can sometimes be seen during the daytime. This curious fact, such as seeing the Moon on Makar Sankranti, leads us to ask why the Moon changes its appearance and how observations of the sky have been used to measure time. This chapter explains the phases of the Moon, how to locate the Moon in the sky, why eclipses do not happen every month, how calendars were created, and why we launch artificial satellites.
2. How Does the Moon's Appearance Change and Why?
Phases of the Moon
The changing shapes of the Moon's illuminated portion, as seen from Earth, are known as the phases of the Moon.
Waning period (Krishna Paksha): After a full Moon, the bright part decreases from a full circle to a half circle in about a week. It then continues to shrink until it disappears in another week. This two-week shrinking period is called the waning period or Krishna Paksha in India.
New Moon (Amavasya): The day on which the Moon is not visible is called the new Moon or Amavasya.
Waxing period (Shukla Paksha): After the new Moon, the bright part grows to a half circle in about a week and becomes a full circle in another week. This two-week growing period is called the waxing period or Shukla Paksha. The waxing and waning of the Moon repeat each month. The complete cycle from one full Moon to the next takes about one lunar month, or about 29.5 days.
Full Moon (Purnima): The day on which the Moon appears as a full bright circle.
New Moon (Amavasya): The day on which the Moon is not visible from Earth.
Waxing: The period during which the illuminated part of the Moon increases, from new Moon to full Moon.
Waning: The period during which the illuminated part of the Moon decreases, from full Moon to new Moon.
Gibbous: A Moon that is more than half but not fully illuminated.
Crescent: A Moon that is less than half illuminated.
The Moon's position in the sky changes every day, even when we observe it at the same clock time.
On full Moon day:
The Moon is nearly opposite the Sun in the sky.
When the Sun rises in the east, the full Moon is almost setting in the west.
After the full Moon:
Each morning at sunrise, the bright part of the Moon becomes smaller and the Moon appears closer to the Sun's position in the sky.
When the Moon looks like a half circle, it is approximately overhead at sunrise. A few days later, the crescent appears closer to the Sun's position.
Observing Guidelines:
Waxing Moon: It is generally best seen in the evening, around sunset.
Waning Moon: It is generally best seen in the morning, around sunrise.
The time of moonrise becomes about 50 minutes later each day. This explains why the Moon can sometimes be seen during daylight, for example, in the afternoon between 2:00 and 4:00 p.m. As the Moon moves ahead in its orbit while Earth rotates, Earth must rotate a little more each day before the Moon is seen in the same position.
After moonrise, waiting for about 30 minutes allows the Moon to climb higher into the sky and makes it easier to observe.
The actual shape of the Moon does not change; only the portion of its sunlit surface visible from Earth changes.
The Moon does not produce its own light; it appears bright because it reflects sunlight.
At any moment, half of the Moon faces the Sun and is illuminated, while the other half faces away from the Sun and is dark.
Appearance from Earth:
As the Moon revolves around Earth, its position and the angle from which we view it change.
Although one half of the Moon is always illuminated by the Sun, the illuminated half is not always fully visible from Earth.
The portion of the Moon that we see from Earth may be fully illuminated, partly illuminated, or not illuminated at all. These changing appearances produce the phases of the Moon.
Full Moon: When the entire sunlit half faces Earth, the Moon appears as a full bright circle.
New Moon: When the dark half faces Earth, the Moon is not visible.
Reason for changing appearance:
The Moon seems to change shape because its position relative to Earth and the Sun constantly changes as it orbits Earth.
We see only the illuminated part that faces us, so the visible fraction changes during the Moon's orbit.
The Moon's Phases with a Simple Model
Model Demonstration:
Use a small ball on a stick to represent the Moon, a torch or lamp as the Sun, and your head as Earth.
Hold the ball slightly above your head and shine the torch towards the ball to represent sunlight.
Turn slowly in a circle. The portion of the ball that is illuminated and visible to your eyes will change, just as the appearance of the Moon changes.
What the model shows:
Full Moon: When the ball is opposite the lamp, the side facing you is fully lit, like a full Moon.
New Moon: When the ball is between your head and the lamp, you see only the dark side, like a new Moon.
Crescent and gibbous phases: As you turn, the visible portion changes. At some positions it is a crescent, which is less than half illuminated, and at other positions it is gibbous, which is more than half illuminated.
The line between the bright and dark parts is curved, except at the half-Moon phase, when it appears straight.
The Science Behind the Phases
At every instant, half of the Moon is lit by sunlight and half is dark.
As the Moon revolves around Earth, the angle between the Sun, Moon and Earth changes. Therefore, the fraction of the illuminated half visible to us changes and produces the phases.
Phase names: Crescent (less than half illuminated), Gibbous (more than half illuminated), Full Moon (the entire visible face is illuminated), New Moon (the illuminated part is not visible).
5. Why Do Moon Phases Occur?
Incorrect idea: Moon phases are NOT caused by Earth's shadow falling on the Moon.
Correct reason: Phases occur because the relative positions of the Sun, Moon and Earth change as the Moon orbits Earth.
Earth's Shadow and Lunar Eclipses
The only time Earth's shadow falls on the Moon is during a lunar eclipse.
A lunar eclipse can occur only on a full Moon day, when Earth comes between the Sun and the Moon.
A solar eclipse can occur only on a new Moon day, when the Moon comes between the Sun and Earth.
Why Do Eclipses Not Happen Every Month?
Eclipses do not occur every month, even though full Moons and new Moons occur every month.
This is because the Moon's orbit is tilted slightly relative to Earth's orbit around the Sun.
In most months, the Sun, Earth and Moon do not line up perfectly. As a result, Earth's shadow does not cover the Moon and the Moon's shadow does not fall on Earth.
6. How Did Calendars Come into Existence?
Natural Cycles and Time Measurement
The apparent daily motion of the Sun, from rising in the east to setting in the west, is caused by Earth's rotation on its axis. This repeating cycle defines the day.
Mean solar day: The average time between one highest Sun position, when the shadow is shortest around noon, and the next such position is about 24 hours. This is called the mean solar day.
Shadow Tracking and the Day
The shortest shadow during the day indicates midday, when the Sun is at its highest position in the sky.
Measuring the time from one noon to the next gives the length of a day.
The average solar day is about 24 hours.
The Month and the Moon
The phases of the Moon form another natural cycle. One complete phase cycle, from one full Moon to the next full Moon, takes about 29.5 days, which is approximately one month.
This lunar cycle forms the basis for measuring a month.
The Year and the Seasons
A year is the time Earth takes to revolve once around the Sun. It is about 365¼ days.
The repetition of seasons, such as spring, summer, autumn and winter, marks the annual cycle.
Multiple Choice Question:
What is the primary unit of timekeeping based on Earth's rotation?
A) Month B) Year C) Day D) Hour
Correct Answer: C (Day). Each day is formed by the natural cycle of the Sun's apparent motion, which is due to Earth's rotation on its axis.
7. Types of Calendars
1. Lunar Calendars
Lunar calendars are based on the phases of the Moon.
Each lunar month is about 29.5 days, and 12 lunar months make a year of about 354 days.
A lunar year is shorter than a solar year. Therefore, the months of a lunar calendar gradually shift with respect to the seasons.
2. Solar Calendars
Solar calendars are based on Earth's revolution around the Sun and the arrival of seasons.
The Gregorian calendar is a solar calendar with a regular year of 365 days.
To correct for the extra quarter of a day in each year, a leap day is added every four years. This day is 29 February.
Leap year rule: A year divisible by 4 is generally a leap year. However, a year divisible by 100 is not a leap year unless it is also divisible by 400.
3. Luni-Solar Calendars
Luni-solar calendars combine lunar months with corrections that keep the calendar in step with the solar year and the seasons.
An extra month, called Adhika Maasa, is added every two or three years in many Indian systems to bring the lunar months into alignment with the solar year.
Names of months in Indian luni-solar calendars include: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha (Agrahayan), Pausha, Magha, Phalguna.
Amant calendars: A month begins after a new Moon and ends on the next new Moon.
Purnimant calendars: A month begins after a full Moon and ends at the next full Moon.
Observations and Heritage
Ancient observers noticed that about twelve cycles of the Moon's phases roughly fit within one annual cycle of seasons.
The position at which the Sun rises shifts northward and southward during the year because of Earth's axial tilt and its revolution around the Sun.
This movement is described in Indian tradition as Uttarayana, the northward movement of the Sun, roughly from December to June, and Dakshinayana, the southward movement, roughly from June to December.
Solstices and equinoxes are important points used to track the Sun's yearly journey and to adjust calendars.
8. The Indian National Calendar
The Indian National Calendar, also called the Saka Calendar, is a solar calendar officially used by the Government of India along with the Gregorian calendar.
It has 365 days in a regular year.
The year begins on 22 March. In a leap year, it begins on 21 March.
Its months are named after traditional Indian months: Chaitra, Vaisakha, Jyestha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Agrahayana, Pausha, Magha, Phalguna.
In a normal year, months 2 to 6 have 31 days, while the remaining months have 30 days. Chaitra has 30 days in a normal year.
In a leap year, one day is added to Chaitra, so the year begins on 21 March.
The Indian National Calendar was adopted for official use in 1957, following the recommendations of the Calendar Reform Committee, which was chaired by the astrophysicist Meghnad Saha.
The calendar follows principles related to ancient Indian astronomical works such as the Surya Siddhanta and helps to unify civil timekeeping across India.
9. Are Festivals Related to Astronomical Phenomena?
Many Indian festivals are linked to the phases of the Moon and are therefore based on lunar or luni-solar calendars.
Examples
Diwali: Celebrated on the new Moon of Kartika.
Holi: Celebrated on the full Moon of Phalguna.
Buddha Purnima: Occurs on the full Moon of Vaisakha.
Eid-ul-Fitr: Celebrated after the crescent Moon is sighted at the end of Ramadan.
Dussehra: Occurs on the tenth day of the waxing period of Ashwina.
These festival dates shift in the Gregorian calendar because lunar years and solar years differ in length.
Luni-solar calendars insert an intercalary month from time to time to keep lunar months aligned with the solar year. Therefore, festival dates drift less than they would in a purely lunar system.
Pure lunar calendars, such as the Islamic calendar, do not add intercalary months. Therefore, festivals such as Eid move through the Gregorian months over the years.
Solar Festivals
Some Indian festivals follow a solar sidereal calendar and occur nearly on the same Gregorian date each year.
Examples: Examples: Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh and Puthandu.
These festivals were traditionally linked with important positions of the Sun, such as solstices or equinoxes. A small difference between sidereal and tropical years causes a very slow drift in some festival dates over centuries. Makar Sankranti is an example.
Variations in Festival Dates
A lunar phase occurs at the same time throughout India. However, local sunrise times and calendar rules can lead to festival dates differing by a day between regions.
To standardise civil dates, the Rashtriya Panchang, or national almanac, is published by the Positional Astronomy Centre of the Government of India. It gives officially recommended festival dates.
Cultural Connection
The Moon and Sun inspire many Indian classical and traditional art forms.
Music: Ragas such as Chandrakauns and Chandranandan are inspired by the Moon.
Dance: Mudras, or gestures, such as Chandrakala and Ardhachandra in Bharatanatyam invoke lunar imagery.
Visual arts: Traditional painting styles such as Madhubani and Warli, as well as sculpture and pottery, frequently depict the Moon and Sun.
10. Why Do We Launch Artificial Satellites in Space?
Natural and Artificial Satellites
Natural Satellite: The Moon is Earth's natural satellite.
Artificial Satellites: Human-made objects placed in orbit around Earth by nations and organisations for practical purposes.
Artificial Satellites: Purpose and Functions
Appearance: From the ground, many satellites look like small, bright, steadily moving dots across the sky. They shine by reflecting sunlight.
Typical low Earth orbit: Many satellites orbit a few hundred kilometres above Earth, for example, around 800 km, and complete one orbit in roughly 100 minutes.
Key uses: Artificial satellites are used for communication, including television, telephone and internet services; navigation, including GPS and map services; weather monitoring; disaster management, including the detection of floods and cyclones; and scientific research on space, the atmosphere and Earth.
Multiple Choice Question:
What is one reason we launch artificial satellites into space?
A) To communicate with animals B) To study weather C) To explore oceans D) To grow plants
Correct Answer: B (To study weather). We launch artificial satellites in space mainly to study weather, navigation, communication, and disaster management.
Satellites and Missions by ISRO (Indian Space Research Organisation)
Cartosat series: High-resolution imaging satellites used for mapping, city planning and disaster response. Platforms such as Bhuvan use Cartosat images to analyse land use, vegetation and terrain.
AstroSat: An observatory satellite for studying stars and other celestial objects.
Chandrayaan 1, 2, 3: Moon missions for exploration and scientific study.
Aditya-L1: A satellite for studying the Sun.
Mangalyaan: The Mars Orbiter Mission.
Student satellites: ISRO encourages student-built small satellites, such as AzaadiSat, InspireSat-1 and Jugnu.
Observing Artificial Satellites
How to spot them: Look for a small, bright, continuously moving dot, often visible shortly after sunset or just before sunrise.
Satellites can often be seen without a telescope.
Mobile applications and websites for satellite tracking provide the times and paths of visible satellites for a particular location.
Space Debris (Space Junk)
When satellites and rocket parts stop working, they can become space debris.
Risks: Space debris can collide with operational satellites and clutter useful orbits.
Disposal and Mitigation
Many small pieces of debris burn up when they enter the atmosphere.
Larger fragments can survive re-entry and fall to Earth.
Agencies work together to reduce debris through controlled re-entry, moving defunct satellites to graveyard orbits, and developing debris-removal technologies.
Key Figures in the Indian Space Programme
Vikram Sarabhai: Pioneer of India's space programme, often called the 'Father of the Indian Space Programme'.
The Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram is named after him.
VSSC works on rocket and launch vehicle technology.
11. Key Points to Remember
Phases of the Moon: The changing shapes of the illuminated part of the Moon include the new Moon, crescent, half Moon, gibbous Moon and full Moon.
Cause of Moon phases: They occur because we see different portions of the Moon's sunlit side as it orbits Earth.
Cycle of Moon phases: A complete sequence takes about one month (29.5 days).
Calendars: Timekeeping systems use natural cycles: the day is related to Earth's rotation, the month to the Moon's phases, and the year to Earth's revolution around the Sun.
Lunar calendar: It follows the phases of the Moon and has about 354 days in 12 lunar months.
Solar calendar: It follows the cycle of the seasons. The Gregorian calendar is an example (365 days).
Luni-solar calendar: It adapts lunar months to the solar year by adding an extra month (Adhika Maasa) occasionally, as in many traditional Indian calendars.
Artificial satellites: These are human-made objects in orbit that provide communication, navigation, weather information and scientific data.
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