CLASS 8 SCIENCE · CHAPTER 11 · STUDY NOTES
Keeping Time with the Skies
Understand keeping time with the skies through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
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The Moon stays round while its bright view changes
Learn waxing, waning and the main views in a cycle.Picture from the NCERT book
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- Phase of the Moon
- The shape of the sunlit part of the Moon visible from Earth.
- Waxing
- The visible bright part increases.
- Waning
- The visible bright part decreases.
The Moon shines mainly by reflecting sunlight. A full Moon looks like a bright circle; a new Moon has its unlit side facing us and is usually not visible. Crescent and gibbous describe views smaller and larger than a half-circle.
A cycle from one new Moon to the next takes about 29.5 days. Waxing is commonly called Shukla Paksha, and waning Krishna Paksha. The Moon itself is not changing into a crescent-shaped object.
Take a moment. What do you think?
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Question
Does a crescent Moon mean most of the Moon has physically disappeared?
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Phases depend on our viewing direction
The sunlit half and the Earth-facing half are not always the same half.Change position, keep the sunlight direction
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- Illuminated hemisphere
- The half of the Moon receiving direct sunlight.
- Lunar eclipse
- An event in which Earth’s shadow falls on the Moon.
Outside eclipses, sunlight illuminates approximately half of the Moon. As the Moon moves around Earth, we see a changing fraction of that lit half. Earth’s shadow does not cause ordinary phases.
A lunar eclipse needs a suitable full-Moon alignment; a solar eclipse needs a suitable new-Moon alignment. The Moon’s orbital tilt is why an eclipse does not happen every month.
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Question
Why is a full Moon possible when the Moon is on the opposite side of Earth from the Sun?
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The Moon can be visible in daylight
Use phase and time rather than assuming “Moon means night.”A daytime Moon is possible
- Apparent motion
- How an object seems to move across the sky from an observer’s viewpoint.
A full Moon is roughly opposite the Sun in the sky, so it rises near sunset and sets near sunrise. A waxing Moon is often convenient to observe around sunset; a waning Moon around sunrise. Exact times depend on phase and location.
The Moon moves along its orbit while Earth rotates. It therefore returns to a similar sky position about 50 minutes later on average each day; the delay is not exactly the same every day. Never look directly at the Sun while searching for the Moon.
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Question
Could you see a Moon above the horizon in the afternoon?
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Different sky cycles give different units of time
Connect day, lunar month and year to repeated events.One stick, different Sun heights
- Mean solar day
- The average time between successive highest apparent Sun positions: 24 hours.
- Lunar month of phases
- One full phase cycle, approximately 29.5 days.
- Solar year
- A year based on the Sun’s seasonal cycle, approximately 365.24 days.
Earth’s rotation gives the daily cycle. The Moon’s changing position relative to the Sun gives the phase-month cycle. Earth’s tilted axis and yearly motion produce the repeating seasons.
A vertical stick’s shortest shadow occurs near local solar noon, which need not be exactly 12:00 on a clock. Record shadows without staring at the Sun.
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Question
Which cycle is most directly used for a lunar month: Earth’s daily rotation or the Moon’s repeated phases?
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Calendars combine cycles in different ways
Explain why twelve lunar months do not make a solar year.Compare one year on the same day scale
- Lunar calendar
- A calendar based on lunar months, with a year commonly of twelve such months.
- Solar calendar
- A calendar whose year follows the seasonal solar cycle.
- Luni-solar calendar
- A calendar using lunar months with adjustments to remain aligned with seasons.
Twelve phase-months total about 12 × 29.5 = 354 days, about 11 days shorter than a solar year. A purely lunar calendar therefore shifts relative to seasons. A luni-solar calendar can add an extra month when its rules require it.
The Gregorian calendar adds leap days. A year divisible by 4 is generally leap, except century years unless divisible by 400: 2100 is not leap, but 2000 was.
The yearly gap
365 − (12 × 29.5) = 11 daysThe real cycles are not exact whole numbers; this is the book’s useful approximation.
Let’s look a little closer.
Go deeper: Why add a month sometimes?
After roughly three years, an 11-day annual shortfall totals about 33 days, near a lunar month. Actual luni-solar rules determine when the intercalary month, often called Adhika Maasa, is inserted. This is not a rule to add exactly one month every third year in all calendars.
Some Indian traditions begin months after the new Moon (Amant), others after the full Moon (Purnimant). They organise the same natural cycle differently.
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Question
Using the approximate values, how far short are twelve lunar months of a 365-day year?
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India’s National Calendar is solar
Traditional month names do not make a calendar lunar.Check the month-length total
- Saka era
- The era used for year numbering in the Indian National Calendar.
The Indian National Calendar is used alongside the Gregorian calendar for certain official purposes. It is a solar calendar: a normal year has 365 days, and its leap years correspond to Gregorian leap years.
Chaitra begins on 22 March in a normal year and 21 March in a leap year. Chaitra has 30 days normally and 31 in a leap year; the next five months have 31 days each and the remaining six have 30.
Normal year total
30 + 155 + 180 = 365 daysCount the first month, next five and last six separately.
Go deeper: Source correction and historical context
The National Calendar took effect for official purposes on 22 March 1957, corresponding to 1 Chaitra 1879 Saka. Meghnad Saha chaired the Calendar Reform Committee.
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Question
Does using the name Chaitra automatically mean the National Calendar follows lunar phases?
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Festival dates can follow different calendar rules
Distinguish a recurring lunar date from a Gregorian date.One event, two calendar labels
- Intercalation
- Adding a day or month according to calendar rules to keep cycles aligned.
Some festivals use lunar or luni-solar dates: for example, Diwali is associated with the new Moon of Kartika and Buddha Purnima with a full Moon. Eid-ul-Fitr follows the end of Ramadan in a lunar calendar and local crescent-sighting or calendar practice.
Their Gregorian dates can vary between years. Solar-calendar festivals tend to stay near the same Gregorian season. Actual observances can differ by region and tradition, so use the relevant official or community calendar for exact dates.
Go deeper: Observations have a long history
Indian calendar traditions drew on repeated observations of sunrise directions, Moon phases and seasons. Uttarayan and Dakshinayan describe the Sun’s apparent northward and southward seasonal movement. Sidereal and tropical year definitions differ slightly; exact festival dates should not be guessed from a simplified diagram.
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Question
Why can a festival keep the same lunar date but change its Gregorian date?
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Satellites serve different purposes in different orbits
Distinguish a natural satellite from human-made equipment.Follow a satellite’s information
- Artificial satellite
- A human-made object placed in orbit.
- Space debris
- Inactive human-made objects and fragments in space.
Satellites support weather observation, communication, navigation, mapping and science. Different missions use different heights and orbital periods; there is no single 800 km orbit for all satellites.
The Moon is Earth’s natural satellite. A bright moving point in the night sky may be a satellite, but aircraft and other objects can also move. Use a trusted observing guide with an adult, rather than identifying every moving light as a satellite.
Go deeper: Examples and responsibility
ISRO’s Cartosat missions provide Earth imagery, while AstroSat observes celestial sources. Bhuvan is an Indian geospatial platform. Disused equipment and fragments create collision risks, so responsible missions plan debris reduction and end-of-life disposal.
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Question
Would a weather satellite and a mapping satellite necessarily have the same orbit?
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