๐ Chapter 11 – Keeping Time with the Skies
๐ 11.1 How Does the Moon’s Appearance Change?
- ๐ The Moon is spherical
- It does not produce light → reflects sunlight
๐ Its shape appears to change → called:
⭐ Phases of the Moon
๐ 11.1.1 Phases of the Moon
๐ฝ Waning Phase (Krishna Paksha)
- Bright part decreases
- Full → Half → Disappears
- Duration ≈ 2 weeks
๐ ๐ New Moon = Amavasya (Moon not visible)
๐ผ Waxing Phase (Shukla Paksha)
- Bright part increases
- Crescent → Half → Full
- Duration ≈ 2 weeks
๐ ๐ Full Moon = Purnima
๐
Moon Cycle:
- One full cycle ≈ 1 month (29.5 days)
๐ Main Phases:
- New Moon ๐
- Crescent ๐
- Half Moon ๐
- Gibbous ๐
- Full Moon ๐
๐งญ 11.1.2 Locating the Moon
๐ Full Moon Day:
- Sun rises in east
- Moon sets in west
๐ Opposite positions
๐ Key Points:
- Moon rises ~50 minutes later each day
- Waxing Moon → seen at sunset
- Waning Moon → seen at sunrise
๐ก 11.1.3 Why Moon Looks Different
๐ Reason:
- Half Moon always illuminated by Sun
- We see only the visible illuminated part
๐ Full Moon:
- Bright side faces Earth
๐ New Moon:
- Dark side faces Earth
๐ Important:
- Phases occur because:
Moon revolves around Earth
⚠️ Clarification:
- ❌ NOT due to Earth’s shadow
- ✔️ Earth’s shadow causes lunar eclipse
๐ Eclipses:
- Lunar eclipse → Full Moon day
- Solar eclipse → New Moon day
๐
11.2 How Did Calendars Come into Existence?
๐ Based on Natural Cycles:
๐ Day:
- Earth rotates once → 24 hours
๐ Month:
- Moon cycle → 29.5 days
☀️ Year:
- Earth revolves around Sun → 365¼ days
๐ 11.2.1 Lunar Calendar
๐ Based on:
- Moon phases
๐ Facts:
- 12 months = 354 days
- Seasons shift every year
☀️ 11.2.2 Solar Calendar
๐ Based on:
- Earth’s revolution + seasons
๐
Example:
- Gregorian Calendar
๐ Features:
- 365 days
- Leap year → every 4 years
⚠️ Special Rule:
- Skip leap year every 100 years
- Add leap year every 400 years
๐ 11.2.3 Luni-Solar Calendar
๐ Combination of:
- Moon phases + Sun seasons
๐ง Problem:
- Lunar year is 11 days shorter
๐ Solution:
- Add extra month:
Adhika Maasa (extra month)
๐ Types:
- Amant (New Moon based)
- Purnimant (Full Moon based)
๐ฎ๐ณ 11.2.4 Indian National Calendar
๐ Features:
- Introduced in 1956
- Solar calendar
๐
Details:
- 365 days
- Starts on 22 March
- Leap year → starts on 21 March
๐️ Months:
- Based on traditional Indian names
๐ 11.3 Festivals & Astronomy
๐ Moon-based Festivals:
- Diwali → New Moon
- Holi → Full Moon
- Buddha Purnima → Full Moon
- Eid → Crescent Moon
☀️ Solar-based Festivals:
- Makar Sankranti
- Pongal
- Bihu
- Vaisakhi
๐ Occur on fixed dates
๐ Important:
- Festival dates shift due to:
- Moon cycle
- Earth’s wobble
๐ฐ️ 11.4 Artificial Satellites
๐ Natural Satellite:
- Moon
๐ Artificial Satellites:
- Man-made objects orbiting Earth
๐ Features:
- Appear as moving light in sky
- Orbit ≈ 800 km above Earth
- One revolution ≈ 100 minutes
๐ก Uses of Satellites
- Communication ๐
- Navigation ๐งญ
- Weather forecasting ๐ฆ️
- Disaster management ๐จ
- Scientific research ๐ฌ
๐ฎ๐ณ ISRO Missions
๐ฐ️ Important Satellites:
- Cartosat → mapping
- AstroSat → space study
- Chandrayaan 1, 2, 3 → Moon missions
- Aditya L1 → Sun study
- Mangalyaan → Mars mission
๐ Student Satellites:
- AzaadiSat
- InspireSat-1
- Jugnu
⚠️ Problems: Space Debris
๐ง What is it?
- Old satellites + rocket parts
❗ Issues:
- Can collide with working satellites
- Large debris may fall to Earth
๐ Solution:
- Countries working to reduce debris
๐ Snapshots (Quick Revision)
⚡ One-Line Points
- Moon phases change daily
- Cycle ≈ 1 month
- Caused by:
- Sun
- Earth
- Moon positions
- Day → Earth rotation
- Month → Moon revolution
- Year → Earth revolution
- Calendars:
- Lunar
- Solar
- Luni-solar
- Satellites:
- Natural → Moon
- Artificial → man-made
๐ฏ Final Conclusion
Sky helps us understand:
- Time ⏰
- Seasons ๐ฆ️
- Calendars ๐
๐ Final Formula:
Sky Observation → Time Measurement → Calendars
๐ง Learning Outcome:
- Understand Moon phases
- Learn calendar systems
- Know importance of satellites
