Notes For All Chapters – Science Curiosity Class 8
Keeping Time with the Skies
11.1 How Does the Moon’s Appearance Change and Why?
- The Moon is spherical and shines by reflecting sunlight.
- Its shape changes every night as seen from Earth.
- The changing visible shapes are called phases of the Moon.
11.1.1 Phases of the Moon
- The bright portion of the Moon decreases from a full circle to a half and then disappears in about 2 weeks → Waning period (Krishna Paksha).
- The Moon is not visible on New Moon day (Amavasya).
- After New Moon, the bright portion increases to a half and then to full in 2 weeks → Waxing period (Shukla Paksha).
- The cycle from one full Moon to the next takes about a month.
- Names of phases: New Moon, Crescent, Half Moon, Gibbous, Full Moon.
11.1.2 Locating the Moon
- On a full Moon day, Sun rises in the east and Moon sets in the west (opposite positions).
- On later days, the Moon appears closer to the Sun.
- Waxing Moon is seen at sunset.
- Waning Moon is seen at sunrise.
- Moon rises and sets at different times than the Sun → rises ~50 minutes later each day.
11.1.3 Why the Moon Looks Different
- The Moon does not emit its own light; it reflects sunlight.
- Half of the Moon facing the Sun is illuminated, the other half is dark.
- We see only the illuminated part facing Earth.
- On Full Moon day, the entire bright side faces Earth.
- On New Moon day, the dark side faces Earth.
- Phases occur because the Moon revolves around Earth.
- Crescent, Gibbous, Half Moon are due to partial visibility of illuminated portion.
- All people on Earth see nearly the same phase on a given day.
Additional Points
- Moon rises ~50 minutes later each day due to its revolution around Earth.
- Phases are not caused by Earth’s shadow. Earth’s shadow causes lunar eclipse.
- Lunar eclipse occurs only on a full Moon day.
- Solar eclipse occurs only on a new Moon day.
- They do not happen every month because Moon’s orbit is slightly tilted.
11.2 How Did Calendars Come into Existence?
- Day, month, and year are based on natural cycles of Sun, Moon, and Earth.
Day:
- Earth rotates once in ~24 hours.
- The time from one highest Sun position to the next is a solar day (24 hrs).
Month:
- The Moon takes about 29.5 days to complete one cycle of phases.
Year:
- Earth takes 365 ¼ days to revolve around the Sun.
- This defines a solar year.
11.2.1 Lunar Calendars
- Based on Moon’s phases.
- 1 lunar month = 29.5 days.
- 12 lunar months = 354 days.
- Seasons shift because lunar year < solar year.
11.2.2 Solar Calendars
- Based on Earth’s revolution around Sun and seasons.
- Gregorian calendar is a solar calendar (365 days).
- Months have 30/31 days; February has 28.
- Leap year: every 4 years, February has 29 days.
- Additional corrections: skip leap years every 100 years, but add every 400 years (e.g., 1600, 2000).
- Gregorian calendar is based on the tropical year.
- Sidereal year (time taken for same stars to rise again) is ~20 minutes longer.
11.2.3 Luni-Solar Calendars
- Combine Moon’s phases and Sun’s seasons.
- 12 lunar months = 354 days → short by ~11 days.
- Every 2-3 years, add an extra month (Adhika Maasa) to match seasons.
- Some calendars start new month after New Moon (Amant).
- Some start after Full Moon (Purnimant).
11.2.4 Indian National Calendar
- Official calendar of India (introduced 1956).
- Solar calendar, 365 days.
- Year begins on 22 March (after spring equinox).
- Months have 30 or 31 days.
- In leap years, Chaitra has 31 days, and year starts on 21 March.
- Names of months taken from traditional Indian calendars.
11.3 Are Festivals Related to Astronomical Phenomena?
- Many Indian festivals depend on phases of the Moon.
- Diwali → New Moon of Kartika.
- Holi → Full Moon of Phalguna.
- Buddha Purnima → Full Moon of Vaisakha.
- Eid-ul-Fitr → Crescent Moon after Ramazan.
- Dussehra → 10th day of Ashwina.
- Dates shift every year in Gregorian calendar.
- In lunar calendars, shift is ~11 days each year.
- In luni-solar calendars, shifts are corrected by adding intercalary month.
- Some festivals like Makar Sankranti, Pongal, Bihu, Vaisakhi follow solar sidereal calendar, so they occur on nearly fixed dates.
- Due to Earth’s wobble, festival dates slowly shift over centuries.
11.4 Why Do We Launch Artificial Satellites in Space?
- Moon is Earth’s natural satellite.
- Artificial satellites are man-made and orbit Earth.
- Appear as moving specks of light in night sky.
- Most orbit ~800 km above Earth and complete one round in ~100 minutes.
Uses of Satellites:
- Communication
- Navigation
- Weather monitoring
- Disaster management
- Scientific research
ISRO Satellites:
- Cartosat series → mapping, disaster management, Bhuvan platform.
- AstroSat → studies stars and celestial bodies.
- Chandrayaan 1, 2, 3 → Moon missions.
- Aditya L1 → studies Sun.
- Mangalyaan → Mars mission.
- Student satellites: AzaadiSat, InspireSat-1, Jugnu.
Problems:
- Old satellites and rocket parts become space debris.
- Can collide with working satellites.
- Small debris burns in atmosphere, big debris may fall on Earth.
- Countries are working to reduce this danger.
Snapshots (Summary)
- The Moon’s illuminated part changes daily → phases of the Moon.
- Phases occur due to changing positions of Sun, Moon, and Earth.
- One full cycle of phases = ~1 month.
- Calendars are based on natural cycles:
- Day → Earth’s rotation.
- Month → Moon’s revolution.
- Year → Earth’s revolution around Sun.
- Lunar calendars = Moon-based.
- Solar calendars = Sun-based.
- Luni-solar calendars = combination.
- Artificial satellites are man-made; help in communication, navigation, weather, research, etc.
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