Notes Chapter 11 Science Curiosity Class 8 CBSE Board

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.