Keeping Time with the Skies Class 8 Notes Science Chapter 11

by

Students can rely on these Class 8 Science Notes Chapter 11 Keeping Time with the Skies Class 8 Notes for complete exam preparation.

Class 8 Science Chapter 11 Keeping Time with the Skies Notes

Class 8 Science Curiosity Chapter 11 Notes

Class 8 Science Chapter 11 Notes – Keeping Time with the Skies Notes Class 8

→ New Moon The Moon is between Earth and Sun, its dark side faces Earth, making it invisible.

→ Full Moon The Earth is between the Moon and Sun, the full face of the Moon is illuminated.

→ Waxing phase The illuminated part of the Moon appears to grow larger each night.

→ Waning Phase The illuminated part of the Moon appears to shrink each night.

→ Lunar Calender Calendar based on the Moon’s phases (12 lunar months ≈ 354 days).

→ Solar Calender Calendar based on Earth’s orbit around the Sun (365/366 days).

→ Leap Year A correction year with 366 days to match Earth’s orbit, occurs every 4 years.

→ Adhika Maasa An extra month added to the lunar calendar to align it with the solar year.

→ Ariticial Satellite A man-made object launched into space to orbit Earth for communication, weather, etc.

→ Luni-Solar Calender A calendar system that blends lunar months and solar year for seasonal.

Keeping Time with the Skies Class 8 Notes Science Chapter 11

→ Riya noticed the Moon was getting bigger every night. Which phase was she observing?

→ When is the Moon exactly between the Earth and the Sun?

→ Why is the Moon not visible on Amavasya?

→ On which side of Earth is the Moon during Full Moon?

→ If today is full Moon, what phase will it be after one week?

→ Why does the Moon sometimes appear in the daytime sky?

→ On which side of Earth is the Moon located during first Quarter phase?

→ How does the Moon’s position change from new Moon to full Moon?

→ Which direction does the Moon rise and set?

→ If the Moon rises late each night and is seen in the early morning, which part of its cycle is it likely in?

→ A calendar with 354 days in a year is based on which celestial body?

→ What type of calendar needs an extra month called Adhika Maasa to match the seasons?

→ Which calendar system adds one day every four years to remain aligned with Earth’s revolution?

→ Why do dates of festivals like Holi and Diwali change every year in the Gregorian calendar?

→ Which calendar keeps festivals like Pongal or Makar Sankranti fixed near the same date?

→ Name two uses of artificial satellites

→ Why do artificial satellites look like moving stars in the night sky?

→ Which Indian organisation is responsible for launching satellites?

→ Which festival is celebrated on a Full Moon night and varies in Gregorian calendar each year?

→ Which festival occurs near the New Moon and is associated with darkness?

→ Name a solar-based festival celebrated when the Sun enters Capricorn (Makara Rashi).

Keeping Time with the Skies Class 8 Notes Science Chapter 11

Phases of the Moon

  • The bright part of the Moon changes every day. These daily changes in the visible part of the Moon are called its phases.
  • The Moon’s shape appears to decrease from a full circle to a half circle in about a week.
    1. It continues to shrink further until it is no longer visible.
    2. This shrinking period is called the waning phase of the Moon.
    3. In India, this waning phase is called Krishna Paksha.
  • In the day, the Moon is not visible at all is called the new Moon day (Amavasya)
  • After new Moon, the Moon’s bright part starts growing again.
    1. It becomes a half circle in about a week and a full circle in another week.
    2. This increasing brightness period is called the waxing phase.
    3. In India, this waxing phase is called Shukla Paksha.
  • In the day, the Moon appears completely bright and circular is called the full Moon day (Purnima)
  • It takes about one month for the Moon to complete one full cycle from one full Moon to the next full moon.
  • The different phases of the Moon include
    1. New Moon : Not visible
    2. Waxing Crescent Moon : Less than half visible
    3. First quarter : Half visible (right side)
    4. Waxing Gibbous Moon : More than half visible
    5. Full Moon : Fully visible
  • Then reverse : Gibbous → Last quarter → Waning Crescent → New Moon

Keeping Time with the Skies Class 8 Notes Science Chapter 11 1

Locating the Moon

  • On a full Moon day, the Moon is almost opposite the Sun, so when the Sun rises in the east, the Moon is seen setting in the west.
  • After the full Moon, the bright part decreases and the Moon appears closer to the Sun each morning.
  • When the Moon is half, it is usually seen overhead at sunrise.
  • A few days later, during the crescent phase, it appears even closer-to the Sun, just before sunrise.
    A waxing Moon is best seen in the evening after sunset, while a waning Moon is visible in the morning before sunrise, helping us know when and where to spot it.

Many people believe that the Moon always rises when the Sun sets, but this is not true. In reality, the Moon rises about 50 minutes later each day.
On some days, the Moon may even rise in the afternoon between 2 : 00 to 4 : 00 p.m. and can be seen in the eastern sky during daylight.

Keeping Time with the Skies Class 8 Notes Science Chapter 11

Making Sense of Our Observations

  • The Moon doesn’t have its own light, it shines because it reflects sunlight.
  • Half of the Moon always faces the Sun and is illuminated, the other half remains dark.

Keeping Time with the Skies Class 8 Notes Science Chapter 11 2

  • We can see only a part of the illuminated side from Earth, which makes the Moon appear different (phases) on different days.
  • On new Moon day, the illuminated portion is not visible at all.
  • On a given day, people around the world see nearly the same phase because Earth’s rotation (1 day) is much shorter than Moon’s revolution (≈ 1 month).
  • The Moon’s position at sunrise/sunset shifts slightly each day as the Moon moves ahead in its orbit.
  • If Earth rotates a little extra each day, then the Moon will appear at the same spot in the sky.

The phases of the Moon are not caused by Earth’s shadow. They occur due to the changing positions of the Sun, Earth and Moon. As the Moon revolves around the Earth, which changes the portion of the illuminated side visible from Earth. Earth’s shadow falls on the Moon only during a lunar eclipse, not during regular phases.

Lunar Calendar and Solar Calendar

Lunar Calendar

  1. Ancient people used Moon phases to measure time.
  2. 1 lunar month = 29.5 days, and 1 lunar year = 12 lunar months = 354 days.
  3. Lunar calendars don’t match seasons because seasons repeat in 365 days, not 354 days.

Solar Calendar

  1. To match seasons (important for agriculture), solar calendars were created.
  2. Gregorian calendar is a solar calendar with 365 days (months have 30/31 days; February has 28 days).
  3. Leap year Every 4 years, 1 extra day is added (Feb has 29 days) to adjust for the extra 1/4 day Earth takes to orbit the Sun.

Leap Year Corrections

  1. Earth’s orbit is slightly less than 365 1/4 days, so leap years are skipped every 100 years (e.g., 1700, 1800, 1900).’
  2. Every 400 years, leap year is added back (e.g., 1600, 2000).
  3. These corrections keep the calendar aligned with seasons.

Tropical Year and Sidereal Year

  1. Tropical year Time between two spring equinoxes; basis of Gregorian calendar.
  2. Sidereal year Time for the same stars to rise again at sunset, longer than tropical year by 20 minutes.
  3. Astronomers use the sidereal year to track Earth’s position in orbit.

Luni-Solar Calendars

  1. 1 lunar year is 11 days shorter than the solar year (365 days)
  2. Every 2 – 3 years, this gap becomes almost 1 month.
  3. To fix this, an extra month called Adhika Maasa (extra month) is added.
  4. This adjustment keeps the calendar in sync with seasons.
  5. Indian month names Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha, Pausha, Magha, Phalguna.
  6. Two ways of counting months
  7. Amant—New Moon to next new Moon.
  8. Purnimant — Full Moon to next full Moon.
  9. These calendars are commonly used in India for festivals and rituals.

Indian National Calendar

  1. It is an official solar.calendar of India, used along with the Gregorian calendar.
  2. 1 year = 365 days, similar to the solar year.
  3. Year starts on 22 March (21 March in leap years).
  4. Months have 30 or 31 days, and the names are taken from traditional Indian calendars (e.g., Chaitra, Vaisakha, etc.).
  5. In a leap year, 1 extra day is added to Chaitra (1st month).
  6. This calendar is used for government and official purposes.

Keeping Time with the Skies Class 8 Notes Science Chapter 11 3
Meghnad Saha was a pioneering astrophysicist of India who studied stars and their temperatures and developed a mathematical equation, famously known as the Saha equation. The Saha Institute of Nuclear Physics, in Kolkata, is named after him. He was also the chairperson of the Calendar Reform Committee.

Keeping Time with the Skies Class 8 Notes Science Chapter 11

Festivals Related to Astronomical Phenomena

  • Many Indian festivals follow lunar or luni-solar calendars, so their dates change every year in the Gregorian calendar.
    Examples 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).
  • Pure lunar calendars (like the one for Eid) do not make adjustments for an extra month, so festivals can fall in different Gregorian months each year.

Festivals Based on Solar Sidereal Calendar

  1. Some festivals follow the solar sidereal calendar (e.g., Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh, Puthandu).
  2. These festivals fall on almost the same date in the Gregorian calendar.
  3. They were originally linked to solstices or equinoxes.
  4. Due to the slow wobble of Earth’s axis (precession), their dates shift slowly over centuries (e.g., Makar Sankranti shifts by 1 day every 71 years).

Regional Date Differences

  1. Sunrise happens earlier in Eastern India and later in Western India, so festival dates can differ by a day between regions.
  2. To avoid confusion, the Positional Astronomy Center (Govt, of India) publishes the Rashtriya Panchang, which officially fixes festival dates.

Why Do We Launch Artificial Satellites?

  1. The Moon is Earth’s natural satellite, while humans have also launched artificial satellites that orbit the Earth.
  2. Most artificial satellites orbit at about 800 km height and complete one orbit in 100 minutes.
  3. At night, they look like moving stars in the sky.

Uses of Artificial Satellites

  1. Communication Telephone, TV and internet signals.
  2. Navigation GPS and route tracking.
  3. Weather monitoring Forecasting and disaster warnings.
  4. Disaster management Flood or cyclone monitoring.
  5. Scientific research Studying space, stars and plane

Vikram Sarabhai, a researcher in space science and nuclear physics is known as the Father of the Indian Space programme. He pioneered the effort to launch the first artificial satellites. The Vikram Sarabhai Space Centre (VSSC), located in Thiruvananthapuram, the ISRO centre that develops rockets and launch vehicle technology, is named after him.