Experts have designed these Class 8 Science Notes Chapter 11 Keeping Time with the Skies Class 8 Notes for effective learning.
Class 8 Science Chapter 11 Keeping Time with the Skies Notes
Class 8 Keeping Time with the Skies Notes
Class 8 Science Chapter 11 Notes – Keeping Time with the Skies Notes Class 8
→ Waning Period: It is the two-week period after the Full Moon during which the bright portion of the Moon keeps shrinking until it becomes completely invisible on the New Moon day.
→ Full Moon Day (Poornima): The day on which the Moon appears as a full bright circle.
→ New Moon Day (Amavasya): The day when the Moon is not visible at all.
→ Waxing Period: It is the two-week period after the New Moon during which the bright portion of the Moon increases, first to a half circle in about a week, and then to a full circle (Full Moon) in the next week.
→ In India, the waning period of the Moon is generally known as Krishna Paksha, while the waxing period is known as the Shukla Paksha.
→ The Moon goes through a waning period followed by a waxing period in a cyclical manner. The cycle from one full Moon to the next takes about a month.
→ The changing shapes of the bright part of the Moon from one day to another as seen from the Earth are called the phases of the Moon.
→ Locating the Moon: The Moon appears in a different part of the sky each day at the same time.
- On a full Moon day, it is nearly opposite the Sun. As it begins to wane after a full Moon, it appears closer to the Sun each morning. A half Moon is overhead at sunrise, and a crescent Moon is even closer to the Sun.
- Knowing the Moon’s phase, and whether it is waxing or waning, helps us predict when and where to see it. A waxing Moon is best seen at sunset, and a waning Moon at sunrise. The Moon rises and sets at different times than the Sun.
→ The Moon does not always rise when the Sun sets. Moonrise times are listed in newspapers or on the India Meteorological Department’s website. The Moon rises about 50 minutes later each day. Sometimes, it rises in the afternoon and can be seen during daylight. You may need to wait about 30 minutes after moonrise for it to be clearly visible.
→ The phases of the Moon happen because we see different parts of the illuminated portion of the Moon as it moves around the Earth.
→ A gibbous phase occurs when more than half of the Moon’s visible surface is illuminated by the Sun, while a crescent phase appears when less than half of the moon’s visible surface is illuminated.
→ The position of Moon at sunrise and sunset appears to be shifted because Moon moves ahead in its orbit while the Earth completes one rotation about its axis in 24 hours. Earth needs to rotate some more for the Moon to appear in almost the same spot in the sky.
→ Moon phases are not caused by Earth’s shadow. They occur because of the changing positions of the Sun, Moon, and Earth as the Moon orbits Earth. The Earth’s shadow on the Moon causes a lunar eclipse.
→ Lunar eclipses only happen on a full Moon, and solar eclipses only on a new Moon. Eclipses don’t happen every month because the Moon’s orbit is slightly tilted compared to Earth’s orbit around the Sun.
→ Mean Solar Day: The average time the Sun takes to return to its highest position in the sky from one day to the next is 24 hours.
→ The Moon takes about 29.5 days to complete all its phases. This cycle forms the basis of a month, a unit used to measure time.
→ Solar Year: A solar year is the time taken by the Earth to complete one full revolution around the Sun, which is about 365 1/4 days long. This period marks the cycle of seasons and is used to define a year in solar calendars.
→ Lunar Calendar: About 12 cycles of the Moon’s phases fit into one cycle of the seasons, forming 12 lunar months. This led to the creation of the lunar calendar, with the day as the smallest unit, a month of about 29.5 days, and a lunar year of 12 months.
→ In a lunar calendar, the seasons do not stay aligned with the same lunar months each year. This is because the seasons follow the solar year of about 365 days, while the lunar year is only 354 days long.
→ Solar Calendar and Leap Year: Solar calendars were created to match the year with the seasons, important for farming.
- The Gregorian calendar, used widely today, is a solar calendar. The months in solar calendars are arranged to add up to 365 days. That’s why some months have 30 days, others have 31 days, and February has 28 days.
- Since Earth takes about 365 1/4 days to orbit the Sun, an extra day is added every four years as a leap year. In these years, February has 29 days to keep the calendar aligned with the seasons.
→ The Earth takes slightly less than 365 1/4 days to complete one orbit. So, adding a leap day every 4 years adds a bit extra over time. To fix this, leap years are skipped every 100 years, except every 400 years when a leap year is added back. This keeps the calendar aligned with the seasons.
→ Tropical Year: Seasons are caused due to the Earth’s revolution around the Sun and its movement from the spring equinox to the next. The time between successive spring equinoxes is called the tropical year, which the Gregorian calendar follows.
→ Sidereal Year: The stars rise at sunset change throughout the year due to Earth’s revolution around the Sun. The time it takes for the same stars to rise again at sunset is called the sidereal year, which can also define a solar calendar. It is about 20 minutes longer than the tropical year. Astronomers use the sidereal year to track Earth’s position in its orbit around the Sun.
→ Our Scientific Heritage: For thousands of years, people, including in India, observed the sky and developed calendars. Even without knowing Earth’s revolution around the Sun, they noticed patterns and found the year to be about 365 days.
- They saw that the Sun rises slightly north of East in summer and south of East in winter. These extremes happen around June 21 and December 21, known as the solstices. The Sun’s northward movement (from December to June) is called Uttarayan, and its southward movement (from June to December) is Dakshinayan. This cycle repeats each year and relates to the changing seasons.
- Ancient Indian texts like the Taittiriya Samhita and Surya Siddhanta recorded these patterns. Different calendars were created in India to track time and festivals.
→ Luni-Solar Calendar: This type of calendar follows the Moon’s phases to count days and months but also makes changes to match the seasons of the solar year. Since 12 lunar months make only 354 days, it falls short by about 11 days as compared to a solar year. So, every few years, an extra month (called Adhika Maasa or intercalary month) is added. This keeps the solar year and lunar cycle in step. Such calendars are used in many parts of India.
→ In Indian luni-solar calendars, the months are named as Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha (or Agrahayan), Pausha, Magha, and Phalguna. In some regions, the new month begins after the new Moon and ends on the next new Moon. This type of calendar is called Amant. In other places, the month starts after the full Moon and ends on the next full Moon. This type of calendar is called Purnimant.
→ The Indian National Calendar: It is a solar calendar with 365 days in a year. The year begins on 22 March, just after the spring equinox. The months are based on traditional Indian calendars and have either 30 or 31 days.
- In a normal year, the second to sixth months have 31 days, and the rest have 30 days.
- In a leap year, an extra day is added to the first month, Chaitra, and the new year then begins on 21 March instead of 22 March.
→ In 1952, the Government of India set up a Calendar Reform Committee (CRC) to create a uniform calendar. The committee recommended ‘Unified National Calendar’ which was adopted on 21 March 1956 CE (1 Chaitra 1878 Saka). The Indian National Calendar is based on the Surya Siddhanta.
→ Meghnad Saha (1893-1956): An Indian astrophysicist who studied stars and their temperatures. He created the Saha equation. The Saha Institute of Nuclear Physics in Kolkata is named after him. He also served as the chairperson of the Calendar Reform Committee.
→ Many Indian festivals are based on the phases of the Moon and follow either lunar or luni-solar calendars.
- Examples of Moon-based festivals:
- Diwali-New Moon of Kartika
- Holi – Full Moon of Phalguna
- Buddha Purnima – Full Moon of Vaisakha
- Eid-ul-Fitr-Crescent Moon at the end of Ramazan
- Dussehra – 10th day of Ashwina
Because of this, these festivals do not fall on the same date every year in the Gregorian calendar.
→ For festivals based on luni-solar calendars, the Gregorian dates shift but usually by less than a month. This is because these calendars add an extra month (intercalary month) every few years to balance the lunar and solar years. Pure lunar calendars do not adjust with the solar year. So, festivals like Eid-ul-Fitr can occur in different months of the Gregorian calendar each year.
→ Some Indian festivals like Makar Sankranti, Pongal, and Bihu follow the solar sidereal calendar. These festivals usually fall on the same date every year in the Gregorian calendar, which is based on the tropical year.
A long time ago, these festivals were tied to either a solstice or an equinox. Due to the small difference in the sidereal and tropical years, the dates of these festivals slowly shift away from the solstices/ equinoxes. This shift is due to slow wobble of the Earth’s axis, similar to the movement of the axis of a wobbling top. This causes the dates of festivals based on the sidereal calendar to move ahead in tropical calendar. For example, Makar Sankranti moves ahead by one day every 71 years.
→ Many Indian festivals are based on the exact lunar phase at sunrise. Since sunrise times vary across India, festival dates can differ by a day in different regions, even in the same year. To keep festival dates the same across the country, the Government of India publishes the Rashtriya Panchang every year, which helps fix official festival dates.
→ The Moon inspires many Indian classical ragas like Chandrakauns and Shubhapantuvarali. Moon-related hand-gestures appear in dances like Bharatanatyam, Kathak, and Octissi. Traditional art styles like Madhubani and Warti, along with tribal crafts from Saura and Gond, often depict the Moon and Sun, showing their importance in daily life.
→ Artificial Satellite: Artificial satellites are human-made objects launched from the Earth. These artificial satellites appear as tiny specks moving in the night sky. Most orbit about 800 km above Earth’s surface and take roughly 100 minutes to complete one orbit. They give us useful information for daily life, communication and space research.
- ISRO’s Cartosat satellites take clear pictures of the Earth to help make maps, plan cities, and manage disasters. The Bhuvan platform uses these images to show land and vegetation.
- AstroSat studies stars and space objects. Other missions include Chandrayaan 1,2 and 3 to the Moon, Aditya Ll to study the Sun, and Mangalyaan to Mars. ISRO also helps students build and launch small satellites like AzaadiSat, InspireSat-1, and Jugnu.
→ Many artificial satellites are sent into space by different countries. After their useful life, some become space junk or debris, which can collide with working satellites. Small debris burns up in the atmosphere, but larger pieces can fall to the Earth. Countries are working together to clean up this debris.
→ Vikram Ambalal Sarabhai (1919-1971): A researcher in space science and nuclear physics, is known as the Father of the Indian Space Programme. He helped launch India’s first satellites. The Vikram Sarabhai Space Centre (VSSC) in Thiruvananthapuram is named after him.
→ Tides are the rise and fall of sea levels. They follow a regular pattern, occurring about 50 minutes later each day. This happens because the Moon also rises about 50 minutes later every day. Tides are closely related to the Moon’s position and phase.