Class 8 › Science › Chapter 11
Keeping Time with the Skies
Why the Moon changes shape, how to find it in the sky, how the day, month and year give rise to lunar, solar and luni-solar calendars, the Indian National Calendar, the link with festivals, and artificial satellites.
1. Phases of the Moon
Phases of the Moon: the changing shapes of the bright (illuminated) portion of the Moon from one day to another, as seen from Earth.
Waxing
Shukla Paksha
Bright part increases from new Moon to full Moon (about 2 weeks).
Waning
Krishna Paksha
Bright part decreases from full Moon to new Moon (about 2 weeks).
Cycle
About a month
One full Moon to the next takes about a month (about 29.5 days).
- Full Moon (Purnima): the Moon appears as a full bright circle.
- New Moon (Amavasya): the Moon is not visible.
- Crescent: less than half of the illuminated portion is seen. Gibbous: more than half of the illuminated portion is seen.
- Sequence: new Moon → crescent → half → gibbous → full Moon → gibbous → half → crescent → new Moon (about a week between each main stage).
- Because the Earth rotates once a day while the Moon's cycle takes about a month, people in different parts of the Earth see nearly the same phase on a given day.
2. Locating the Moon in the Sky
- Full Moon: nearly opposite the Sun. When the Sun rises in the East, the Moon is almost setting in the West.
- On successive mornings at sunrise, as the bright part decreases, the Moon appears closer to the Sun in the sky. At a half-circle shape (waning) it is overhead at sunrise; the crescent Moon is even closer to the Sun.
- New Moon: appears closest to the Sun. Full Moon: appears farthest from the Sun.
| Moon | Easiest to spot |
|---|---|
| Waxing | At sunset |
| Waning | At sunrise |
- Moonrise is about 50 minutes later each day. The Moon moves ahead in its orbit while Earth completes one rotation in 24 hours, so Earth must rotate a bit more for the Moon to appear at the same spot.
- Myth: "the Moon rises when the Sun sets" is not always true. Sometimes moonrise is in the afternoon (about 2 to 4 p.m.), so the Moon can be seen in the eastern sky during daylight (wait about 30 minutes after listed moonrise).
- The Moon therefore always rises and sets at different times from the Sun.
3. Why Do Phases Occur?
Explanation
1. The Moon is a sphere and does not emit its own light; it shines by reflecting sunlight.
2. The half facing the Sun is illuminated; the half facing away is non-illuminated.
3. The Moon revolves around the Earth in about a month, and only one half of it always faces Earth.
4. The half facing Earth is not always the illuminated half, so we see different fractions of the illuminated part on different days. The Moon's shape does not change; only what we see changes.
Moon's orbit (not to scale). A to C to E: waning; E to G to A: waxing. At A the whole lit side faces Earth (full Moon); at E only the dark side faces Earth (new Moon).
- Ball-and-lamp activity: a ball on a stick (Moon), a lamp or torch (Sun) and your head (Earth). Holding the ball opposite the lamp shows a fully lit face (full Moon); holding it towards the lamp shows only the dark side (new Moon). In between, the line separating lit and dark parts looks curved, like the Moon's phases.
4. Phases vs Eclipses
| Phases of the Moon | Lunar eclipse | |
|---|---|---|
| Cause | Changing positions of Sun, Moon and Earth as the Moon revolves; we see different parts of its lit half | Earth's shadow falls on the Moon |
| Frequency | Every day, repeating monthly | Rare and brief |
- Misconception: Earth's shadow does not cause phases.
- Lunar eclipses happen only on a full Moon day; solar eclipses only on a new Moon day.
- They do not occur every month because the Moon's orbit is slightly tilted relative to Earth's orbit around the Sun.
5. Day, Month and Year: Units from Natural Cycles
| Unit | Natural cycle it is based on | Length |
|---|---|---|
| Day | Sun returning to its highest point due to Earth's rotation | 24 hours (mean solar day) |
| Month | Cycle of Moon's phases due to the Moon's revolution around Earth | About 29.5 days |
| Year | Cycle of seasons due to Earth's revolution around the Sun | About 365¼ days |
- Mean solar day: the average time the Sun takes to go from its highest position in the sky on one day to its highest position the next day.
- Finding the highest position: the shadow of a vertical stick is shortest when the Sun is highest. Mark the shadow tip every minute and find the time of the shortest shadow on consecutive days; the difference is about 24 hours.
6. Lunar, Solar and Luni-solar Calendars
| Type | Based on | Features | Example |
|---|---|---|---|
| Lunar | Moon's phases | Month about 29.5 days; year = 12 lunar months = 354 days; seasons do not stay in the same months (year is about 11 days short of 365) | Islamic (Hijri) calendar |
| Solar | Cycle of seasons | Months adjusted to add up to 365 days; stays in step with seasons; leap day added | Gregorian calendar |
| Luni-solar | Moon's phases for months, adjusted for seasons | An extra month (Adhika Maasa, intercalary month) added every 2 to 3 years to make up the ~11-day yearly gap | Many Indian calendars |
- Indian luni-solar months: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha (Agrahayan), Pausha, Magha, Phalguna.
- Amant: month starts the day after the new Moon and ends on new Moon. Purnimant: month starts the day after the full Moon and ends on full Moon.
- Gregorian months have 30 or 31 days, and February has 28, so that the year adds up to 365 days.
- Heritage: ancient people deduced the year (about 365 days) by careful sky observation. The Sun rises a little north of East in summer and a little south of East in winter, with extremes at the solstices (about 21 June and 21 December). Apparent northward movement (Dec to June) is Uttarayan; southward (June to Dec) is Dakshinayan (recorded in the Taittiriya Samhita; Surya Siddhanta tracks it by constellations). On equinoxes the Sun rises exactly East and sets exactly West.
7. Leap Years
Why leap years
Earth takes about 365¼ days to orbit the Sun, so the extra quarter days add up to about 1 day every 4 years, which is added as 29 February.
Gregorian leap year rule (with the century correction)
1. A year divisible by 4 is a leap year (February has 29 days).
2. Century years are skipped (1700, 1800, 1900 were not leap years) because 365¼ is slightly too much (the year between spring equinoxes is a bit shorter).
3. Every 400 years a leap year is added back (1600, 2000 were leap years).
- Tropical year: time between successive spring equinoxes (Gregorian calendar is based on it). Sidereal year: time for the same stars to rise again at sunset; about 20 minutes longer than the tropical year. Astronomers use the sidereal year to track Earth's position in its orbit.
8. The Indian National Calendar
- A solar calendar of 365 days, used with the Gregorian calendar for official purposes.
- Year begins on 22 March (day after the spring equinox); in leap years it begins on 21 March. A leap day is added to Chaitra, the first month.
- Months have 30 or 31 days. In a regular year months 2 to 6 have 31 days and the rest have 30. Month names come from traditional Indian calendars.
- Calendar Reform Committee set up in 1952 (chairperson: Meghnad Saha). The unified calendar was adopted from 21 March 1956 CE (1 Chaitra 1878 Saka). It follows the principles of the Surya Siddhanta.
- Be a scientist, Meghnad Saha (1893 to 1956): astrophysicist who studied stars and their temperatures; the Saha equation is named after him, as is the Saha Institute of Nuclear Physics, Kolkata.
9. Are Festivals Related to Astronomical Phenomena?
| Festival | Linked to |
|---|---|
| Diwali | New Moon of the month of Kartika |
| Holi | Full Moon of Phalguna |
| Buddha Purnima | Full Moon of Vaisakha |
| Dussehra | Tenth day of Ashwina |
| Eid-ul-Fitr | Sighting of crescent Moon at the end of Ramazan |
| Makar Sankranti, Pongal, Bihu, Vaisakhi, Poila Baisakh, Puthandu | Solar sidereal calendar: nearly the same Gregorian date every year |
- Why dates change every year: lunar and luni-solar festivals follow Moon phases, so their Gregorian dates shift. In luni-solar calendars the shift is typically less than a month (because of the intercalary month). In purely lunar calendars it keeps moving, so Eid-ul-Fitr comes about 11 days earlier each year and can fall in any Gregorian month.
- Sidereal festivals slowly drift: because of the slow wobble of Earth's axis (like a wobbling top), Makar Sankranti moves ahead by one day every 71 years relative to the tropical calendar.
- Rashtriya Panchang: published yearly by the Positional Astronomy Centre (Govt of India) for a central location; used to fix festival dates for holidays, since lunar-phase-at-sunrise dates can differ by a day between east and west India.
- Culture: the Moon and Sun inspire ragas (Chandrakauns, Chandranandan, Shubhapantuvarali), mudras (Chandrakala, Ardhachandran) and art (Warli, Madhubani, Dhokra).
10. Why Do We Launch Artificial Satellites?
- The Moon is the natural satellite of Earth; man-made artificial satellites also orbit it. They look like fast-moving specks of light at dusk or dawn.
- Most orbit about 800 km above the surface and take roughly 100 minutes per orbit, so their motion is also periodic.
- Uses: communication, navigation, weather monitoring, disaster management, scientific research, mapping.
- ISRO: Cartosat (high-quality Earth images for maps, city planning and disasters; used in the Bhuvan platform), AstroSat (observes stars and celestial objects). Missions: Chandrayaan 1, 2, 3 (Moon), Aditya L1 (Sun), Mangalyaan (Mars). Student satellites: AzaadiSat, InspireSat-1, Jugnu.
- Space debris: dead satellites and rocket parts crowd space and may collide with working satellites. Small pieces burn up in the atmosphere; larger ones can crash on the ground.
- Be a scientist, Vikram Sarabhai (1919 to 1971): Father of the Indian Space Programme. VSSC (Thiruvananthapuram), which develops rockets and launch vehicles, is named after him.
- Tides: high or low tide repeats about 50 minutes later each day, like moonrise, showing that tides are linked to the Moon's position and phase.
11. Quick Revision
| Fact | Value |
|---|---|
| Mean solar day | 24 hours |
| Cycle of Moon's phases | About 29.5 days |
| Solar (tropical) year | About 365¼ days |
| Lunar year (12 lunar months) | 354 days (about 11 days short) |
| Moonrise delay per day | About 50 minutes |
| Artificial satellite orbit | About 800 km, about 100 minutes |
| Indian National Calendar adopted | 21 March 1956 (1 Chaitra 1878 Saka) |
Remember
Where to look
Waxing Moon: evening, near sunset. Waning Moon: morning, near sunrise. Crescent's bright edge always faces the Sun.
Remember
Three calendars
Lunar = Moon; solar = seasons; luni-solar = Moon months + extra month every 2 to 3 years.
Common mistakes:
• Saying phases are caused by Earth's shadow (that is a lunar eclipse).
• Saying the Moon's shape actually changes (only the visible lit part changes).
• Believing the Moon is visible only at night or always rises at sunset.
• Mixing up waxing (growing, after new Moon) and waning (shrinking, after full Moon).
• Confusing crescent (less than half lit) with gibbous (more than half lit).
• Thinking eclipses occur every month (the Moon's orbit is tilted).
• Treating all festivals alike: lunar (Eid) shifts through the year; sidereal (Makar Sankranti) stays near the same date.
• Saying phases are caused by Earth's shadow (that is a lunar eclipse).
• Saying the Moon's shape actually changes (only the visible lit part changes).
• Believing the Moon is visible only at night or always rises at sunset.
• Mixing up waxing (growing, after new Moon) and waning (shrinking, after full Moon).
• Confusing crescent (less than half lit) with gibbous (more than half lit).
• Thinking eclipses occur every month (the Moon's orbit is tilted).
• Treating all festivals alike: lunar (Eid) shifts through the year; sidereal (Makar Sankranti) stays near the same date.