This MCQ module is based on: Moon Phases, Month and Calendars
TOPIC 40 OF 50
Moon Phases, Month and Calendars
🎓 Class 8
Science
CBSE
Theory
Ch 11 — Friction
⏱ ~30 min
🌐 Language: [gtranslate]
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[myaischool_lt_science_assessment grade_level="class_8" science_domain="physics" difficulty="basic"]
Probe and Ponder
Step outside on several nights in a row and look at the Moon. One evening it is a thin, shy sliver. A week later it is a half circle. Another week and it is a bright, full silver disc. Then it begins to shrink again — half, a sliver, and finally disappears completely for a night or two. And then the whole cycle begins again.
- Why does the Moon's shape seem to change so dramatically night after night?
- Does the Moon actually change shape, or do we only see different parts of it lit up?
- Does one full cycle of the Moon's phases give us a unit we already know — the month?
- Why do Eid and Diwali fall on different dates every year, but Republic Day always on 26 January?
11.6 The Changing Moon
The Moon is our nearest neighbour in space and Earth's only natural satellite. It does not produce light of its own. Everything we see at night is sunlight bouncing off the Moon's grey rocky surface. As the Moon orbits the Earth, the angle between the Sun, the Moon and us keeps changing, so we see different portions of its lit half — these are the phases of the Moon.
Amavasya, Purnima and the Two Pakshas
In the Indian tradition, two nights in every lunar cycle have special names:
- Amavasya — the night with no visible Moon (new moon).
- Purnima — the night of the bright full Moon.
The fortnight from Amavasya to Purnima, when the Moon is "growing" in size, is called Shukla Paksha (the bright half). The fortnight from Purnima to the next Amavasya, when the Moon is "shrinking", is called Krishna Paksha (the dark half). Each paksha contains 15 lunar days, called tithis. Two pakshas together make one lunar month.
1 Lunar Month ≈ 29.5 Days: the time from one Amavasya to the next, or one Purnima to the next. This gave humanity the first idea of a "month".
11.7 Why Does the Moon Change Phase?
The Moon always has exactly half of its surface lit by the Sun — a permanent "day side" — and the other half in darkness. But depending on where the Moon is in its orbit around Earth, we see only a portion of that lit half.
Why Does an Eclipse Not Happen Every Month?
You might think that at every Amavasya the Moon should block the Sun (solar eclipse) and at every Purnima the Earth should block the Sun from the Moon (lunar eclipse). But the Moon's orbit is tilted by about 5° with respect to Earth's orbit around the Sun, so most of the time the three bodies don't line up in a perfect straight line. Eclipses happen only a handful of times a year when the alignment is just right.
11.8 Calendars Built on the Moon
If you count days between one full moon and the next, you get about 29–30 days — the lunar month. Early human cultures built calendars around this. Twelve lunar months give approximately 354 days — 11 days shorter than the solar year of 365.25 days. This difference is what creates three different families of calendars.
The Chaitra Maas and Hindu Lunar Months
In the Hindu calendar, the lunar month is named after the nakshatra (star group) in which the full moon occurs. The 12 lunar months begin with Chaitra (March–April) and include familiar names like Vaisakha, Jyaishtha, Ashadha, Shravana, Bhadrapada, Ashvina, Kartika, Margashirsha, Pausha, Magha and Phalguna. Each month is divided into its two pakshas of 15 tithis.
Did You Know? The Indian new year starts on the first day of the Shukla Paksha of Chaitra — celebrated as Ugadi in Karnataka and Andhra Pradesh, Gudi Padwa in Maharashtra, and Cheti Chand by the Sindhi community. It usually falls in late March or early April.
The Three Kinds of Calendars
| Calendar Type | Follows | Year Length | Example |
|---|---|---|---|
| Solar | The Sun's position | 365.25 days | Gregorian (civil), Iranian Persian |
| Lunar | Only the Moon's phases | ≈ 354 days (12 lunar months) | Islamic Hijri |
| Lunisolar | Both Moon (month) and Sun (year) | Adjusted with extra month | Hindu Panchang, Jewish, Chinese |
The Islamic Hijri Calendar — Pure Lunar
The Hijri calendar is a pure lunar calendar. Each month starts when the first thin crescent Moon is sighted after Amavasya. Twelve such months form one year of ~354 days. Because this is 11 days short of the solar year, Islamic festivals slide earlier each year with respect to the Gregorian calendar. That is why Ramzan in 2024 was in March, in 2027 will be in February, and in 2030 will be in January.
The Hindu Panchang — Lunisolar Genius
The Panchang is a lunisolar calendar — it uses lunar months but keeps them in step with the solar year. Since 12 lunar months are 11 days short, the mismatch piles up to a whole month every 2–3 years. To fix this, the Hindu calendar adds an extra lunar month, called Adhik Maas, roughly every third year. This ensures that festivals like Holi (Phalguna Purnima) always remain in spring, and Diwali (Kartika Amavasya) always in autumn, even though their Gregorian dates change by a few weeks each year.
Why Different Calendars Have Different Lengths:
• A purely lunar year (354 days) slips behind the seasons, so festivals drift across months and years.
• A purely solar year (365.25 days) keeps the seasons fixed but ignores the Moon's beautiful monthly rhythm.
• A lunisolar calendar is the best of both worlds — months follow the Moon, but an extra month is inserted now and then to stay aligned with the Sun.
• A purely lunar year (354 days) slips behind the seasons, so festivals drift across months and years.
• A purely solar year (365.25 days) keeps the seasons fixed but ignores the Moon's beautiful monthly rhythm.
• A lunisolar calendar is the best of both worlds — months follow the Moon, but an extra month is inserted now and then to stay aligned with the Sun.
🌙 Activity 11.2 — Your Own Moon Diary
You will need: a notebook, a pencil, and 30 minutes of clear night sky for 30 days in a row.
- Every clear evening, go outside around 8 p.m. Look for the Moon (if you can't see it, note "not visible — maybe new moon or below horizon").
- Draw a small circle on your page and shade in the dark portion so that the remaining lit part matches what you see.
- Write the date and, if you can, the approximate direction (east, overhead, west).
- Do this for a full month. At the end, flip through your pages quickly — you will have created a "Moon flipbook".
🔍 Predict: How many days will pass between the night when you saw no Moon and the night the Moon is fullest? How many days from full Moon back to no Moon?
New Moon to Full Moon: about 14–15 nights (the Shukla Paksha). The lit part grows a little each night — this is "waxing".
Full Moon to New Moon: another 14–15 nights (the Krishna Paksha). The lit part shrinks a little each night — this is "waning".
Total cycle: about 29–30 days — the origin of our "month".
You will also notice that the Moon rises a little later every night (about 50 minutes later each day) and appears in a slightly different part of the sky. That's because it is quietly orbiting Earth while you watch.
11.9 A Quick Look at Units of Time So Far
Day (24 h)
Given by the Earth's rotation about its own axis.
Month (~29.5 d)
Given by the Moon's revolution around the Earth — one full phase cycle.
Year (~365.25 d)
Given by Earth's revolution around the Sun.
Panchang
Indian lunisolar almanac blending Moon and Sun into one unified system.
🎯 Competency-Based Questions
Fatima keeps a little "festival diary" at home. She notices that Eid-ul-Fitr fell on 22 April in 2023, on 11 April in 2024, and on 31 March in 2025 — each year the date shifts back by about 11 days. Her brother Rehan keeps track of Diwali, which was on 24 October in 2022, 12 November in 2023, and 1 November in 2024 — sometimes it moves forward, sometimes back, but never by more than a couple of weeks.
Q1. L1 Remember Define "Amavasya" and "Purnima".
Answer: Amavasya is the new-moon night — the Moon's dark side faces Earth and no Moon is visible in the sky. Purnima is the full-moon night — Earth sits between Sun and Moon, so the entire sunlit face of the Moon is visible as a bright disc.
Q2. L2 Understand Why does the shape of the Moon appear to change night after night?
Answer: The Moon itself does not change shape. As it moves around Earth, the angle between Sun, Moon and us keeps changing, so the portion of the sunlit half visible from Earth also keeps changing. When the lit half fully faces us, we see Purnima; when it fully faces away, we see Amavasya; in between, we see crescents and gibbous shapes.
Q3. L3 Apply Explain, using Fatima and Rehan's observations, why Eid shifts back 11 days per year but Diwali stays close to late October / early November.
Answer: Eid follows the Islamic Hijri calendar, which is purely lunar (12 lunar months ≈ 354 days). Because the Hijri year is 11 days shorter than the Gregorian (solar) year, every Islamic festival slips 11 days earlier in the Gregorian calendar each year. Diwali follows the Hindu lunisolar Panchang, which inserts an extra month (Adhik Maas) every 2–3 years. The extra month keeps lunar months aligned with the solar year, so Diwali stays locked to autumn.
Q4. L4 Analyse Analyse whether a purely lunar calendar would be suitable for a farmer who needs to sow crops at the right time every year.
Answer: A purely lunar calendar would be unsuitable for farming. Its year of 354 days slips 11 days per year against the seasons, so over 3 years the calendar drifts a whole month. Sowing would shift from spring into winter over a decade, ruining crops. Farming needs a calendar locked to the seasons — either a solar calendar (like the Gregorian) or a lunisolar calendar (like the Panchang) with its Adhik Maas correction.
Q5. L5 Evaluate A classmate claims that "Adhik Maas is a religious invention with no scientific basis." Evaluate this statement.
Answer: The claim is incorrect. Adhik Maas has a solid astronomical basis: 12 lunar months (~354 days) are shorter than one solar year (~365.25 days) by about 11 days. Over 2–3 years this accumulates to nearly one full lunar month. Adding one extra lunar month at this point is simply the minimum correction needed to keep the calendar synchronised with the solar year and the seasons. Most ancient lunisolar calendars (Hindu, Jewish, Chinese) use similar schemes. Its ritual significance came later; the astronomy came first.
🔗 Assertion–Reason Questions
Assertion (A): The Moon shines at night.
Reason (R): The Moon is a hot, glowing body like the Sun.
Answer: C. The Moon does appear bright at night (A true), but it has no light of its own — it merely reflects sunlight off its rocky surface. R is false.
Assertion (A): The Islamic month of Ramzan shifts about 11 days earlier every year.
Reason (R): The Hijri calendar is a lunar calendar of 354 days, 11 days shorter than the solar (Gregorian) year.
Answer: A. The 11-day gap between a lunar year and a solar year is exactly why Hijri months slide earlier each Gregorian year.
Assertion (A): The Hindu calendar adds an Adhik Maas every few years.
Reason (R): Adding an extra lunar month keeps the lunar months aligned with the solar seasons.
Answer: A. The Adhik Maas is specifically the mechanism by which the Panchang (a lunisolar calendar) stays locked to the solar year.
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