ScienceCBSE Class 8 · Keeping Time with the Skies

Phases of the Moon and the Moon's Changing Position: Class 8 Science

Updated 17 October 2026 · Fact-checked

The Moon shines by reflecting sunlight, and half of it is always lit. As the Moon revolves around the Earth in about a month, we see different fractions of its lit half, giving the phases from new Moon to full Moon and back. The Moon rises about 50 minutes later each day, and the Earth's shadow causes eclipses, not phases.

Understand Phases of the Moon and the Moon's Changing Position

Watching the Moon (Activity 11.1). Start from the first sunrise after a full Moon, noting the date, when you saw the Moon and the shape of the bright part. For the first 15 days or so you watch at sunrise, and for the next 15 days at sunset. The bright part decreases from a full circle to a half circle in about a week and then to nothing in another week: this two-week period is the waning period (Krishna Paksha). After the new Moon the bright part grows to a half circle in about a week and then to a full circle: the waxing period (Shukla Paksha). The day with a full bright circle is full Moon (Purnima) and the day with none is new Moon (Amavasya). The cycle from one full Moon to the next takes about a month (about 29.5 days). The changing shapes of the bright part from day to day are the phases of the Moon. Names: crescent (bright part less than half of the Moon's disc), half (half of the disc bright), gibbous (bright part more than half of the disc but not full).

Locating the Moon. On a full Moon day the Moon is nearly opposite the Sun: as the Sun rises in the East, the Moon is about to set in the West. On the following mornings the Moon at sunrise appears closer to the Sun as its bright part shrinks; when the bright part is a half circle, the Moon is overhead at sunrise; the crescent appears still closer to the Sun. Knowing the phase tells you where to look.

Moonrise is later each day. Moonrise occurs about 50 minutes later each day. Sometimes the Moon rises in the afternoon (around 2 to 4 p.m.), so you can see it in the eastern sky in daylight, about 30 minutes after the listed moonrise. The reason: in one day the Moon revolves a bit further in its orbit, so the Earth has to rotate somewhat more (about 50 minutes) before the Moon is in nearly the same spot in the sky.

Why phases happen (Activity 11.2, ball and lamp). The Moon does not give light of its own, and the half facing the Sun is lit while the other half is dark. The same side of the Moon always faces the Earth, but the lit part of it is not always the part facing us. When the whole lit half faces the Earth we see a full Moon; when only the dark half faces us it is new Moon; at other positions we see part of the lit half, with a curved line separating lit and dark parts. Gibbous phases appear at the positions before and after full Moon, and crescents at positions near new Moon. On new Moon day the Moon appears closest to the Sun in the sky, and on full Moon day farthest. Because the Earth rotates in one day, much faster than the Moon's one-month revolution, people everywhere see nearly the same phase on a given day.

Eclipses are different. The Earth's shadow on the Moon causes a lunar eclipse, not the phases. Lunar eclipses occur only on a full Moon day and solar eclipses only on a new Moon day. They do not happen every month because the Moon's orbit is slightly tilted with respect to the Earth's orbit around the Sun.

Culture. Ragas such as Chandrakauns and Chandranandan carry the Moon in their names, and hand gestures (mudras) such as Chandrakala are named after it.

Calendars built from the Moon's cycle are in the next topic.

Phases of the Moon and the Moon's Changing Position: formulas and key facts

Waxing and waning: Waxing (Shukla Paksha): bright part increases | Waning (Krishna Paksha): bright part decreases; Each lasts about two weeks.
Special days: Full Moon (Purnima): full bright circle | New Moon (Amavasya): bright part not visible; Full Moon is nearly opposite the Sun.
Cycle length: About 29.5 days from one full Moon to the next; Basis for the month.
Moonrise delay: About 50 minutes later each day; Because the Moon moves ahead in its orbit.
Eclipses: Lunar eclipse: full Moon day only | Solar eclipse: new Moon day only; Not every month because the Moon's orbit is tilted.

How to solve Phases of the Moon and the Moon's Changing Position questions

Use this method for Moon-phase questions.

1Remember that half of the Moon is always lit by the Sun.
2Decide how much of the lit half faces the Earth at the given position.
3Name the phase: none = new Moon, less than half = crescent, half, more than half = gibbous, all = full.
4For waxing or waning, check whether the bright part is increasing or decreasing day by day.
5For timing, add 50 minutes to the moonrise time for each day later.
6For eclipses, use the rule: lunar on full Moon, solar on new Moon.

Phases of the Moon and the Moon's Changing Position in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Predicting moonrise

Suppose the Moon rises at 5:40 p.m. on a full Moon day in a town, and it rises about 50 minutes later each day (use exactly 50 minutes for this problem).

(a) Find the moonrise time two days later. (b) On which day after full Moon does the moonrise first pass 10:00 p.m.? (c) What phase is it near then? (d) Is the Moon visible at sunset (about 6:00 p.m.) two days later?

Try it first, then show the solutionSolution

Two days later: 5:40 p.m. + 2 × 50 minutes = 5:40 + 1 h 40 min = 7:20 p.m.
Day 1: 6:30; day 2: 7:20; day 3: 8:10; day 4: 9:00; day 5: 9:50; day 6: 10:40 p.m. So the sixth day after.
About six days after the full Moon the Moon is in its waning gibbous phase heading toward the half phase (about a week after full Moon).
No; moonrise is 7:20 p.m., so it has not risen at 6:00 p.m.

Answer: (a) 7:20 p.m.; (b) the sixth day; (c) waning, between full and half; (d) not yet risen.

Memory hook: Fifty minutes late each night, like a slowly slipping clock.

Ball-and-lamp model

Imagine you hold a ball at arm's length and turn slowly in a dark room with a lamp on one side, as in Activity 11.2.

(a) When do you see the whole lit side of the ball? (b) When do you see none of it? (c) What do you see at other times? (d) What does your head represent?

Try it first, then show the solutionSolution

When the ball is opposite the direction of the lamp, you see the entire lit side: like full Moon.
When the ball is towards the lamp, you see only the dark side: like new Moon.
At other positions a curved boundary separates the lit and dark parts: crescent and gibbous shapes.
The Earth.

Answer: (a) Ball opposite the lamp; (b) ball towards the lamp; (c) crescents and gibbous shapes; (d) the Earth.

Memory hook: Light is always half; what changes is how much of that half faces you.

Tricks to remember Phases of the Moon and the Moon's Changing Position

Short memory aids. Say each one aloud once, then use it on a question the same day.

Shukla grows, Krishna shrinks
Shukla Paksha: bright (shukla) part grows. Krishna Paksha: dark (krishna) part grows.
Use it when: Waxing and waning.
Gibbous is Great, crescent is Cut
Gibbous = more than half; crescent = a thin cut.
Use it when: Naming shapes.
Full Moon opposite Sun
Sun sets, Moon rises on full Moon: opposite sides.
Use it when: Position of the full Moon.
Lunar on Purnima, Solar on Amavasya
Lunar eclipse on full Moon; solar eclipse on new Moon.
Use it when: Eclipse questions.

Phases of the Moon and the Moon's Changing Position: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Define the phases · 3 marks · Short answerBoard level

Define phases of the Moon, waxing and waning, and give their Indian names.

Try it first, then show the solutionSolution

Phases are the changing shapes of the bright part of the Moon seen from the Earth from day to day. (1)
Waxing: the period in which the bright part increases (Shukla Paksha). (1)
Waning: the period in which the bright part decreases (Krishna Paksha). (1)

Answer: Phases are the changing shapes of the bright part; waxing is Shukla Paksha; waning is Krishna Paksha.

Example 2: Which day is the Moon not visible · 1 mark · MCQBoard level

On which day is the illuminated part of the Moon not visible from the Earth?
(a) Full Moon day
(b) New Moon day
(c) First quarter
(d) Gibbous day

Try it first, then show the solutionSolution

On new Moon day only the non-lit half faces the Earth.

Answer: (b)

Example 3: Moon phases and positions · 5 marks · Long answerHarder board level

(a) Explain why we see phases though half the Moon is always lit. (2)
(b) Where is the Moon in the sky at sunrise on the first day after full Moon, and why? (1)
(c) Why does the Moon rise about 50 minutes later each day? (2)

Try it first, then show the solutionSolution

(a) The Moon revolves around the Earth, so the part of the lit half that faces us changes: sometimes the entire lit part, sometimes a part, sometimes none. (2)
(b) Just before setting in the west, since the full Moon is nearly opposite the Sun and the next day is only slightly past it. (1)
(c) In a day the Moon moves a bit ahead in its orbit, so the Earth must rotate some more (about 50 minutes) for the Moon to be in nearly the same place again. (2)

Answer: (a) The lit portion facing us changes; (b) near the western horizon; (c) the Moon moves ahead and Earth needs more time.

Example 4: Misconceptions about phases · 4 marks · Case-basedHarder board level

Read the passage and answer.

Ajay says: 'The Moon's phases happen because the Earth's shadow falls on the Moon.' Bela says: 'The Moon changes shape through the month.'
(a) Correct Ajay's statement. (1)
(b) Correct Bela's statement. (1)
(c) What does the Earth's shadow on the Moon cause? (1)
(d) Why do we not have a lunar eclipse every full Moon? (1)

Try it first, then show the solutionSolution

(a) Phases are caused by the changing orientation of the Sun, Moon and Earth as the Moon revolves around the Earth. (1)
(b) The Moon's shape does not change; only the part of the lit half that we see changes. (1)
(c) A lunar eclipse. (1)
(d) The Moon's orbit is slightly tilted with respect to the Earth's orbit around the Sun. (1)

Answer: (a) Phases are due to geometry, not the shadow; (b) only the visible lit part changes; (c) lunar eclipse; (d) tilted orbit.

Example 5: Daytime Moon · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

On a certain day the Moon rises at 2:40 p.m. A student sees a bright part that is more than half but less than full, and then says the Moon should not be visible in daytime.
(a) Is a daytime Moon possible? Explain. (1)
(b) At what time can she expect the Moon to be easily visible (the chapter suggests waiting about 30 minutes past moonrise)? (1)
(c) Is it waxing or waning if the Moon rose in the afternoon and shows more than half bright? (1)
(d) Predict moonrise on the next day. (1)

Try it first, then show the solutionSolution

(a) Yes. The Moon can rise in the afternoon and be seen in the eastern sky in daylight, since it shines by reflected sunlight. (1)
(b) 2:40 p.m. + 30 minutes = 3:10 p.m. (1)
(c) Waxing: a Moon rising in the afternoon is seen moving away from the Sun toward full Moon (an inference: full Moon rises near sunset, and a Moon rising earlier is before full). (1)
(d) About 50 minutes later: 3:30 p.m. (1)

Answer: (a) Yes; (b) about 3:10 p.m.; (c) waxing (gibbous); (d) about 3:30 p.m.

Where students lose marks in Phases of the Moon and the Moon's Changing Position

The slips examiners see most often, and what to write instead.

Mistake: Explaining phases using the Earth's shadow.
A common wrong idea.
Wrong working
Phases are caused by the Earth's shadow covering part of the Moon.
Right working
Phases are due to the changing relative positions of the Sun, Moon and Earth; the Earth's shadow causes a lunar eclipse.
Do this: Say 'orientation' and 'revolution'.
Usually costs: 2 marks.
Mistake: Saying the Moon gives its own light.
It looks bright.
Wrong working
The Moon shines because it produces light.
Right working
The Moon shines by reflecting sunlight.
Do this: Always add 'reflected'.
Usually costs: 1 mark.
Mistake: Reversing waxing and waning.
Names are confusing.
Wrong working
Waxing is the period when the bright part decreases.
Right working
Waxing: bright part increases; waning: bright part decreases.
Do this: Wax = grow.
Usually costs: 1 mark.
Mistake: Saying the Moon rises exactly when the Sun sets every day.
Only true on full Moon day roughly.
Wrong working
The Moon always rises at sunset.
Right working
Moonrise is about 50 minutes later each day and can even be in the afternoon.
Do this: Use the 50-minute fact.
Usually costs: 1 mark.

Exam tips

Draw the Sun, Earth and Moon positions with the lit half shaded when asked to explain phases.
State that the Moon shines by reflected sunlight and that half of it is always lit.
Use both Indian and English names (Purnima, Amavasya; Shukla, Krishna).
Quote 29.5 days for the cycle and about 50 minutes for the daily delay in moonrise.
Separate the causes of phases and eclipses explicitly.

Practice questions on Phases of the Moon and the Moon's Changing Position

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Name the phase of the Moon in which (a) the entire lit half faces the Earth, (b) no lit part is visible.
Hint
Purnima and Amavasya.
Answer
(a) Full Moon (Purnima) (1). (b) New Moon (Amavasya) (1).
Q2 · 1 mark · MCQBoard level
A solar eclipse can occur only on
(a) full Moon day
(b) new Moon day
(c) any day of waxing
(d) any day of waning
Hint
Moon between the Earth and the Sun.
Answer
(b)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): People in different parts of the Earth see nearly the same Moon phase on a given day.
Reason (R): The Earth rotates once a day, which is much faster than the Moon's revolution of about a month around the Earth.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Is the explanation the chapter's?
Answer
(a) The chapter gives R as the reason for A.
Q4 · 4 marks · Short answerHarder board level
The Moon rises at 6:10 p.m. on one day. Assume it rises exactly 50 minutes later each day. (a) Find the moonrise on the 3rd day after. (b) After how many days does the moonrise first fall after midnight (12:00 a.m.)? (c) How many hours later in total is the moonrise after 6 days? (d) What phase would you expect around the day of 6:10 p.m. moonrise if it is near sunset?
Hint
Add 50 minutes per day.
Answer
(a) 6:10 p.m. + 150 min = 8:40 p.m. (b) Day 7 gives 6:10 p.m. + 350 min = exactly 12:00 midnight; day 8 gives 12:50 a.m., the first moonrise after midnight. (c) 6 × 50 = 300 min = 5 hours. (d) A Moon rising near sunset is a full Moon (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A student draws the Moon at sunset on successive days and notes a thin crescent near the western horizon on day 1. A friend says the crescent will be larger and further from the Sun on later days.
(a) Is the Moon waxing or waning? (b) Is the friend right about the shape and position? (c) Describe the Sun-Moon separation on the full Moon day. (d) Name the Indian term for this period.
Hint
Thin crescent just after sunset follows new Moon.
Answer
(a) Waxing. (b) Yes: the bright part grows and the Moon moves farther from the Sun in the sky. (c) On full Moon day the Moon is nearly opposite the Sun. (d) Shukla Paksha (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is Purnima?Show answer
The full Moon day.
2.What is the waxing period called in India?Show answer
Shukla Paksha.
3.How much later does the Moon rise each day?Show answer
About 50 minutes.
4.What causes a lunar eclipse?Show answer
The Earth's shadow falling on the Moon.
5.How long is the cycle of Moon phases?Show answer
About 29.5 days.

Phases of the Moon and the Moon's Changing Position: frequently asked questions

Why do we always see the same side of the Moon?

The chapter says only one portion of the Moon always faces the Earth; but the lit part of it changes as the Moon revolves.

Is the Moon visible only at night?

No. It can be seen in the daytime, for example when it rises in the afternoon.

Which Moon is closest to the Sun in the sky?

The new Moon is closest to the Sun, and the full Moon is farthest.

Why are there not eclipses every month?

The Moon's orbit is slightly tilted with respect to the Earth's orbit around the Sun.