Phases of the Moon
On Diwali night the sky has no Moon at all, yet about two weeks later a bright round Moon rises. Neha wonders why the Moon looks different from night to night.
The changing shapes of the bright part of the Moon seen from the Earth are called the phases of the Moon. The whole cycle takes about 29.5 days.
Key idea 1: The cycle of phases — After the new Moon day (Amavasya) the bright part grows for about 15 days (waxing, Shukla Paksha) up to the full Moon day (Purnima). Then it shrinks for about 15 days (waning, Krishna Paksha) back to the new Moon. A shape with less than half of the lit part visible is a crescent; more than half is gibbous.
- Easy: On Purnima the whole bright disc is seen; on Amavasya the Moon cannot be seen.
- Medium: About a week after the new Moon we see a half Moon; one more week gives the full Moon.
- Exam-type: If the Moon is full on 3 March, the next full Moon comes about 29.5 days later, around 1 April.
In the figure: The top row shows the bright part growing from the new Moon; the bottom row starts at the full Moon and shrinks back towards the new Moon.
Key idea 2: When to look for the Moon — A waxing Moon stands in the evening sky after sunset. A waning Moon is in the morning sky before sunrise. The Moon rises about 50 minutes later every day, and it can often be seen in the daytime too.
- Easy: Look for a growing Moon in the sky just after sunset.
- Medium: The Moon on Purnima rises near sunset, while the new Moon rises with the Sun, so we cannot see it.
- Exam-type: If the Moon rises at 7:00 pm today, it will rise at about 7:50 pm tomorrow and about 8:40 pm the day after.
In the figure: Look in the evening for the growing Moon and in the morning for the shrinking Moon.
The phases repeat about every 29.5 days. Purnima is the full Moon; Amavasya is the new Moon.
Thinking the Moon can be seen only at night. It is often visible in the daytime sky too.
Ask whether the Moon is growing or shrinking. Memory clue: WAXing = "Wait till evening"; WANing = "Wake up early".
Spot the difference
| Phase | Seen when | Bright part |
|---|---|---|
| Waxing Moon | evening | growing |
| Waning Moon | morning | shrinking |
| Full Moon | all night | whole disc |
Practice questions
- 1What is the full Moon day called?
- 2What is the new Moon day called?
- 3Name the shape seen when less than half of the lit part is visible.
- 4The Moon was full on 10 May. Roughly when will it be full again?
- 5At what time of day is a waning Moon easy to spot?
- 6The Moon rises at 6:30 pm on Purnima. About when will it rise the next day?
- 7Aarav says he saw a thin crescent Moon high in the sky at midnight. Is this likely? Why?
- 8Name, in order, the shapes seen after the full Moon until the new Moon.
Show answers (try them yourself first)
- 1Purnima.
- 2Amavasya.
- 3A crescent.
- 4Around 8 to 9 June, about 29.5 days later.
- 5In the morning, before sunrise.
- 6At about 7:20 pm, about 50 minutes later.
- 7Not likely. A crescent Moon stays close to the Sun, so it is seen only low near the horizon soon after sunset or before sunrise.
- 8Waning gibbous, half Moon, waning crescent, and then the new Moon.
Why the Moon has phases
In a dark room a torch lights one half of a football, wherever you stand. As you walk around the ball you see a bigger or smaller part of that lit half.
The Moon has no light of its own; it shines by reflecting sunlight. Half of the Moon always faces the Sun and is lit. As the Moon goes around the Earth once a month, we see more or less of the lit half, and these are the phases.
Key idea 1: The ball-and-lamp model — Let a lamp be the Sun, a ball be the Moon and your head be the Earth. Move the ball in a circle around your head and you will see a new, crescent, half, gibbous and full ball in turn.
- Easy: A ball held between you and the lamp shows only its dark side, like the new Moon.
- Medium: A ball held opposite the lamp shows its whole lit face to you, like the full Moon.
- Exam-type: At the two half positions you see exactly half the ball lit with a straight dividing line; at the crescent and gibbous positions the line looks curved.
In the figure: The lit half of the Moon always faces the Sun. From the Earth we see all of it at the full Moon and none of it at the new Moon.
Why the Moon has phases
In the app you can move this figure with your finger.
Open in the appKey idea 2: Phases are not the Earth's shadow — The Earth's shadow falling on the Moon causes a lunar eclipse, only on a full Moon day. The Moon's shadow falling on the Earth causes a solar eclipse, only on a new Moon day. Eclipses do not happen every month because the Moon's orbit is slightly tilted.
- Easy: Phases come from our angle of view; eclipses come from shadows.
- Medium: A lunar eclipse can happen only on Purnima; a solar eclipse only on Amavasya.
- Exam-type: Earth's shadow touches the Moon for only a few hours in an eclipse, but the phases change slowly through the whole month, so the shadow cannot be their cause.
In the figure: Phases happen all month; eclipses need a shadow and happen only on full or new Moon days.
The Moon shines by reflected sunlight, and half of it is always lit.
Believing that the phases are caused by the Earth's shadow.
Picture lamp, ball and head. Memory clue: Phase = Position of the Moon; Eclipse = a shadow blocks the light.
Spot the difference
| Event | Cause | Happens on |
|---|---|---|
| Phases | changing angle of view | every month |
| Lunar eclipse | Earth's shadow on the Moon | Purnima only |
| Solar eclipse | Moon's shadow on the Earth | Amavasya only |
Practice questions
- 1Why does the Moon shine?
- 2In the ball model, what does the lamp stand for?
- 3On which day can a solar eclipse happen?
- 4Why can we not see the Moon on Amavasya?
- 5Why do eclipses not happen every month?
- 6Which part of the ball model stands for the Earth?
- 7Meena says the half Moon she saw yesterday was caused by the Earth's shadow. Correct her in two lines.
- 8If the Moon takes a whole cycle of 29.5 days, about how many days does it take to go from the new Moon to the full Moon?
Show answers (try them yourself first)
- 1It reflects sunlight; it has no light of its own.
- 2The Sun.
- 3On Amavasya, the new Moon day.
- 4Only its dark half faces the Earth; the lit half faces away from us.
- 5The Moon's orbit is slightly tilted, so the Sun, Earth and Moon rarely line up exactly.
- 6Your head.
- 7Phases come from the changing angle of the Sun, Moon and Earth. The Earth's shadow causes only a lunar eclipse on Purnima.
- 8About 15 days, which is half of the 29.5-day cycle.
Day, month, year and calendars
Priya's birthday comes once a year, her timetable changes every day and the library card renews every month. We count time with three natural clocks: the Earth's spin, the Moon's cycle and the Earth's trip around the Sun.
A day is the time the Earth takes to rotate once, about 24 hours. A lunar month is one cycle of the Moon's phases, about 29.5 days. A year is the time the Earth takes to go once around the Sun, about 365¼ days. A calendar arranges days into months and years.
Key idea 1: The solar calendar and the leap year — A solar calendar follows the Earth's journey around the Sun. A normal year has 365 days, but the Earth needs a quarter day more. So every fourth year gets one extra day, 29 February, and has 366 days. A century year is a leap year only if it is divisible by 400.
- Easy: 2028 is a leap year because 4 divides 2028; 2027 is not.
- Medium: Four quarter days make 4 × ¼ = 1 whole day.
- Exam-type: Without leap years, after 40 years the calendar would be about 40 × ¼ = 10 days behind the seasons.
In the figure: Each natural clock measures a different length of time: a day, a month and a year.
Key idea 2: The lunar calendar drifts — A purely lunar calendar has 12 months of about 29.5 days. That makes about 354 days, nearly 11 days shorter than a solar year. So its festivals come about 11 days earlier in the seasons every year.
- Easy: 12 × 29.5 = 354 days in a lunar year.
- Medium: 365 − 354 = 11 days of difference every year.
- Exam-type: After 3 years the shift is 3 × 11 = 33 days, about a whole month.
In the figure: The lunar year is about 11 days shorter than the solar year.
A solar year is about 365¼ days and a lunar year about 354 days.
Thinking that 12 lunar months make exactly one solar year.
A leap year is divisible by 4; a century year needs 400. Memory clue: "Leap = Four": 2024, 2028 yes; 2000 yes, 1900 no.
Spot the difference
| Calendar | Based on | Length of year |
|---|---|---|
| Solar | Earth's orbit | 365 or 366 days |
| Lunar | Moon's phases | about 354 days |
| Leap year | extra day, 29 February | 366 days |
Practice questions
- 1How many days are there in a leap year?
- 2Which month gets the extra day in a leap year?
- 3What does one day measure?
- 4Is 2032 a leap year?
- 5Find the number of days in 12 lunar months of 29.5 days each.
- 6By how many days is a lunar year shorter than a solar year of 365 days?
- 7How many leap years are there from 2041 to 2060, both included?
- 8A purely lunar festival fell on 20 June. About when will it fall two years later?
Show answers (try them yourself first)
- 1366 days.
- 2February, which then has 29 days.
- 3The time the Earth takes to rotate once, about 24 hours.
- 4Yes, because 4 divides 2032.
- 512 × 29.5 = 354 days.
- 6365 − 354 = 11 days.
- 75: the years 2044, 2048, 2052, 2056 and 2060.
- 8About 2 × 11 = 22 days earlier, around 29 May.
Day, month, year and calendars
In the app you can move this figure with your finger.
Open in the appLuni-solar calendars, the Indian calendar and festivals
Diwali moves between October and November from year to year, yet it always stays in the same season. A festival that follows only the Moon would slip about 11 days earlier every year.
A luni-solar calendar uses the Moon for its months and adds an extra month now and then to stay in step with the Sun and the seasons. India's official National Calendar follows the Saka era and was adopted in 1957.
Key idea 1: Luni-solar calendars and festivals — A lunar month of 29 or 30 days has two parts of about 15 days, Shukla Paksha and Krishna Paksha. The yearly gap of 11 days grows to about a month in three years, so an extra month called adhika masa is added. Festivals such as Holi (full Moon) and Diwali (new Moon night) follow lunar days, so their Gregorian dates change but their seasons stay the same.
- Easy: The extra month of the Indian luni-solar calendar is called adhika masa.
- Medium: Each lunar month has two pakshas of about 15 days.
- Exam-type: Holi falls on a full Moon day and Diwali on a new Moon night.
In the figure: Only the luni-solar calendar follows the Moon and still keeps festivals in their season.
Key idea 2: The Indian National Calendar — It has 12 months and starts with Chaitra, around 22 March (21 March in a leap year). A normal year has 365 days: Chaitra 30, the next five months 31 each and the last six 30 each. In a leap year Chaitra gets 31 days. National days such as Republic Day (26 January) follow the solar calendar and have fixed dates.
- Easy: The first month is Chaitra and there are 12 months.
- Medium: 30 + 5 × 31 + 6 × 30 = 30 + 155 + 180 = 365 days.
- Exam-type: In a leap year Chaitra becomes 31 days, so the year has 366 days.
In the figure: The twelve months in order, starting with Chaitra.
A luni-solar calendar means lunar months plus an extra month now and then.
Thinking that a festival such as Diwali has a fixed Gregorian date.
If a date moves each year, it follows the Moon; if it is fixed, it follows the Sun. Memory clue: Moon Moves, Sun Stays.
Spot the difference
| Calendar | Months follow | Extra needed |
|---|---|---|
| Lunar | Moon | none, so it drifts |
| Solar | Earth's orbit | leap day |
| Luni-solar | Moon and Sun | extra month |
Practice questions
- 1What is the extra month of the Indian luni-solar calendar called?
- 2Which is the first month of the Indian National Calendar?
- 3How many months does the National Calendar have?
- 4On what kind of Moon day is Holi celebrated?
- 5Why does the Gregorian date of Diwali change every year?
- 6Around which Gregorian date does Chaitra begin in a normal year?
- 7Why does a luni-solar festival stay in its season while a purely lunar festival does not?
- 8In a leap year, which month of the National Calendar gets the extra day, and how many days does it then have?
Show answers (try them yourself first)
- 1Adhika masa.
- 2Chaitra.
- 3Twelve.
- 4A full Moon day (Purnima).
- 5Diwali follows lunar days, but the Gregorian calendar follows the Sun.
- 6Around 22 March.
- 7A luni-solar calendar adds an extra month about every three years to cover the 11-day yearly gap; a purely lunar calendar does not.
- 8Chaitra, which then has 31 days.
Artificial satellites
The TV dish on Anil's roof always points to the same spot in the sky, and his mother's phone shows cloud pictures with the weather forecast. Both depend on machines circling far above our heads.
A satellite is an object that goes around a larger object. The Moon is a natural satellite of the Earth. Machines built by people and sent up by rockets are artificial satellites. India's first was Aryabhata, launched in 1975, and ISRO builds and launches India's satellites.
Key idea 1: What satellites do — Communication satellites carry TV, phone and internet signals. Weather satellites photograph clouds and cyclones. Navigation satellites (GPS, NavIC) show positions on maps. Remote sensing satellites photograph crops, forests and rivers from far above.
- Easy: Communication satellites help TV and phone signals reach far places.
- Medium: Weather satellites show a cyclone coming, so coastal villages get an early warning.
- Exam-type: Satellite photos of fields let officials estimate how much land is under a crop without visiting each farm.
In the figure: Each kind of satellite does one job: carrying signals, watching weather or finding positions.
Key idea 2: Orbits and seeing satellites — A satellite in a low orbit goes around the Earth in about 90 minutes and crosses our sky in a few minutes. A geostationary satellite goes around once in 24 hours in the direction of the Earth's spin, so it seems fixed in the sky. Satellites stay up because their great sideways speed balances the pull of gravity, and they fly above the air, which would slow them. After sunset a satellite may look like a steady moving dot lit by sunlight.
- Easy: A TV dish points at a geostationary satellite.
- Medium: A low satellite goes around the Earth about 16 times a day, since 24 hours ÷ 1.5 hours = 16.
- Exam-type: A steady light that moves across the sky without blinking is a satellite; stars twinkle and move slowly.
In the figure: The inner orbit is quick and low; the outer orbit takes 24 hours, so that satellite stays over one place.
An artificial satellite is man-made; the Moon is a natural satellite.
Thinking that satellites float because there is no gravity in space. Gravity is still pulling them.
Fixed in the sky means a 24-hour orbit; a quickly moving dot means a low orbit. Memory clue: Geo-stationary = "Goes with the Earth".
Spot the difference
| Satellite | Orbit time | Seen from the ground |
|---|---|---|
| Low orbit | about 90 minutes | moves quickly across the sky |
| Geostationary | 24 hours | fixed in the sky |
| Moon (natural) | about a month | shows phases |
Practice questions
- 1Name India's first artificial satellite.
- 2What carries satellites into space?
- 3Name one use of a weather satellite.
- 4Why does a TV dish point at one fixed place in the sky?
- 5Why can we see a satellite as a bright dot after sunset?
- 6Which is a natural satellite, the Moon or INSAT?
- 7A steady, non-blinking light crosses the sky in about 4 minutes just after sunset. What is it likely to be, and how do you know?
- 8Why do satellites keep going around the Earth instead of falling down?
Show answers (try them yourself first)
- 1Aryabhata, launched in 1975.
- 2Rockets.
- 3It shows clouds and cyclones for forecasts and early warnings.
- 4The TV satellite is geostationary, so it stays above one place all the time.
- 5It reflects sunlight.
- 6The Moon.
- 7A satellite, because stars twinkle and move slowly, while a steady dot that moves fast is a satellite lit by the Sun.
- 8They move sideways at great speed while gravity pulls them in, so they follow a curved path around the Earth.