Motions of the Moon
We always see the same face of the moon and never the far side. Behind this is a neat match of the moon’s two motions.
The moon rotates on its axis and also revolves around the earth. Both take the same time, so we always see the same side of the moon.
Key idea 1 : Two motions of the moon — Like the earth the moon has axial and orbital motions. The moon rotates around itself, revolves around the earth and the earth revolves around the sun. So though the moon does not independently go around the sun, indirectly it does. One revolution around the earth and one rotation take the same time, which is why we see the same side.
Key idea 2 : Moon’s orbit and distance — The moon’s orbit is elliptical like the earth’s. So the distance between moon and earth is not the same everywhere. When the moon is nearest to the earth it is in perigee, about 3,56,000 km; when farthest it is in apogee, about 4,07,000 km.
Key idea 3 : Playground activity — Select three students as sun, earth and moon. Make the sun stand in the middle and draw an elliptical orbit around. The student as earth rotates west to east around himself and simultaneously revolves anti-clockwise around the sun. The student as moon also revolves around the earth. Draw a diagram of the whole activity in the notebook.
- Easy : How many sides of the moon do we see? — Only one.
- Medium : Perigee means? — Nearest to earth.
- Exam-type : Apogee means? — Farthest from earth.
In the figure: Rotation and revolution time are equal.
Perigee = near; apogee = away.
It is wrong that the moon does not rotate; it does, but equal time shows the same side.
Memory clue: Two motions = one time = one side.
Spot the difference
| Position | Distance | Meaning |
|---|---|---|
| Perigee | About 3,56,000 km | Nearest |
| Apogee | About 4,07,000 km | Farthest |
Practice questions
- 1What are the moon’s two motions?
- 2Why is only one side of the moon seen?
- 3What is the shape of the moon’s orbit?
- 4What are perigee and apogee?
- 5How does the moon go around the sun indirectly?
- 6What does each do in the sun-earth-moon activity?
- 7What would be seen if the moon did not rotate?
- 8How big is the difference in the moon’s distance?
Show answers (try them yourself first)
- 1Rotation and revolution.
- 2Both take equal time.
- 3Elliptical.
- 4Perigee = nearest; apogee = farthest.
- 5Because it goes with the earth around the sun.
- 6Sun stands, earth rotates and circles sun, moon circles earth.
- 7We would have seen all sides of the moon.
- 8About 51,000 km (4,07,000 − 3,56,000).
Phases of the Moon
The moon does not look the same daily; sometimes round, sometimes half, sometimes a thin sliver. These changes are due to sunlight.
Phases are the different shapes of the moon’s lit part as seen by us. From new moon to full moon it waxes and from full moon to new moon it wanes.
Key idea 1 : The lit part — Phases are the illuminated portions of the moon. They are visible due to sunlight reflected from the moon. The moon is not self-luminous.
Key idea 2 : New moon, full moon and quarters — On a full moon day the moon is on the side opposite to the sun. On a new moon day the moon is between sun and earth. On the first and third quarter days the moon, earth and sun make an angle of 90°; then we see half the lit moon, which appears semicircular.
Key idea 3 : Angles — On a new moon day the angle between the earth-sun and earth-moon lines is 0° and on a full moon 180°. It is 90° on the first quarter and 270° on the third quarter. In a month there are four main stages: new moon, two quarters and full moon.
- Easy : Angle on full moon? — 180°.
- Medium : Angle on new moon? — 0°.
- Exam-type : How much looks lit on a quarter? — Half.
In the figure: Four main phases.
0° new, 90° quarter, 180° full, 270° quarter.
Phases are not caused by the earth’s shadow; they are how the lit part looks.
Memory clue: New = between; full = opposite.
Spot the difference
| Day | Moon position | Appearance |
|---|---|---|
| New moon | Between sun and earth | Not visible |
| Quarter | 90° angle | Half lit |
| Full moon | Opposite the sun | Full disc |
Practice questions
- 1What are the phases of the moon?
- 2Why do phases appear?
- 3Where is the moon on a full moon day?
- 4Where is the moon on a new moon day?
- 5Why does the moon look semicircular on a quarter?
- 6Write the angles of all four stages.
- 7How many times in a month is the 90° angle formed?
- 8Is the moon self-luminous? Explain.
Show answers (try them yourself first)
- 1The shapes of the lit parts we see.
- 2Because sunlight is reflected from the moon.
- 3Opposite the sun.
- 4Between the sun and the earth.
- 5Because only half the lit part faces us.
- 60°, 90°, 180°, 270°.
- 7Twice.
- 8No; it reflects sunlight.
Solar and Lunar Eclipses
If you hold a ball before a lamp a shadow falls on the wall. When the moon’s or earth’s shadow falls we get an eclipse.
In a solar eclipse the moon comes between the sun and the earth; in a lunar eclipse the earth comes between the sun and the moon. In an eclipse all three are in one line and one plane.
Key idea 1 : Why no eclipse every new moon? — The earth’s orbit and the moon’s orbit are not in the same plane; the moon’s orbit makes an angle of about 5° with the earth’s. So in one revolution the moon crosses the plane of the earth’s orbit twice. Hence on every new moon or full moon the three bodies do not fall in one line and plane and there is no eclipse. Sometimes on a new moon or full moon they do fall in a line; then an eclipse occurs.
Key idea 2 : Solar eclipse — When the moon comes between the sun and the earth its shadow falls on the earth. The central part of the shadow is dark and the periphery light. In the dark shadow area the sun becomes completely invisible: total solar eclipse. Where the shadow is lighter the sun disc appears partly covered: partial solar eclipse. A total eclipse is seen in a very limited region.
Key idea 3 : Annular and lunar eclipse — When the moon is in apogee its deep shadow does not reach the earth. From a very small region only the bright edge of the sun is seen like a ring: an annular eclipse, which is rare. A lunar eclipse occurs when the moon enters the earth’s shadow; then the earth is between the sun and the moon and all three are in one plane. If the moon is fully in the shadow it is a total lunar eclipse; if partly, a partial one.
- Easy : Solar eclipse on which day? — New moon.
- Medium : Lunar eclipse on which day? — Full moon.
- Exam-type : Annular eclipse when? — Moon in apogee.
In the figure: Types of eclipses.
Solar = new moon; lunar = full moon.
A solar eclipse does not occur on every new moon; due to the 5° tilt of orbits the three rarely align.
Memory clue: Sun-new moon, moon-full moon.
Spot the difference
| Point | Solar eclipse | Lunar eclipse |
|---|---|---|
| Day | New moon | Full moon |
| Comes between | Moon | Earth |
| Shadow falls on | Earth | Moon |
Practice questions
- 1When does a solar eclipse occur?
- 2When does a lunar eclipse occur?
- 3Why is there no eclipse every new moon?
- 4State the difference between total and partial solar eclipse.
- 5How does an annular eclipse occur?
- 6State the types of lunar eclipse.
- 7How will you make a model of a solar eclipse with balls and a torch?
- 8Why is an annular lunar eclipse not seen?
Show answers (try them yourself first)
- 1On a new moon when sun, moon, earth align in one plane.
- 2On a full moon when sun, earth, moon align.
- 3The moon’s orbit is tilted by 5°.
- 4Total hides the sun fully; partial only partly.
- 5With the moon at apogee the sun’s edge looks like a ring.
- 6Total and partial.
- 7A small ball moon on clay, a bigger ball earth, torch sun.
- 8Because the earth is much bigger than the moon its shadow covers the moon.
Eclipse as an Astronomical Event
An eclipse is not a regular event so people are curious. But it is only an event due to positions of three bodies in the sky.
An eclipse is an astronomical event; nothing is auspicious or inauspicious about it. Scientists study eclipses and one must protect the eyes while observing a solar eclipse.
Key idea 1 : Characteristics — A solar eclipse occurs only on a new moon day but not on every new moon, and only when sun, moon and earth are in one plane and line. The maximum duration of a total solar eclipse is 7 minutes 20 seconds (440 seconds). A lunar eclipse occurs only on a full moon day but not on every full moon. The maximum duration of a total lunar eclipse is 107 minutes.
Key idea 2 : No superstition — A solar or lunar eclipse is only an astronomical event; nothing ‘auspicious’ or ‘inauspicious’ about it. It happens as sun, earth and moon come into particular positions. As it is not regular people are curious. Astronomers from all over the world visit places where total or annular eclipses occur and study them. Birds and animals get confused by untimely darkness; note their behaviour.
Key idea 3 : Safety, occultation and transit — While observing a solar eclipse view the sun disc through dark glasses or special goggles; otherwise the intense light can harm the naked eye. Occultation: while revolving the moon hides a star or planet and that body appears to hide behind it; a total solar eclipse is a kind of occultation. Transit: when an inner planet like Mercury or Venus comes in line between earth and sun a small dot moves across the sun disc; transit is a type of solar eclipse.
- Easy : Maximum total solar eclipse time? — 7 min 20 s.
- Medium : Maximum total lunar eclipse time? — 107 min.
- Exam-type : A transit of Mercury looks like? — A small dot on the sun.
In the figure: Eclipse is an astronomical event.
Never look at a solar eclipse with bare eyes; use dark glass or special goggles made for the purpose. (as per the textbook)
Eclipse = event, not superstition; goggles = safety.
An eclipse is not inauspicious; it is due to positions of three bodies.
Memory clue: Astronomical event = science; no superstition.
Spot the difference
| Term | Meaning | Example |
|---|---|---|
| Occultation | Moon hides a star/planet | Total solar eclipse |
| Transit | Inner planet crosses sun | Mercury |
Practice questions
- 1Why is an eclipse an astronomical event?
- 2What is the maximum duration of a total solar eclipse?
- 3What is the maximum duration of a total lunar eclipse?
- 4What care should be taken while observing a solar eclipse?
- 5Are ‘auspicious-inauspicious’ beliefs correct? Why?
- 6What is occultation?
- 7What is transit?
- 8How do birds and animals behave during an eclipse?
Show answers (try them yourself first)
- 1Because it is due to positions of sun, earth and moon.
- 27 minutes 20 seconds.
- 3107 minutes.
- 4View through dark glass or special goggles.
- 5No; it is superstition, not science.
- 6Moon hiding a star or planet.
- 7An inner planet passing before the sun.
- 8They get confused due to sudden darkness.