Glossary of Astronomical Terms
Astronomical Unit (A.U.)
A unit of distance equal to the mean distance between the Earth and the Sun (~1.5 × 10⁸ km).
Asteroid
Small rocky bodies orbiting the Sun, primarily found in the asteroid belt between Mars and Jupiter.
Big Dipper (Ursa Major)
A prominent northern constellation resembling a ladle; contains the “pointer stars” to locate Polaris.
Black Hole
A region in space with gravity so strong that nothing, not even light, can escape.
Celestial Body
Any natural object in space (e.g., stars, planets, moons).
Comet
An icy body that, when near the Sun, develops a glowing coma and tail due to vaporization.
Constellation
A group of stars forming a recognizable pattern, often named after mythological figures.
Crater
A bowl-shaped depression on a planet or moon’s surface, often caused by meteor impacts.
Dwarf Planet
A celestial body (e.g., Pluto) that orbits the Sun but has not cleared its orbital path of debris.
Electromagnetic Induction
Generation of electric current in a conductor by changing the magnetic field around it (discovered by Faraday).
Electrolysis
Chemical decomposition of a substance (e.g., water) by passing electric current through it.
Galaxy
A massive system of stars, gas, and dust bound by gravity (e.g., the Milky Way).
Geostationary Satellite
An artificial satellite orbiting Earth at ~36,000 km, matching Earth’s rotation to remain fixed over one spot.
Greenhouse Effect
Trapping of heat by atmospheric gases (e.g., CO₂), raising a planet’s temperature (notable on Venus).
Halley’s Comet
A periodic comet visible from Earth every ~76 years (last seen in 1986).
Jovian Planets
Gas giants (Jupiter, Saturn, Uranus, Neptune) with no solid surfaces, composed mainly of hydrogen/helium.
Kuiper Belt
A region beyond Neptune containing icy bodies, including dwarf planets like Pluto.
Light Year
The distance light travels in one year (~9.46 trillion km); used to measure stellar distances.
Meteor (Shooting Star)
A streak of light caused by a meteoroid burning up in Earth’s atmosphere.
Meteorite
A meteoroid that survives atmospheric entry and lands on Earth’s surface.
Milky Way
Our spiral galaxy, containing ~200 billion stars, including the Sun.
Moon (Natural Satellite)
A celestial body orbiting a planet (e.g., Earth’s Moon).
Nebula
A cloud of gas and dust in space, often a star-forming region.
Orbit
The elliptical path of a celestial body around another (e.g., Earth around the Sun).
Phases of the Moon
Cyclic changes in the Moon’s appearance due to its position relative to Earth and the Sun.
Pole Star (Polaris/Dhruva Tara)
A star nearly aligned with Earth’s rotational axis, appearing stationary in the northern sky.
Satellite (Artificial)
Human-made objects orbiting Earth (e.g., INSAT for communication, Aryabhata for research).
Solar System
The Sun and all objects gravitationally bound to it (planets, moons, asteroids, comets).
Terrestrial Planets
Rocky planets (Mercury, Venus, Earth, Mars) with solid surfaces.
Twinkling (Scintillation)
Apparent flickering of stars due to atmospheric turbulence (planets do not twinkle).
Something To Know
A. Fill in the blanks.
- The spiral shaped galaxy, in which our solar system is located, is known as the __________ .
Answer – Milky Way.
- The various shapes, of the bright part of the moon, as seen by us during the month, are called __________ of the moon.
Answer – phases
- The temperature on moon ranges from __________ at ‘noon’ to__________ at ‘night’.
Answer – 100°C, -173°C
- The planet, known to have the largest number of natural satellites, as at present, is __________.
Answer – Jupiter
- The comet, which ‘visits’ the earth every 76 years, is the __________.
Answer – Halley’s Come
- Asteroids are found between the orbits of __________ and __________.
Answer – Mars, Jupiter
B. Match the following.
- Star nearest to the Earth (a) Orion
- Brightest star in the night sky (b) Sun
- Star which appear stationary from the earth (c) Ceres
- Constellation (d) Pole Star
- The largest asteroid (e) Geo-stationary Satellite
- Global communication (f) Sirius
Answer –
Star nearest to the Earth → (b) Sun
Brightest star in the night sky → (f) Sirius
Star which appears stationary from the Earth → (d) Pole Star
Constellation → (a) Orion
The largest asteroid → (c) Ceres
Global communication → (e) Geo-stationary Satellite
C. Tick (✓) the correct option.
L A star is twenty light years away from the earth. Suppose it brightens up suddenly today. After how much time shall we see this change?
20 years
5 years
10 years
4.3 years
Answer – 20 years
Explanation: Light takes 1 year to travel 1 light-year. Since the star is 20 light-years away, the light (and brightness change) will take 20 years to reach Earth.
- Three stars A, B and C are observed in the night sky. The colours of these stars are:
Star A : Blue colour Star
B : Red colour Star
C : White colour
The correct sequence, in terms of the increasing order of their surface temperature, is—
Star A > Star B > Star C
Star A = Star B = Star C
Star B > Star C > Star A
Star B < Star C < Star A
Answer – Star B < Star C < Star A
Explanation:
Red stars (B) are the coolest (~3,000 K).
White stars (C) are hotter (~6,000–10,000 K).
Blue stars (A) are the hottest (>10,000 K).
- Stars, like the Sun, appear to move from east to west around the earth. This means that earth rotates from—
west to east
north to south
east to west
south to north
Answer – west to east
Explanation: The apparent motion of stars (east to west) is due to Earth’s actual rotation from west to east.
- Phases of moon are observed because—
the moon does not reflect sunlight at all.
it is only sunlight, reflected by the some parts of the moon that reaches the earth.
the shadow of the earth falls on the moon.
only some parts of moon can emit light.
Answer – it is only sunlight, reflected by some parts of the moon, that reaches the earth.
Explanation: The Moon’s phases change based on how much of its sunlit side is visible from Earth as it orbits.
- The change in seasons on the earth occurs because—
the distance between the sun and earth is not constant.
the axis of rotation of the earth is tilted with respect to its orbit
earth’s atmosphere has an ozone layer.
the axis of rotation of the earth is perpendicular to the plane of its orbit
Answer – the axis of rotation of the earth is tilted with respect to its orbit.
Explanation: Earth’s 23.5° tilt causes varying sunlight distribution, leading to seasons.
- The diagram shows the various positions of a comet in its orbit. In which position will the tail of the comet will appear to be longest?
position A
position C
position B
position D
Answer – position A
Explanation: A comet’s tail lengthens as it approaches the Sun (due to increased solar radiation vaporizing its ice). If A is closest to the Sun, the tail will be longest there.
D. Answer the following questions in brief:
- Name the first two stars closest to us.
Answer – Sun (closest star to Earth)
Proxima Centauri (next closest, 4.24 light-years away)
- Name the brightest star in the sky,
(a) during day time.
Answer – Sun
(b) during night.
Answer – Sirius (also called the “Dog Star”)
- Identify the planet with the help of the given clue.
(a) largest planet
Answer – Jupiter
(b) closest to the sun
Answer – Mercury
(c) this planet has two moons named, Phobos and Deimos
Answer – Mars
(d) red planet
Answer – Mars
(e) has a unique and special system of rings
Answer – Saturn
(f) has the largest number of moons
Answer – Jupiter (95 confirmed moons as of 2024)
(g) often known as morning or evening star
Answer – Venus
(h) named after the Roman Sea god
Answer – Neptune
4- Define the following.
(a) Milky way
Answer – Our spiral-shaped galaxy, containing ~200 billion stars, including the Sun.
(b) Celestial objects
Answer – Natural objects in space (e.g., stars, planets, moons, comets).
(c) Pole star
Answer – A star (Polaris) nearly aligned with Earth’s axis, appearing stationary in the northern sky.
(d) Constellation
Answer – A group of stars forming a recognizable pattern (e.g., Orion, Ursa Major).
(e) Asteroids
Answer – Small rocky bodies orbiting the Sun, mostly found in the asteroid belt (between Mars and Jupiter).
(f) Comets
Answer – Icy bodies that develop glowing tails when near the Sun (e.g., Halley’s Comet).
(g) Geo-stationary Satellites
Answer – Artificial satellites orbiting Earth at ~36,000 km, matching Earth’s rotation (used for communication).
(h) Light year
Answer – Distance light travels in one year (~9.46 trillion km).
(i) Meteors
Answer – “Shooting stars”—small rocks burning up in Earth’s atmosphere.
(j) Meteorites
Answer – Meteors that survive and hit Earth’s surface.
- Name the visible layer of the sun. Give an estimate of its temperature?
Answer – Visible layer: Photosphere
Temperature: ~5,500°C (or ~5,800 Kelvin)
E. Answer the following questions.
- How is Pole Star located in the sky? Explain with a diagram.
Answer – The Pole Star (Polaris) is located by using the Ursa Major (Great Bear/Saptarishi) constellation.
The two stars at the end of Ursa Major’s “bowl” (called Dubhe and Merak) point directly to Polaris.
Method:
Draw an imaginary line through Dubhe and Merak.
Extend this line about 5 times the distance between them.
The bright star at the end is Polaris, which stays fixed in the northern sky.
Diagram:
Polaris (Pole Star)
/
/
(Dubhe & Merak in Ursa Major)
(Note: A labeled diagram would show Ursa Major’s “pointer stars” aligning with Polaris.)
- Differentiate between the following, giving examples:
(a) stars and constellations
Answer –
(a) Stars vs. Constellations
Stars | Constellations |
Single luminous celestial bodies (e.g., Sun, Sirius). | Groups of stars forming patterns (e.g., Orion, Ursa Major). |
Emit their own light. | Do not emit light; just visual arrangements. |
Example: Sirius (brightest star). | Example: Orion (has Betelgeuse & Rigel). |
(b) stars and planets
Answer –
Stars | Planets |
Emit their own light (nuclear fusion). | Reflect sunlight (no light of their own). |
Twinkle due to atmospheric refraction. | Do not twinkle (appear steady). |
Example: Sun. | Example: Earth, Jupiter. |
(c) terrestrial planets and jovian planets
Answer –
Terrestrial | Jovian |
Rocky, solid surfaces (e.g., Earth, Mars). | Gas giants, no solid surface (e.g., Jupiter, Saturn). |
Smaller in size. | Very large (e.g., Jupiter is 1,300x Earth’s volume). |
Few/no moons. | Many moons (e.g., Jupiter has 95+). |
(d) artificial and natural satellite
Answer –
Artificial | Natural |
Man-made (e.g., INSAT, Hubble). | Naturally occurring (e.g., Moon). |
Used for communication, weather, etc. | Orbits planets due to gravity. |
- Identify, and name the constellations X, Y and Z and answer the following questions.
(a) Name the constellation that can be used to locate star Sirius?
Answer – Orion (follow the line through Orion’s belt southeast to find Sirius).A = Betelgeuse (red supergiant, top-left).
B = Rigel (blue-white star, bottom-right).
(b) Name the constellation that can be used to locate the Pole Star.
Answer – Ursa Major (use the “pointer stars” Dubhe and Merak to trace a line to Polaris).
(c) Name the constellations visible during the—
Answer – Ursa Major (use the “pointer stars” Dubhe and Merak to trace a line to Polaris).
(i) spring season (ii) winter season
Answer – (i) Spring: Ursa Major, Leo
(ii) Winter: Orion, Taurus
(d) Name the stars A and B in the constellation marked as ‘X’
Answer – A = Betelgeuse (red supergiant, top-left shoulder).
B = Rigel (blue-white star, bottom-right foot).
- Give reasons for the following.
(a) Uranus is observed to have a ‘blue-green’ colour.
Answer – Due to methane gas in its atmosphere, which absorbs red light and reflects blue-green.
(b) Venus is the hottest planet even though Mercury is closest to the sun.
Answer – Venus has a thick CO₂ atmosphere (96%), causing a runaway greenhouse effect, trapping heat. Mercury has no atmosphere to retain heat.
(c) The shape and size of bright side of moon appears to change (gradually) everyday.
Answer – Due to phases of the Moon—different portions of its sunlit side become visible as it orbits Earth.
(d) A star appears to rise four minutes earlier in the sky everyday than the previous day.
Answer – Earth orbits the Sun, so its position changes slightly each day, making stars appear to shift.
(e) Comets become visible only when they are near the sun.
Answer – Solar heat vaporizes their ice, creating glowing tails (invisible when far in outer space).
- State, in brief, how artificial satellites are used in transmitting television programmes over large distances.
Answer – Signal Uplink: TV signals are sent from a ground station to a geostationary satellite (~36,000 km above Earth).
Amplification & Relay: The satellite receives, amplifies, and retransmits the signal at a different frequency.
Global Coverage: Multiple satellites ensure worldwide broadcast (e.g., live sports, news).
Downlink: Receivers (dishes) on Earth capture the signal for TV networks.
Example: INSAT (India) enables nationwide TV and weather services.
Value Based Ques
Sumit was an extremely lively boy, always respectful towards his elders and teachers. He was also academically brilliant and always scored well. However, when he was in Class IX, his behaviour would often reflect some undesirable qualities. The school counsellor decided to talk to him. She told him that just as the moon reflects the light of the sun to illuminate itself, we too, can also idolise inspirational sources and try to imbibe and radiate their positive energy. This way we cannot only brighten up our lives but also of the ones around us.
Sumit understood her point and promised to tread on the path to improvement.
- State the values displayed by the school counsellor.
Answer – The school counsellor displayed the values of guidance, wisdom, and positivity. She encouraged Sumit to idolize inspirational sources and to imbibe and radiate positive energy, which shows her belief in the power of role models and personal improvement.
- Name any two celestial objects, other than moon, which do not have their own light.
Answer – Planets (e.g., Earth, Mars)
Asteroids
- Have a group discussion on the importance of ‘having the right company’ specially during the ‘teen years’.
Answer – During the teen years, it is crucial to have the right company as it plays a significant role in shaping one’s personality, behaviour, and choices. Having good friends who share positive values and encourage academic, emotional, and social growth can help teens make wise decisions, avoid negative influences, and enhance their overall well-being. The right company can inspire confidence, provide support during challenges, and encourage personal development, which is important for mental and emotional growth.
Something To Do
1- Visit any of the ancient astronomical observation built by Maharaja Jai Singh (of Jaipur) at Delhi (Jantar Mantar), Jaipur, Mathura, Ujjain or Varanasi. Try to learn about the instruments and techniques used by astronomers of those times.
Answer – The Jantar Mantar observatories built by Maharaja Jai Singh II were designed with large instruments to observe celestial bodies. These instruments include the Samrat Yantra (a large sundial), Jaiprakash Yantra (to measure the altitude of stars), Ram Yantra (used to measure the position of stars), and Chandrashekhara Yantra (for astronomical observations). The observatories use techniques based on heliocentric models and geometry. These instruments helped astronomers of the time to make accurate astronomical observations without modern technology.
- Obtain information about the different Indian satellites and their significant uses.
Answer – India has launched various satellites under its Indian Space Research Organisation (ISRO), including:
Aryabhata: India’s first satellite, launched in 1975, for studying cosmic rays, solar x-rays, and the Earth’s ionosphere.
INSAT Series: A series of multipurpose geostationary satellites for telecommunications, broadcasting, and meteorology.
IRS (Indian Remote Sensing) Satellites: Used for applications in agriculture, forestry, and disaster management.
GSAT Series: These satellites provide services in communication, broadcasting, and weather forecasting.
Mangalyaan (Mars Orbiter Mission): India’s first interplanetary mission, which successfully entered Mars’ orbit in 2014, providing valuable data about Mars.
NavIC: A regional navigation satellite system providing accurate position information to users in India and surrounding areas.
These satellites contribute to national development in areas like telecommunications, broadcasting, weather prediction, and space exploration.
- If possible, visit a planetarium. There are planetariums in many cities. In a planetarium, one can see the motion of stars, planets and constellations on a large dome.
Answer – A visit to a planetarium can provide a visual and immersive experience where one can learn about celestial bodies, constellations, planetary motions, and astronomical phenomena. Many planetariums offer shows and programs that explain the workings of the universe in an easily understandable way. This could help foster a deeper interest in astronomy and allow for the visual observation of stars, planets, and galaxies.
- Observe the sky on a clear cloudness night and try to locate the pole star and some other star patterns you know of. Also, learn to identify planets visible to the unaided eye.
Answer – On a clear, cloudless night, one can locate the Pole Star (North Star) in the Ursa Minor constellation. It’s the brightest star in the constellation and marks the direction of True North. Some common star patterns visible to the naked eye include:
Orion: A prominent constellation that can be seen in the winter.
Big Dipper (part of Ursa Major): A distinctive group of stars that form part of a larger constellation.
Venus, Jupiter, Mars: These planets are often visible to the naked eye. Venus is visible just before sunrise or after sunset, Jupiter and Mars can be seen in the night sky.
- Select any one planet from the eigth listed planets. Collect as much information about this planet as you can. (Use internet resources/library/encyclopedia, etc.). Prepare a report. Try to give details about the following in your report:
(a) discovery of this planet.
Answer – Mars has been known since ancient times, and its reddish appearance made it one of the most observed celestial objects. It was first recorded by Babylonian astronomers as early as 4000 BCE.
(b) its special features, if any.
Answer – Mars is known for its red color, which is due to iron oxide (rust) on its surface. It has the tallest volcano in the solar system, Olympus Mons, and the deepest canyon, Valles Marineris.
(c) would human beings be able to survive on this planet?
Answer – Mars has a very thin atmosphere, composed mostly of carbon dioxide, and its surface temperature can range from -125°C to 20°C. It is unlikely that humans could survive there without advanced life-support systems. However, Mars is considered one of the most promising planets for future human colonization due to the presence of water ice and its proximity to Earth.
Additional Questions and Answers
Multiple Choice Questions (MCQs):
Question – Which is the hottest planet in our solar system?
- a) Mercury
- b) Venus ✅
- c) Mars
- d) Jupiter
Question – The Great Red Spot is a massive storm on:
- a) Saturn
- b) Jupiter ✅
- c) Neptune
- d) Uranus
Question – Light from the Sun takes approximately ____ to reach Earth.
- a) 8 seconds
- b) 8 hours
- c) 8 minutes ✅
- d) 8 days
Question – Which of these is NOT a Jovian planet?
- a) Saturn
- b) Uranus
- c) Mars ✅
- d) Neptune
Question – The asteroid belt lies between:
- a) Earth and Mars
- b) Mars and Jupiter ✅
- c) Jupiter and Saturn
- d) Venus and Earth
True/False Questions:
Question – The Pole Star is part of the Orion constellation.
False ✅ (It’s near Ursa Minor).
Question – All stars in a constellation are at the same distance from Earth.
False ✅ (They only appear aligned from Earth).
Question – Pluto is classified as a dwarf planet.
True ✅
Question – The Moon’s gravity causes tides on Earth.
True ✅
Question – Halley’s Comet is visible every 76 years.
True ✅
Short Answer Questions:
Question – Why does the Moon have craters?
Due to impacts from asteroids and meteoroids over billions of years.
Question – Name the galaxy closest to the Milky Way.
Andromeda Galaxy.
Question – What causes a solar eclipse?
When the Moon passes between the Sun and Earth, blocking sunlight.
Question – Why is Venus called Earth’s “twin”?
Similar size and mass, but vastly different conditions.
Question – What is a light-year?
Distance light travels in one year (~9.46 trillion km).
Long Answer Questions:
Question – Explain why we always see the same side of the Moon.
The Moon is tidally locked to Earth, meaning its rotation period (27.3 days) matches its orbital period, keeping one face permanently turned toward Earth.
Question – Describe the greenhouse effect on Venus.
Venus’s thick CO₂ atmosphere traps solar heat, raising surface temperatures to 467°C—hotter than Mercury despite being farther from the Sun.
Question – How do geostationary satellites aid communication?
Orbit Earth at 36,000 km, matching Earth’s rotation to stay fixed over one spot, enabling uninterrupted TV, weather, and GPS signals.
Question – Compare terrestrial and Jovian planets.
Terrestrial (Mercury, Venus, Earth, Mars): Rocky, small, few moons.
Jovian (Jupiter, Saturn, Uranus, Neptune): Gaseous, large, ring systems, many moons.
Question – What evidence suggests water existed on Mars?
Dry riverbeds, polar ice caps, and mineral traces (e.g., hematite) detected by rovers like Curiosity.
Diagram-Based Question:
Question – Label the phases of the Moon in order (New Moon to Full Moon).
(Answer: New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon.)
Fill in the Blanks:
The Sun is a __________ (star/planet).
star ✅
The largest moon of Saturn is __________.
Titan ✅
__________ is the only planet rotating clockwise.
Venus ✅