Solar System and the Earth: Planets, Moon and Motions
Notes on the solar system for exams: the Sun, the eight planets and their records, the Moon, the rotation and revolution of the Earth, seasons and eclipses.
By GK24 Editorial Team· Published · 4 min read
The solar system is the family of the Sun: the eight planets that go round it, their satellites, the dwarf planets, and the asteroids, comets and meteoroids that fill the space between them. It was formed about 4.6 billion years ago from a cloud of gas and dust, an idea first put forward as the nebular hypothesis by Immanuel Kant and later reworked by Laplace. Questions from this chapter are almost always about records and order, so learn the sequence of the planets first and the extremes after it.
The Sun, the centre of the system
The Sun is a medium-sized star made mostly of hydrogen and helium. It produces its energy by nuclear fusion, in which hydrogen nuclei join to form helium and release enormous heat and light. Its outer visible surface, the photosphere, is around 5,500 degrees Celsius, while the core is measured in millions of degrees. The Sun holds the overwhelming share of the mass of the solar system, which is why its gravity keeps every planet in orbit. It lies about 149.6 million kilometres from the Earth, a distance called one astronomical unit, and its light takes about 8 minutes and 20 seconds to reach us. The nearest star to the Sun is Proxima Centauri, about 4.2 light years away. A light year is a unit of distance, not of time: it is the distance light travels in one year.
The eight planets in order
In order of distance from the Sun the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The first four are the inner or terrestrial planets, small, rocky and dense. Beyond the asteroid belt, which lies between Mars and Jupiter, come the outer planets: Jupiter and Saturn are gas giants and Uranus and Neptune are usually called ice giants. Pluto was counted as the ninth planet until the International Astronomical Union redefined a planet in 2006 and placed it among the dwarf planets.
| Planet | What it is known for |
|---|---|
| Mercury | Smallest planet, nearest the Sun, fastest revolution of about 88 days, no satellite |
| Venus | Hottest planet, twin of the Earth in size, rotates east to west, the morning and evening star |
| Earth | The only planet known to support life; the blue planet, because water covers most of its surface |
| Mars | The red planet, from iron oxide in its soil; Olympus Mons, the tallest volcano; moons Phobos and Deimos |
| Jupiter | Largest planet, fastest rotation of about ten hours, the Great Red Spot, the moon Ganymede |
| Saturn | Bright rings and the lowest density of all the planets; its largest moon is Titan |
| Uranus | Rotates on its side with an axis tilted about 98 degrees; discovered by William Herschel in 1781 |
| Neptune | Farthest from the Sun, the windiest planet, a revolution of about 165 Earth years |
The Earth and its two motions
The Earth is the third planet from the Sun and the fifth largest. It is not a perfect sphere: it bulges at the equator and is flattened at the poles, a shape called an oblate spheroid or geoid, so the equatorial radius is about 21 kilometres greater than the polar radius. The Earth has two motions. Rotation is its spin on its own axis from west to east, completed once in about 24 hours, and it causes day and night. Revolution is its journey around the Sun, completed in about 365 days and six hours; the extra six hours are saved up and added as a leap day in February every fourth year.
Why the Earth has seasons
Seasons are not caused by the changing distance between the Earth and the Sun, a favourite trap in examinations. They are caused by the tilt of the Earth's axis, which is about 23.5 degrees from the vertical, and by the fact that the tilt keeps pointing the same way as the Earth revolves. So each hemisphere leans towards the Sun for part of the year and away from it for the rest.
- 21 June: the Sun is vertically overhead at the Tropic of Cancer. This is the summer solstice, the longest day in the northern hemisphere.
- 22 December: the Sun is vertically overhead at the Tropic of Capricorn. This is the winter solstice, the shortest day in the northern hemisphere.
- 21 March and 23 September: the Sun is vertically overhead at the equator and day and night are equal everywhere. These are the spring and autumn equinoxes.
The Earth is in fact nearest the Sun in early January, a position called perihelion, and farthest in early July, called aphelion, which shows again that distance does not decide the seasons.
The Moon and eclipses
The Moon is the Earth's only natural satellite. It is about 384,400 kilometres away, so its light reaches us in about 1.3 seconds. It takes about 27.3 days to complete one rotation and one revolution, and because these two periods are equal, the same side of the Moon always faces the Earth. The Moon has no atmosphere and its gravity is about one sixth of the Earth's, which is why astronauts bounce as they walk. A solar eclipse happens only on a new moon day, when the Moon comes between the Sun and the Earth; a lunar eclipse happens only on a full moon day, when the Earth comes between the Sun and the Moon.
Exam Point of View
Most questions are one-line records: smallest, largest, hottest, brightest, least dense, fastest rotation, farthest planet. The second group is about the Earth, and here the traps are sharp. Seasons are caused by the tilt of the axis, not by the distance from the Sun; rotation gives day and night while revolution gives the year; a light year measures distance and not time; and the Earth is closest to the Sun in January, in the northern winter. Also learn which planets have no satellite (Mercury and Venus), which two rotate east to west (Venus and Uranus), and that a solar eclipse falls on a new moon and a lunar eclipse on a full moon.
Important Facts
| Age of the solar system | About 4.6 billion years |
|---|---|
| Source of the Sun's energy | Nuclear fusion of hydrogen into helium |
| Distance of the Sun from the Earth | About 149.6 million km, called one astronomical unit |
| Time taken by sunlight to reach the Earth | About 8 minutes 20 seconds |
| Nearest star after the Sun | Proxima Centauri, about 4.2 light years away |
| Smallest and largest planets | Mercury and Jupiter |
| Hottest planet | Venus, because of its thick carbon dioxide atmosphere |
| Planet with the lowest density | Saturn |
| Planets rotating east to west | Venus and Uranus |
| Asteroid belt | Between the orbits of Mars and Jupiter |
| Pluto reclassified | 2006, as a dwarf planet, by the International Astronomical Union |
| Tilt of the Earth's axis | About 23.5 degrees from the vertical |
| Solstices | 21 June at the Tropic of Cancer; 22 December at the Tropic of Capricorn |
| Equinoxes | 21 March and 23 September, when the Sun is overhead at the equator |
| Distance of the Moon | About 384,400 km; moonlight reaches the Earth in about 1.3 seconds |
Practice MCQs on this topic
Which is the hottest planet of the solar system?
- A.Mercury
- B.Venus
- C.Mars
- D.Jupiter
Show answer
Venus is wrapped in a very thick atmosphere of carbon dioxide, which lets sunlight in but does not let the heat escape.
Read the full explanation →Approximately how long does sunlight take to reach the Earth?
- A.8 minutes 20 seconds
- B.1.3 seconds
- C.4.2 years
- D.24 hours
Show answer
The Sun is about 149.6 million kilometres from the Earth, a distance called one astronomical unit, and light travels about three lakh kilometres in a second.
Read the full explanation →Which is the largest planet of the solar system?
- A.Saturn
- B.Neptune
- C.Jupiter
- D.Uranus
Show answer
Jupiter is the fifth planet from the Sun and the largest of all by both diameter and mass; it is a gas giant of hydrogen and helium.
Read the full explanation →Apart from Uranus, which planet rotates on its axis from east to west, that is in a direction opposite to most planets?
- A.Mercury
- B.Venus
- C.Mars
- D.Saturn
Show answer
Almost every planet, the Earth included, rotates from west to east, which is why the Sun appears to rise in the east.
Read the full explanation →The occurrence of seasons on the Earth is caused mainly by which of the following?
- A.The changing distance between the Earth and the Sun
- B.The rotation of the Earth on its axis
- C.The tilt of the Earth's axis along with its revolution around the Sun
- D.The gravitational pull of the Moon
Show answer
The axis of the Earth is tilted about 23.5 degrees from the vertical and keeps pointing in the same direction throughout the year.
Read the full explanation →Which planet has the lowest density, so low that it would float if placed in a large enough body of water?
- A.Jupiter
- B.Saturn
- C.Neptune
- D.Mercury
Show answer
Saturn is a huge ball of hydrogen and helium spread over an enormous volume, and its average density is less than that of water.
Read the full explanation →The asteroid belt of the solar system lies between the orbits of which two planets?
- A.Earth and Mars
- B.Mars and Jupiter
- C.Jupiter and Saturn
- D.Saturn and Uranus
Show answer
The asteroid belt is a broad band of rocky bodies, from dust-sized grains to bodies hundreds of kilometres across, going round the Sun between the orbit of Mars and that of Jupiter.
Read the full explanation →Light from the Moon takes about how much time to reach the Earth?
- A.1.3 seconds
- B.8 minutes 20 seconds
- C.27.3 days
- D.1 hour
Show answer
The Moon is on average about 384,400 kilometres from the Earth. Light covers about three lakh kilometres in a second, so it crosses that distance in a little over a second.
Read the full explanation →In which year did the International Astronomical Union reclassify Pluto as a dwarf planet?
- A.1996
- B.2003
- C.2006
- D.2011
Show answer
In that year the International Astronomical Union adopted a formal definition of a planet: a body that orbits the Sun, is massive enough for its own gravity to pull it into a nearly round shape, and has cleared the neighbourhood around its orbit.
Read the full explanation →On 21 June the rays of the Sun fall vertically over which line of latitude?
- A.The equator
- B.The Tropic of Cancer
- C.The Tropic of Capricorn
- D.The Arctic Circle
Show answer
On 21 June the northern hemisphere is tilted towards the Sun, whose rays fall vertically on the Tropic of Cancer at 23.5 degrees north.
Read the full explanation →Ganymede, the largest satellite in the solar system, is a moon of which planet?
- A.Saturn
- B.Mars
- C.Jupiter
- D.Neptune
Show answer
Ganymede is one of the four large moons of Jupiter first seen by Galileo, the others being Io, Europa and Callisto, and it is larger than the planet Mercury, which makes it the biggest satellite in the solar system.
Read the full explanation →Frequently Asked Questions
Why is Venus hotter than Mercury although Mercury is nearer the Sun?
Venus has a very thick atmosphere of carbon dioxide that traps the heat it receives, a runaway greenhouse effect. Mercury has almost no atmosphere, so the heat of the day escapes into space at night and its surface swings between extremes. The trapped heat makes Venus the hottest planet in the solar system.
What causes the seasons on the Earth?
The tilt of the Earth's axis, about 23.5 degrees from the vertical, combined with the revolution of the Earth around the Sun. Because the axis always points in the same direction, each hemisphere leans towards the Sun for part of the year, receiving more direct rays and longer days, and away from it for the other part. The changing distance from the Sun plays almost no role.
Why do we always see the same side of the Moon?
Because the Moon takes the same time, about 27.3 days, to rotate once on its axis as it takes to revolve once around the Earth. This is called synchronous rotation. As a result the same face is always turned towards us and the other face, often loosely called the dark side, is never visible from the Earth.
Is a light year a unit of time or of distance?
It is a unit of distance. A light year is the distance that light, travelling at about three lakh kilometres per second, covers in one year. It is used for measuring distances between stars, which are far too large for kilometres. Within the solar system the astronomical unit, the mean distance between the Earth and the Sun, is used instead.
Why is Pluto no longer called a planet?
In 2006 the International Astronomical Union laid down three conditions for a planet: it must orbit the Sun, be massive enough to take a nearly round shape, and have cleared the neighbourhood around its orbit. Pluto meets the first two but shares its zone with other bodies of the Kuiper belt, so it was placed in the new class of dwarf planets.
Sources
- The Earth: Our Habitat (Class VI), chapters on the solar system and the motions of the Earth — NCERT
- Fundamentals of Physical Geography (Class XI) — NCERT
