Solar System and Motions of the Earth Notes for UPSC

Our Solar System is the Sun and all the objects that move around it: eight planets, their moons, dwarf planets, asteroids, comets and dust. The Earth is a part of this system, and its motions, rotation and revolution, give us day and night, seasons and time zones. This is a very important chapter of World Geography. In UPSC, questions come on the planets and their special features, the shape and tilt of the Earth, solstices and equinoxes, latitudes and longitudes, time zones and eclipses. This note explains each idea in simple words with easy tables.

The Universe and the Milky Way

Scientists explain the origin of the universe with the Big Bang theory: about 13.8 billion years ago, everything was packed in a tiny point which exploded and began to expand. Stars gather together in huge groups called galaxies. Our galaxy is the Milky Way, a spiral galaxy with hundreds of billions of stars. The nearest large galaxy is Andromeda. The Sun is just one star in the Milky Way and is about 4.6 billion years old, just as old as the solar system.

The Sun and the Planets

The Sun is a star at the centre of the solar system and holds about 99.8 percent of its mass. Its light takes about 8 minutes and 20 seconds to reach the Earth. There are eight planets, divided into two groups:

• Inner (terrestrial) planets: Mercury, Venus, Earth and Mars. They are small, rocky and dense.
• Outer (giant) planets: Jupiter, Saturn (gas giants), Uranus and Neptune (ice giants). They are huge, made mostly of gases and have rings and many moons.

In 2006, the International Astronomical Union reclassified Pluto as a dwarf planet, with Ceres, Eris, Haumea and Makemake. A dwarf planet is round but has not cleared its orbit of other objects.

Other Members of the Solar System

• Asteroids: rocky bodies mostly found in the asteroid belt between Mars and Jupiter.
• Comets: icy bodies with long orbits that grow a bright tail near the Sun. Halley's Comet returns about every 76 years (it was last seen in 1986).
• Meteoroids, meteors and meteorites: small rocks in space are meteoroids, their glowing streaks when they burn in the atmosphere are meteors and the pieces that reach the ground are meteorites.
• Kuiper Belt and Oort Cloud: icy regions beyond Neptune, and the far edge of the solar system.

Shape and Size of the Earth

The Earth is not a perfect sphere. It is slightly flattened at the poles and bulging at the equator, and its shape is called an oblate spheroid or geoid. The equatorial diameter is about 12,756 km, which is slightly larger than the polar diameter of about 12,714 km, and the circumference at the equator is about 40,075 km. The Earth's axis is tilted by 23.5 degrees from the vertical (or 66.5 degrees to the plane of its orbit), and this tilt is the main cause of the seasons.

Rotation of the Earth

The Earth spins on its axis from west to east, and one full turn takes about 24 hours (precisely 23 hours 56 minutes). At the equator the surface moves at about 1,670 km per hour. Rotation causes day and night, the rise and fall of the tides, the deflection of winds and currents (the Coriolis effect) and the difference in local time between places. Since the Earth turns 360 degrees in 24 hours, it moves 15 degrees of longitude every hour, or 1 degree every 4 minutes.

Revolution of the Earth and the Seasons

The Earth also moves around the Sun in an elliptical orbit in about 365 days, 5 hours and 48 minutes, at a speed of about 30 km per second. Because of the extra quarter day, we add one day to February every four years, creating a leap year. The Earth is closest to the Sun (perihelion) around 3 January and farthest (aphelion) around 4 July, but this does not cause the seasons. Seasons come from the tilt of the axis combined with revolution: when the northern half is tilted towards the Sun, it gets direct rays and longer days, so it is summer there and winter in the south.

Latitudes, Longitudes and Time

Latitudes are imaginary circles parallel to the equator (0 degrees). Important ones are the Tropic of Cancer (23.5 degrees N), Tropic of Capricorn (23.5 degrees S), Arctic Circle (66.5 degrees N) and Antarctic Circle (66.5 degrees S). Longitudes are lines running from pole to pole, and the Prime Meridian (0 degrees) passes through Greenwich in London. The International Date Line is near 180 degrees and bends to avoid land. The world has 24 time zones. Indian Standard Time (IST) is based on 82.5 degrees East and is 5 hours 30 minutes ahead of GMT. India spans about 30 degrees of longitude, so the sun rises about two hours earlier in the east than in the west.

The Moon and Eclipses

The Moon is the Earth's only natural satellite, about 384,400 km away. It takes about 27 days to revolve around the Earth and rotates once in the same time, so we always see the same face. The phases of the Moon (waxing and waning) repeat every 29.5 days.

• Solar eclipse: occurs on a new moon day when the Moon comes between the Sun and the Earth and blocks the sunlight. It can be total, partial or annular (a ring of light around the Moon).
• Lunar eclipse: occurs on a full moon day when the Earth comes between the Sun and the Moon, and the Moon enters the Earth's shadow, often looking red.

Eclipses do not occur every month because the Moon's orbit is tilted by about 5 degrees to the Earth's orbit.

Mains Angle: Why Are There Seasons and Differences in Day Length?

Explain in four steps. First, the Earth's axis is tilted by 23.5 degrees and stays pointed in the same direction as the Earth revolves. Second, because of this tilt, each hemisphere is turned towards or away from the Sun at different times of the year. Third, the hemisphere tilted towards the Sun gets more direct rays and longer days, making it warmer. Fourth, at the equinoxes, both hemispheres get equal light. Mention that without the tilt there would be no seasons, and that the latitude makes a bigger difference in day length near the poles, with the midnight sun beyond the Arctic Circle in summer.

Practice Statements and Common Mistakes

• "Venus is the hottest planet because it is closest to the Sun." Wrong. Mercury is closest, but Venus is hottest because of its thick atmosphere.
• "Seasons are caused by the Earth's distance from the Sun." Wrong. They are caused by the tilt of the axis.
• "A solar eclipse occurs on a new moon day." Correct.
• Do not mix up rotation (spinning, causing day and night) with revolution (orbiting, causing seasons).

Quick Revision

• Eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune; Pluto is a dwarf planet.
• Rotation: 24 hours, west to east; revolution: 365 days; tilt: 23.5 degrees.
• Equinoxes: 21 March and 23 September; solstices: 21 June and 22 December; IST: 82.5 degrees East.

The solar system and the motions of the Earth are easy to learn with diagrams. Draw the orbit with the tilted axis and the four dates, memorise the planet table and practise explaining eclipses in simple words, and this topic will bring you sure marks.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• Climatic Regions of the World Notes for UPSC
• Major Mountains, Rivers, Lakes and Deserts of the World Notes
• Geomorphology Notes for UPSC
• All World Geography notes

Frequently Asked Questions (FAQ)

How many planets are there in the solar system?

There are eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto has been a dwarf planet since 2006.

Which is the largest planet of the solar system?

Jupiter is the largest planet. It is a gas giant with a Great Red Spot storm and more than 90 known moons.

Which planet is the hottest?

Venus is the hottest planet, with surface temperatures of more than 450 degrees Celsius because its thick carbon dioxide atmosphere traps heat, even though Mercury is closer to the Sun.

Why is Pluto not a planet?

In 2006 the International Astronomical Union classified Pluto as a dwarf planet because it has not cleared the neighbourhood around its orbit of other objects.

What is the difference between rotation and revolution of the Earth?

Rotation is the spinning of the Earth on its axis in 24 hours, causing day and night. Revolution is the Earth’s movement around the Sun in about 365 days, which together with the tilt causes seasons.

What causes the seasons?

Seasons are caused by the 23.5-degree tilt of the Earth’s axis combined with its revolution around the Sun, which changes the amount of direct sunlight each hemisphere gets during the year.

What are equinoxes and solstices?

Equinoxes (21 March and 23 September) are days when day and night are equal. Solstices (21 June and 22 December) are the days of the longest and shortest day in a hemisphere.

What is a leap year?

A leap year has 366 days, with an extra day on 29 February added every four years to adjust for the extra quarter day that the Earth takes to go around the Sun.

On which longitude is Indian Standard Time based?

Indian Standard Time is based on the 82.5 degrees East longitude and is 5 hours 30 minutes ahead of Greenwich Mean Time.

What is a solar eclipse?

A solar eclipse occurs on a new moon day when the Moon passes between the Sun and the Earth and blocks the Sun’s light from part of the Earth.

What is a lunar eclipse?

A lunar eclipse occurs on a full moon day when the Earth lies between the Sun and the Moon and the Moon moves into the Earth’s shadow.

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

The Moon takes about the same time to rotate once on its axis as it does to revolve around the Earth, so the same side always faces us.