Earth as a Planet Chapter Notes | Science for Grade 7 PDF Download

Introduction

• In ancient times, people believed Earth was flat.
• In India, astronomer Aryabhatta, and in Europe, Pythagoras and Aristotle, were among the first to propose that Earth is spherical.

Proofs of Earth's Spherical Shape

• Sunrise and Sunset: The time of sunrise and sunset differs across locations, indicating a curved surface. If Earth were flat, all places would experience these events simultaneously.
• Sighting of a Ship: When a ship approaches the shore, it becomes visible gradually from the top down, suggesting Earth's surface is curved.

• Bedford Level Experiment: Three poles of equal length were placed 5 km apart in the Bedford Level Canal in Britain, rising to the same height above water, showing alignment consistent with a curved surface.

• Pole Star Position: The Pole Star appears directly overhead at the North Pole, but its angle of elevation decreases as one moves south, indicating a spherical Earth.
• Stars at Night: Different constellations are visible as you move north or south. For example, the constellation Orion appears upside down in Australia compared to the Northern Hemisphere.
• Circular Horizon: The horizon appears circular, and its extent widens with increased altitude, a feature unique to spherical bodies.
• Spherical Earth Horizon Expansion: At higher altitudes, the circular horizon expands from AB to CD, observable only on a spherical Earth.

• Circumnavigation: Ferdinand Magellan’s journey around the world, traveling west and returning to the starting point without encountering an edge, confirms Earth’s spherical shape.
• Varying Time Zones: Different regions experience different times of day due to the uneven distribution of sunlight, supporting the idea of a spherical Earth.
• Lunar Eclipse: During a lunar eclipse, Earth’s shadow on the Moon is consistently oval, indicating a spherical shape. This shadow remains consistent as Earth rotates.
• Varying Star Constellations: As one moves away from the Equator, known constellations shift toward the horizon and are replaced by new ones, a phenomenon not possible on a flat Earth.
• Shadows and Sticks: Shadows cast by sticks in different locations vary in length and angle, unlike what would be expected on a flat surface where shadows would be identical.
• Satellite Pictures: Photographs from space, including those taken by astronauts on the International Space Station, clearly show Earth’s curved surface.

Shape of the Earth

• Earth is not a perfect sphere but an oblate spheroid, bulging at the equator and flattened at the poles.
• This shape is referred to as a "Geoid," meaning "Earth-shaped."
• Earth is the fifth largest planet in the Solar System, with an average diameter of approximately 12,735 km.
• Greek philosopher Eratosthenes made an early attempt to measure Earth’s circumference.

Earth’s Unique Features

• Distance from the Sun: Earth is located 150 million km from the Sun, providing an ideal temperature averaging 17°C, neither too hot nor too cold.
• Atmosphere: Earth has a thick atmosphere with 21% oxygen and 75% nitrogen, supporting life.
• Ozone Layer: The atmosphere contains ozone, which protects life by absorbing harmful ultraviolet rays.
• Water Presence: About 71% of Earth’s surface is covered by water, earning it the names "Blue Planet" or "Watery Planet."
• Lithosphere: The Earth’s rocky surface, called the lithosphere, provides soil for plant life and mineral resources.
• Natural Satellite: The Moon is Earth’s only natural satellite.
• Home to Life: Earth’s unique conditions make it the only known planet capable of supporting human life.

Key Words

• Circumnavigation: Traveling around the world, typically by sailing or other means.
• Lunar Eclipse: When Earth passes between the Sun and Moon, casting a shadow on the Moon.
• Oblate Spheroid: A sphere that is slightly flattened at the poles and bulging at the equator, like an orange.
• Geoid: A term for Earth’s shape, resembling an oblate spheroid due to its rotation.
• Optimum Distance: The ideal distance between objects, such as Earth and the Sun, that supports life.
• Ozone: A gas made of three oxygen atoms that protects Earth by absorbing harmful ultraviolet radiation.
• Ultraviolet Rays: Sunlight radiation that can darken skin and is harmful in excess.