Detailed Notes: Earth & Its Natural Satellites (Part 2)

The Moon

Introduction

The Moon is the natural satellite of the Earth. It is made of rock and dust and has no atmosphere that can support life; therefore it has no air and no liquid water on its surface. The Moon does not produce its own light. Instead, it reflects sunlight, which is why we see it shining in the night sky. The Moon appears to change shape from night to night; these changes are called phases of the Moon. The gravity of the Moon also affects the Earth by causing the rise and fall of sea levels known as tides. This chapter explains the Moon's features, phases, effects on Earth, eclipses, artificial satellites and a short note on space travel.

Basic facts about the Moon

• The Moon is the natural satellite of the Earth.
• The Moon is essentially lifeless for humans: it has no breathable air and no liquid water on its surface.
• The Moon does not have its own light; it shines because it reflects sunlight.
• Because there is almost no air on the Moon, sound cannot travel there.
• The surface of the Moon is covered with rocks and fine dust (called regolith).
• The Moon has tall mountains and large flat regions called plains.
• There are many large round holes on the Moon called craters, formed by impacts of meteorites.
• The Moon's surface shows light and dark areas; the dark areas are mostly plains and the lighter areas are higherlands or mountains.
• We can see the Moon's craters and surface details with the help of a telescope.
• The first successful human landing on the Moon was Apollo 11 on 21 July 1969. The spacecraft carried three American astronauts: Neil Armstrong, Edwin "Buzz" Aldrin and Michael Collins.

Chandrayaan-1 was India's first spacecraft sent to study the Moon. It was launched on 22 October 2008 from Sriharikota by ISRO. The mission successfully studied the Moon's surface and operated until 30 August 2009, when contact was lost after most objectives had been achieved.

Phases of the Moon

The appearance of the Moon changes each night because of the changing positions of the Sun, Earth and Moon. As the Moon goes around the Earth, we see different portions of its sunlit half. The sequence of appearances is called the phases of the Moon.

• The Moon takes about one month (approximately 27-29 days for visible cycle, about 29.5 days for the full cycle of phases) to revolve once around the Earth.
• The Moon rotates on its axis once in the same time it takes to revolve around the Earth; this is why we always see nearly the same side (the near side) from Earth.
• The main phases are New Moon, Waxing Crescent, First Quarter (half-moon), Waxing Gibbous, Full Moon, Waning Gibbous, Last Quarter, and Waning Crescent, then back to New Moon.
• On a New Moon we cannot see the Moon because the side facing Earth receives very little sunlight.
• After New Moon we see a thin bright curve called the crescent.
• About one week after New Moon, we see the First Quarter when half of the Moon's visible side is lit.
• Around the fourteenth day we see the Full Moon, when the side of the Moon facing Earth is fully illuminated by the Sun.
• After Full Moon the visible illuminated part decreases through the waning phases until New Moon returns.

Phases of Moon

Effect on Tides

Tides are the regular rise and fall of sea levels along coastlines. They are mainly caused by the gravitational pull of the Moon and, to a lesser extent, the Sun.

• High tide is when the sea level rises to a higher level than average; low tide is when the sea level falls to a lower level than average.
• The Moon's gravity pulls the water of the oceans toward the side of Earth nearest the Moon, causing a bulge of water and a high tide there.
• On the opposite side of Earth there is another high tide due to the balance of forces (sometimes described as caused by inertia of the water).
• The Sun also pulls on Earth's water; when Sun and Moon align with Earth (during Full Moon or New Moon), their combined pull causes spring tides, which have higher high tides and lower low tides.
• When the Sun and Moon are at right angles relative to Earth (during First Quarter and Last Quarter), their pulls partly cancel, producing neap tides, which are less extreme.

Effect of Tides

Eclipses

Light from the Sun travels in straight lines. When an opaque object (one that does not let light pass through) blocks the path of sunlight, it casts a shadow. When the Sun, Earth and Moon line up, one body can fall into the shadow of another, producing an eclipse.

Types of Eclipse

Lunar Eclipse

• A lunar eclipse happens when the Earth comes between the Sun and the Moon, and the Earth's shadow falls on the Moon.
• When the Moon passes completely into the darkest part of Earth's shadow, called the umbra, it is a total lunar eclipse.
• When only a part of the Moon enters the umbra, it is a partial lunar eclipse.
• The lighter, outer part of the shadow that blocks only some of the Sun's light is called the penumbra; when the Moon is only in the penumbra, the eclipse is a penumbral lunar eclipse.

Lunar Eclipse

Solar Eclipse

• A solar eclipse occurs when the Moon comes between the Sun and the Earth and casts its shadow on Earth, blocking sunlight in some places.
• If the Moon completely blocks the Sun for a small area on Earth, observers there see a total solar eclipse.
• If only part of the Sun is hidden, observers see a partial solar eclipse.
• Solar eclipses are visible only from limited regions of Earth and usually last for a few minutes at a given location.

Solar Eclipse

Artificial Satellites

Artificial satellites are man-made objects placed into orbit around the Earth (or other bodies) to perform useful tasks. They remain in orbit because of a balance between their forward speed and the pull of Earth's gravity.

• Satellites are used for many purposes such as communication, weather forecasting, remote sensing, navigation and scientific study of space.
• The first artificial satellite was Sputnik 1, launched by the Soviet Union on 4 October 1957.
• India's first satellite was Aryabhata, launched in 1975.
• Cartosat-2 is a remote-sensing satellite launched by India on 12 January 2018.
• India launched the educational satellite EDUSAT in September 2004 to support education through satellite communication.
• Other Indian satellites include Bhaskara, Rohini, INSAT-2A, and satellites from the CARTOSAT series.
• Communication satellites such as GSAT-15 (launched on 11 November 2015) help transmit telephone and television signals across large areas.
• Weather satellites provide images and data that help forecast weather and issue early warnings for storms and cyclones.
• Remote sensing satellites take pictures of Earth to study land, water and natural resources.
• Some satellites are designed to study outer space and help scientists learn about other planets and stars.

Space Travel

• Space is a harsh environment with no air and very low pressure; people travelling to space must live inside spacecraft or suits that provide air, pressure and temperature control.
• Astronauts in space are exposed to higher levels of radiation than on Earth.
• Life in microgravity can affect the human body; therefore vigorous training and medical checks are required before space missions.
• People of Indian origin who have travelled to space include Rakesh Sharma, Kalpana Chawla and Sunita Williams.

Summary

The Moon is a rocky natural satellite that orbits Earth, reflects sunlight and shows changing phases. Its gravity causes tides on Earth and, when lined up with the Sun and Earth, it can cause eclipses. Human-made satellites orbit Earth to help with communication, weather forecasting, mapping and space science. Travel into space allows us to explore beyond Earth but requires careful preparation because of the difficult conditions there.