Chapter Notes

Earth, Moon, and the Sun

Rotation of the Earth

The Sun appears to rise in the East and set in the West due to the Earth's rotation which is the spinning of the Earth on its axis.

The Earth rotates on its axis which is an imaginary line passing through the geographic North Pole and South Pole. One complete rotation takes about 24 hours. When viewed from the top of the North Pole, the Earth rotates in an anti-clockwise direction, from West to East.

Rotation is the motion of an object in which all its parts move in circles around an imaginary line that passes through it. This line is called the axis of rotation.

The Earth's rotation causes the day-night cycle. As the Earth rotates, different parts of its surface face the Sun. The side facing the Sun experiences daytime, while the opposite side experiences night. Sunrise occurs as a location moves into the light, and sunset occurs as it moves into darkness.

The Sun appears to rise in the East, move across the sky from East to West, and set in the West because the Earth rotates from West to East.

Foucault Pendulum

Observing Stars

The Earth's axis of rotation points very close to the Pole Star (Dhruva Tara) in the Northern Hemisphere. This makes the Pole Star appear nearly stationary in the sky. All other stars appear to move around it. Just like the Sun, the Moon also appears to rise in the eastward direction and set in the westward direction because the Earth rotates from West to East.

Star trails are arcs of a circle that are recorded as apparent motion of the stars by astrophotographers who take long exposure photographs, keeping the camera's shutter open for a long time.

Ancient Indian Astronomy

Ancient Indian astronomers, including Aryabhata, had also noticed the daily apparent motion of celestial objects. Aryabhata's value for the time taken by the Earth to complete one full rotation about its axis is around 23 hours 56 minutes 4.1 seconds, which is close to the currently accepted value.

Revolution of the Earth

While rotating on its own axis, the Earth also revolves around the Sun. Revolution is the motion of an object around another object. The path an object takes while revolving around another object is called its orbit. The orbit of the Earth around the Sun is nearly circular and the Earth completes one revolution around the Sun in about 365 days and 6 hours.

Changing View of Night Sky

The stars seen in the night sky gradually change over a year because the Earth revolves around the Sun. As the Earth moves to different positions in its orbit, we look in different directions and see different stars.

Seasons on the Earth

The Earth's axis of rotation is not upright with respect to its orbit; it is tilted. The Earth maintains this tilt as it orbits around the Sun. The tilt of the Earth's axis and the spherical shape of the Earth give rise to seasons.

In June, the Northern Hemisphere is tilted towards the Sun, while the Southern Hemisphere is tilted away. This causes sunlight to be more concentrated in the Northern Hemisphere, leading to summer. The Northern Hemisphere also receives sunlight for more than 12 hours in June. In December, the situation is reversed, and the Northern Hemisphere experiences winter with shorter days.

The seasons and length of daytime are reversed in the Southern Hemisphere compared to the Northern Hemisphere. It is winter in June and summer in December in the Southern Hemisphere.

• When the Northern Hemisphere tilts towards the Sun, it is closer to the Sun.
• The orbit of the Earth is an oval with the Sun slightly displaced from its centre so the Earth is at different distances from the Sun over the year.

The difference in distances in either of these cases is very small and these are not the reasons why seasons occur on the Earth. In fact, the Earth is closest to the Sun in January.

In the Northern Hemisphere, the longest day occurs around June 21, known as summer solstice. The shortest day and longest night occur around December 22, known as winter solstice. Around March 21 and September 23, the daytime lasts for 12 hours; these days are called the spring and autumn equinox, respectively.

At the North Pole, the Sun rises on March 21 (equinox) and remains continuously in the sky for six months, setting on September 22. The South Pole experiences the opposite.

On the equator, there are always 12 hours of sunlight and 12 hours of darkness, with little difference in the intensity of sunrays throughout the year.

Eclipses

Solar Eclipse

A solar eclipse occurs when the Moon comes between the Sun and the Earth, obstructing the Sun's light.

During a solar eclipse, the Moon's shadow falls on a small area on the Earth's surface. Observers in this area witness a total solar eclipse. In areas where the Moon partially blocks the Sun, observers see a partial solar eclipse.

During a total solar eclipse, it turns dark for a few minutes. The Moon's shadow moves across the Earth's surface due to the Earth's rotation and the Moon's motion.

Lunar Eclipse

A lunar eclipse occurs when the Earth blocks sunlight from reaching the Moon. On such days, we see the Earth's shadow falling on the full disc of the Moon.

When the Moon is completely in the Earth's shadow, it is a total lunar eclipse. The Moon appears dark red. When part of the Moon is in the Earth's shadow, it is a partial lunar eclipse. Unlike solar eclipses, lunar eclipses can be safely watched with the naked eye.