Chapter 12 — Earth, Moon and the Sun

Chapter 12, titled 'Earth, Moon, and the Sun', covers Earth's rotation (which causes day and night and the apparent movement of the Sun and stars), Earth's revolution around the Sun (which leads to changing night-sky views and seasons), and how solar and lunar eclipses occur.

Rotation is the spinning of the Earth on its own axis — the Earth completes one rotation in about 24 hours. Revolution is the motion of the Earth around the Sun — the Earth completes one revolution in about 365 days and 6 hours. Rotation causes day and night; revolution (combined with axial tilt) causes seasons.

The Sun does not actually move across the sky; it only appears to do so because the Earth is rotating from West to East. As the Earth turns, observers on its surface see the Sun appear in the East, move across the sky, and disappear in the West.

The Earth rotates from West to East. When viewed from above the North Pole, this direction appears anti-clockwise. The Earth completes one full rotation in about 24 hours.

Seasons occur because Earth's axis of rotation is tilted with respect to its orbit around the Sun, and because of Earth's spherical shape. In June, the Northern Hemisphere is tilted towards the Sun, so it receives more intense sunlight for more hours — causing summer. In December, the opposite occurs and the Northern Hemisphere experiences winter.

No. The difference in distance from the Sun across the year is very small and is not the cause of seasons. In fact, the Earth is closest to the Sun in January, which is winter in the Northern Hemisphere. Seasons are caused by the tilt of Earth's axis and the spherical shape of Earth, not by varying distance from the Sun.

In the Northern Hemisphere, the summer solstice (longest day) occurs around 21 June, and the winter solstice (shortest day and longest night) occurs around 22 December. The spring equinox falls around 21 March and the autumn equinox around 23 September; on these days, daytime lasts 12 hours.

A solar eclipse occurs when the Moon comes between the Sun and the Earth, blocking sunlight from reaching the Earth. The Moon's shadow falls on a small area of Earth's surface, where observers experience a total solar eclipse (complete darkness for a few minutes). Where the Moon only partially blocks the Sun, observers see a partial solar eclipse.

The apparent size of an object depends on both its actual size and its distance from the observer. Although the Moon is much smaller than the Sun, it is also much closer to Earth, so its apparent size as seen from Earth is similar to the Sun's apparent size. This allows the Moon to appear to cover the entire Sun.

A lunar eclipse occurs when the Earth comes between the Sun and the Moon, blocking sunlight from reaching the Moon. During a total lunar eclipse, the Moon appears dark red. Unlike a solar eclipse — which is visible from only a small area of Earth's surface — a lunar eclipse can be seen from a large part of Earth. Also, it is safe to watch a lunar eclipse with the naked eye, unlike a solar eclipse.

Even during a solar eclipse, the Sun is intense enough to damage the eyes and cause blindness. Direct viewing must be strictly avoided — including through sunglasses, binoculars, or telescopes. Safe options include using specially designed solar eclipse goggles, projecting the Sun's image using a mirror onto a screen, or attending a viewing event organised by a planetarium or astronomy club.

As the Earth revolves around the Sun over the course of a year, it faces different directions in space after sunset. This means that different groups of stars become visible in the night sky in different months — the Earth's revolution around the Sun is responsible for this changing view.

The Earth's axis of rotation points very close to the Pole Star (Dhruva Tara) in the Northern Hemisphere. Because of this, the Pole Star appears nearly stationary in the sky. All other stars appear to move around it due to the Earth's rotation.

Aryabhata, a mathematician and astronomer who wrote the Aryabhatiya around the fifth century CE, explained the apparent westward motion of stars as the result of the Earth's own rotation — comparing it to a man in a moving boat seeing stationary objects appear to move backwards. His calculated value for the time taken by Earth to complete one rotation was about 23 hours 56 minutes 4.1 seconds, which is impressively close to the currently accepted value.

Yes, the Chapter 12 PDF from the Curiosity Class 7 Science textbook is completely free to download on cbseprepmaster.com. No sign-up or account is required.