Class 7 Science Chapter 12: Earth, Moon, and the Sun Notes

What is this chapter about?

In Kanniyakumari, Rashmika notices that coconut-tree shadows are long when she cycles to school and short when she rides home. Does the Sun move across the sky, or does the Earth move? This last chapter of the book answers her. It shows how the Earth spins and travels, and how those two motions explain day and night, the turning of the stars, the seasons and eclipses.

The Earth rotates

On a merry-go-round turning anti-clockwise, the world around you seems to turn clockwise. The Earth is the same. It rotates on its axis, a line through the North and South Poles, once in about 24 hours, turning anti-clockwise as seen from above the North Pole, that is, from West to East. The half facing the Sun has day and the other half has night, and each place has sunrise as it turns into the light. Because we turn West to East, the Sun, the Moon and the stars seem to rise in the East and set in the West. Read more in rotation and revolution of the Earth.

The sky seems to turn

At night the stars also move across the sky, all except the Pole Star (Dhruva Tara), which stays nearly still because the Earth’s axis points almost at it. Saptarishi, the Big Dipper, circles around it through the night. Long-exposure photographs show the stars drawing arcs called star trails. Aryabhata explained this around the 5th century CE: from a boat moving forward, things on the shore seem to move back, and in the same way the fixed stars seem to move west. A 22-metre Foucault pendulum in India’s new Parliament building shows the Earth’s rotation too.

The Earth revolves

While it spins, the Earth also revolves around the Sun in a nearly circular orbit, taking about 365 days and 6 hours. Its night side faces a different part of space as the months pass, so the stars that rise at sunset change through the year. The Bhil and Pawara communities of the Tapi valley used the appearance of certain stars to know that the monsoon was near.

Why seasons happen

The Earth’s axis is tilted, and it keeps the same tilt all the way round its orbit. In June the Northern Hemisphere leans towards the Sun, so its sunlight is more concentrated and its days are longer than 12 hours: it is summer there, and winter in the Southern Hemisphere. In December the situation reverses. The longest northern day, around 21 June, is the summer solstice; day and night are equal at the equinoxes in March and September. Distance from the Sun is not the cause: the Earth comes nearest the Sun in January. See seasons on the Earth.

Eclipses

Mercury and Venus pass between us and the Sun but look far too small to hide it. The Moon, small but near, looks about as big as the Sun, so it can cover it. In a solar eclipse the Moon blocks the Sun’s light from a small area of the Earth, which sees a total eclipse, while a wider area sees a partial one. Never look at it directly. In a lunar eclipse the Earth’s shadow falls on the full Moon, which can turn dark red, and it is safe to watch. Read solar and lunar eclipses, and find answers to every question in the NCERT solutions for Chapter 12.