Earth, Moon, and the Sun

1. The Sun — Our Nearest Star

The Sun is a huge, glowing ball of hot gases at the centre of our solar system. It is the nearest star to the Earth. The Sun is luminous — it gives out its own light and heat. Without the Sun there would be no light, no warmth, no food (plants need sunlight) and no life on Earth.

Light from the Sun takes about 8 minutes to reach the Earth, even though light is the fastest thing known. We must never look directly at the Sun, especially through a telescope or binoculars, as it can permanently damage the eyes.

2. The Earth’s Rotation and Day and Night

The Earth is shaped like a slightly flattened ball (a sphere). It is always spinning around an imaginary line passing through its centre, called the axis. This spinning is called rotation.

2.1 How Day and Night Are Caused

The Earth is a non-luminous body. The Sun can light up only one half of the Earth at a time — the half facing the Sun has day, while the half facing away is in darkness and has night. As the Earth rotates, different parts come into the sunlight and then move into the shadow, so day and night follow each other continuously.

Because the Earth rotates from west to east, the Sun appears to rise in the east and set in the west. This apparent daily movement of the Sun and stars across the sky is actually caused by the Earth’s own rotation, not by the Sun moving.

3. The Earth’s Revolution and the Seasons

Besides spinning on its axis, the Earth also moves around the Sun in a fixed path called an orbit. This movement around the Sun is called revolution.

3.1 The Tilt of the Earth’s Axis

The Earth’s axis is not straight up and down; it is tilted at an angle of about 23.5° from the vertical. This tilt stays in the same direction as the Earth revolves around the Sun. Because of the tilt, the two halves of the Earth (the Northern and Southern hemispheres) receive different amounts of sunlight at different times of the year.

3.2 How Seasons Are Caused

When the Northern Hemisphere is tilted towards the Sun, it receives more direct sunlight and has summer, while the Southern Hemisphere has winter. Six months later, when the Northern Hemisphere is tilted away from the Sun, it gets slanting, weaker sunlight and has winter, while the Southern Hemisphere has summer. This is why the seasons are opposite in the two hemispheres.

4. The Moon — Earth’s Natural Satellite

The Moon is the natural satellite of the Earth — a body that revolves around the Earth. It is the brightest object in the night sky, but it is non-luminous: it has no light of its own and shines only because it reflects the light of the Sun.

The surface of the Moon has plains, mountains and many bowl-shaped pits called craters, formed by the impact of meteorites long ago. Because the Moon has almost no atmosphere, its sky always looks black and there is no sound on it. The first humans landed on the Moon in 1969 (the Apollo 11 mission). India’s Chandrayaan missions have also explored the Moon, with Chandrayaan-3 landing near the lunar south pole in 2023.

5. Phases of the Moon

Every night the Moon seems to change its shape — sometimes a thin crescent, sometimes a half disc, sometimes a full bright circle, and sometimes not visible at all. These different shapes of the bright part of the Moon are called the phases of the Moon.

5.1 Why the Phases Happen

The Moon does not really change shape. Exactly half of the Moon is always lit by the Sun. As the Moon revolves around the Earth, we see different amounts of its lit half. When the lit side faces away from us, we cannot see the Moon — this is the new moon. As the Moon moves on, more and more of the lit part becomes visible (the Moon is said to be waxing) until the whole lit face is seen — the full moon. After that the bright part shrinks (the Moon is waning) back to a new moon.

6. Eclipses

Sometimes the Sun, the Earth and the Moon come in a straight line, and the shadow of one falls on another. Such an event is called an eclipse. There are two kinds.

6.1 Solar Eclipse

During a total solar eclipse, the Moon completely covers the Sun for a short while and it becomes dark like night in the daytime. A solar eclipse must never be viewed with the naked eye or through binoculars — only with special safe solar filters, or by using a pinhole projection.

6.2 Lunar Eclipse

A lunar eclipse is completely safe to watch with the naked eye. It can be seen from the whole night-side of the Earth, so it lasts longer and is visible to more people than a solar eclipse.

7. Exploring the Sky

On a clear, dark night away from city lights, we can see thousands of stars. Stars are huge, hot, glowing balls of gas like our Sun, but they look like tiny points because they are extremely far away. Stars appear to twinkle because their light is disturbed as it passes through the moving layers of the Earth’s atmosphere.

People long ago imagined patterns among groups of stars and named them constellations, such as the Great Bear (Saptarshi or Ursa Major) and Orion (the Hunter). The bright Pole Star appears fixed in the north and was used by travellers and sailors to find direction.

Planets (like Venus, Mars, Jupiter and Saturn) also appear in the sky but, unlike stars, they do not have their own light — they shine by reflecting sunlight and do not twinkle. To study the sky in detail, scientists use telescopes, which make distant objects look bigger and clearer.