The Solar System, Stars and Artificial Satellites

Astronomy closes the physics block and gives 1 question. It overlaps with the geography chapter of Part-II, but here the treatment is physical rather than locational — the paper asks about phases, eclipses and satellites rather than about latitude.

The Solar System

Eight planets orbit the Sun, held by gravitational attraction. In order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

The inner four are rocky planets and the outer four gas giants. Asteroids lie between Mars and Jupiter.

A meteor burns up in the atmosphere; if part survives and reaches the ground it is a meteorite. A comet's tail always points away from the Sun.

The Moon and Its Phases

The Moon has no light of its own and shines by reflecting sunlight. Its phases occur because we see different portions of its lit half as it orbits the Earth.

• The same side of the Moon always faces the Earth, because its rotation period equals its revolution period.
• The Moon has no atmosphere, so there is no sound, no weather and extreme temperature differences.
• Neil Armstrong was the first person to set foot on the Moon, in 1969.

✗ The Moon has phases because the Earth's shadow falls on it  |  ✓ Phases are caused by our changing view of the lit half; the Earth's shadow causes an eclipse

That distinction between a phase and an eclipse is the single most common misconception in this topic and a teacher is expected to correct it.

Eclipses

The Moon is in the middle during a solar eclipse and the Earth in the middle during a lunar one. A solar eclipse must never be viewed with the naked eye.

Stars and Artificial Satellites

• Stars are self-luminous and twinkle; planets shine by reflected light and do not.
• The Pole Star appears fixed because it lies almost above the Earth's axis of rotation.
• Distances between stars are measured in light years, the distance light travels in one year.

An artificial satellite is a human-made object placed in orbit, used for weather forecasting, communication, remote sensing and navigation. Aryabhata, launched in 1975, was India's first satellite.

A light year measures distance, not time — the wording is designed to catch a hurried reading, and that option always appears.

Frequently Asked Questions

What causes the phases of the Moon?

We see different fractions of the Moon's sunlit half as it moves around the Earth. It is not caused by the Earth's shadow, which is what produces a lunar eclipse instead. Confusing phases with eclipses is the commonest misconception in this topic.

What is the difference between a solar and a lunar eclipse?

In a solar eclipse the Moon comes between the Sun and the Earth, so it occurs on a new moon day. In a lunar eclipse the Earth comes between the Sun and the Moon, so it occurs on a full moon day. Ask which body sits in the middle.

Is a light year a unit of time?

No, it is a unit of distance, specifically the distance light travels in one year. The word year in its name makes it look like a time unit, and that option is placed in questions deliberately. It is used for the vast distances between stars.

Why do stars twinkle while planets do not?

Stars are extremely far away and act as point sources, so refraction through moving layers of air makes their light flicker. Planets are much closer and appear as tiny discs, so the fluctuations from different parts of the disc average out and the light stays steady.