Revision Notes — Chapter 12: Earth, Moon and the Sun

This study module summarises Chapter 12 from NCERT for Class 7 Science. It covers the movements of the Earth, their effects, the behaviour of the Moon, celestial events such as eclipses, and practical ideas useful for board exam answers. Read carefully — diagrams and labelled sketches (normal ray diagrams or orbit sketches) are often asked in exams, so practice drawing simple figures to support your answers.

1. Earth’s Movements — Rotation and Revolution

Earth performs two primary movements: rotation on its axis and revolution around the Sun. Rotation means the Earth spins about an imaginary line called its axis (runs from the North Pole to the South Pole). This rotation takes approximately 24 hours, and it causes day and night. The side of the Earth facing the Sun receives sunlight and experiences day, while the opposite side is in shadow and experiences night. The apparent daily motion of the Sun across the sky is a consequence of Earth’s rotation.

Revolution is the motion of Earth around the Sun along an almost circular path called its orbit. One complete revolution takes roughly 365.25 days — this fraction leads to the addition of a leap day every four years. Revolution, combined with the tilt of the Earth’s axis, is responsible for the seasons.

2. Why Day and Night Occur

As the Earth rotates, different parts of the planet move into and out of sunlight. At any moment, half the Earth is illuminated (day) and half is in darkness (night). The boundary between the day side and the night side is called the terminator. Sunrise and sunset occur when a location moves across the terminator. The speed of rotation is fastest at the equator and reduces towards the poles, which explains why the length of daylight varies with latitude and season.

3. Seasons — Cause and Characteristics

The Earth’s axis is tilted at about 23.5° to the plane of its orbit. Because of this tilt, during different parts of the year, either the Northern Hemisphere or the Southern Hemisphere is tilted towards the Sun. The hemisphere tilted towards the Sun receives more direct sunlight (the Sun’s rays are more vertical) and experiences summer; the opposite hemisphere receives slanted rays and experiences winter. When neither hemisphere is particularly tilted towards the Sun, both receive similar sunlight and we have spring or autumn (equinoxes).

Key points for exams:

• Summer: Sun’s rays fall more directly → longer days, shorter nights, warmer temperatures.
• Winter: Sun’s rays are oblique → shorter days, longer nights, cooler temperatures.
• Equinoxes: Day and night are nearly equal (around March 21 and September 23).
• Solstices: Longest day (around June 21 in Northern Hemisphere) and shortest day (around December 22 in Northern Hemisphere).

4. The Moon — Motion, Phases and Tides

The Moon revolves around the Earth and also rotates about its own axis. Moon’s rotation period is equal to its revolution period (about 27.3 days), which is why we always see nearly the same face of the Moon — this phenomenon is called synchronous rotation. However, the phases of the Moon are observed with a cycle of about 29.5 days (the synodic month) because phases depend on the relative positions of the Sun, Earth and Moon.

Phases of the Moon: As the Moon orbits the Earth, we see varying portions of the illuminated part of the Moon. The main phases in order are:

• New Moon — Moon between Earth and Sun; near side not lit (invisible).
• Waxing Crescent — small sliver visible after new moon.
• First Quarter — half of the Moon’s near side illuminated.
• Waxing Gibbous — more than half illuminated.
• Full Moon — Earth between Sun and Moon; near side fully illuminated.
• Waning Gibbous, Last Quarter, Waning Crescent — phases leading back to New Moon.

Tides: The gravitational pull of the Moon (and to a lesser extent, the Sun) on Earth’s oceans causes tides. High tides occur on the side of the Earth nearest the Moon and on the opposite side; low tides occur at right angles to these. When Sun, Earth and Moon are aligned (new or full moon), spring tides (higher high tides) occur; when they form right angles, neap tides (lower high tides) occur.

5. Eclipses — Solar and Lunar

Eclipses are special events that happen when three bodies (Sun, Earth and Moon) move into a straight line. They are predictable and are fascinating topics in examinations.

Solar Eclipse: Occurs at new moon when the Moon comes between the Sun and the Earth. The Moon’s shadow falls on parts of the Earth, causing daytime darkness in those regions. Depending on distances and alignments, eclipses can be total (Moon fully covers Sun for a narrow path), partial (only part of the Sun is covered), or annular (Moon covers the Sun’s centre leaving a ring). Important exam point: Solar eclipses can be seen only from specific locations on Earth and last for a short time at any given place.

Lunar Eclipse: Occurs at full moon when the Earth is between the Sun and the Moon. Earth’s shadow falls on the Moon, and observers on Earth can see the Moon darken or take on a reddish colour (due to scattering of sunlight by Earth’s atmosphere). Lunar eclipses are visible from the entire night side of Earth and last longer than solar eclipses.

Why don’t eclipses happen every month? The Moon’s orbit around the Earth is tilted by about 5° to the Earth’s orbit around the Sun (the ecliptic). Because of this tilt, the Moon usually passes above or below the Sun-Earth line, so perfect alignment (syzygy) required for eclipses happens only occasionally when the Moon’s orbital plane crosses the ecliptic near new or full moon.

6. Seasons on Other Planets — Short Note

Not all planets have seasons like Earth. Seasons depend on axial tilt. For example, Mars has a tilt similar to Earth and shows seasons, while planets with very small tilt show little seasonal change. This is an interesting point for extension questions and shows understanding beyond NCERT basics.

7. Practical Observations and Classroom Activities

These activities are simple and often asked as practical descriptions in exams:

• Model demonstration of day and night: Use a globe and a torch (representing the Sun). Rotate the globe slowly to show different regions experience day and night.
• Model of seasons: Tilt the globe by 23.5° and revolve it around the torch while keeping the tilt direction fixed to show how one hemisphere receives more direct sunlight (summer) and the other less (winter).
• Moon phases demonstration: Use a small ball (Moon), a lamp (Sun) and yourself (Earth) to show illuminated portion changing as the Moon orbits you.
• Observational tasks: Note sunrise and sunset times for a week, observe Moon phases nightly, and record shadow lengths at different times to link to Sun’s altitude.

8. Common Exam Questions & How to Answer

Write succinct definitions first, then explain with simple labelled diagrams where needed. For numerical or observational questions, list materials, procedure, observations and conclusion. For conceptual questions (e.g., why seasons occur), emphasise axial tilt and directness of Sun’s rays — include dates of equinoxes/solstices if asked.