Our planet Earth is part of the solar system, orbiting the Sun. The Moon orbits Earth. Understanding the motions of these three bodies helps explain day and night, seasons, the phases of the Moon, solar and lunar eclipses, and tides.
Key Concepts and Definitions
The Sun is the nearest star to Earth — a massive ball of hot gases (mostly hydrogen and helium) that generates energy by nuclear fusion. Its diameter is about 1.4 million km. The Sun is the source of almost all energy on Earth.
The Earth orbits the Sun in an elliptical path called a revolution — it takes approximately 365.25 days (one year). Earth also rotates on its own tilted axis once every 24 hours, causing day and night. Earth's axis is tilted at about 23.5° to its orbital plane — this tilt causes the seasons.
The Moon is Earth's only natural satellite. It orbits Earth in approximately 27.3 days (sidereal period). The Moon takes the same time to rotate on its axis as it takes to revolve around Earth, so we always see the same face of the Moon.
Phases of the Moon: As the Moon orbits Earth, the illuminated portion visible from Earth changes. The cycle repeats approximately every 29.5 days (lunar month / synodic period):
New Moon → Waxing Crescent → First Quarter → Waxing Gibbous → Full Moon → Waning Gibbous → Last Quarter → Waning Crescent → New Moon.
Solar eclipse: Occurs when the Moon comes between the Sun and Earth, blocking sunlight. A total solar eclipse occurs only at new Moon. The shadow of the Moon falls on Earth.
Lunar eclipse: Occurs when Earth comes between the Sun and the Moon, and Earth's shadow falls on the Moon. It occurs at full 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 Earth facing the Moon and on the opposite side; low tides occur at right angles. Spring tides (very high and very low) occur at new and full Moon when the Sun, Earth, and Moon are aligned.
Worked Examples
Why do we have day and night?
Earth rotates on its axis from west to east (anticlockwise when viewed from the North Pole) once every 24 hours. The side of Earth facing the Sun experiences daytime; the side away from the Sun is in darkness (night).
Why do seasons occur?
Earth's axis is tilted 23.5°. As Earth revolves around the Sun, the tilt causes different parts of Earth to receive sunlight at different angles at different times of year. When the Northern Hemisphere tilts toward the Sun (June), it is summer there; when it tilts away (December), it is winter.
Why does the Moon always show the same face to Earth?
The Moon's rotation period equals its revolution period around Earth (both about 27.3 days). This synchronous rotation means the same hemisphere of the Moon always faces Earth.
On which day of the lunar cycle does a total solar eclipse occur, and why?
A solar eclipse can only occur at New Moon, when the Moon is between the Sun and Earth. However, not every new Moon causes an eclipse because the Moon's orbit is tilted about 5° to Earth's orbital plane, so the Moon usually passes slightly above or below the line connecting the Sun and Earth.
Distinguish between a solar and a lunar eclipse.
Solar eclipse: Moon blocks sunlight from reaching Earth; occurs at new Moon; Moon's shadow falls on Earth. Lunar eclipse: Earth's shadow falls on the Moon; occurs at full Moon; the Moon may appear reddish (due to refraction of sunlight around Earth).
Why are spring tides higher than normal tides?
At new Moon and full Moon, the Sun, Earth, and Moon are nearly in a straight line. The gravitational pulls of the Sun and Moon add together (combined pull), producing exceptionally high tides called spring tides and correspondingly very low tides.
Calculate how many times Earth revolves around the Sun in 10 years.
Earth completes 1 revolution per year. In 10 years = 10 revolutions.
In days: 10 x 365.25 = 3652.5 days.
- Key Facts
- Distance from Earth to Sun: approximately 150 million km (1 Astronomical Unit).
- Distance from Earth to Moon: approximately 3,84,400 km.
- The Moon has no atmosphere, so it has extreme temperature variations and no weather.
- The Sun is about 400 times farther from Earth than the Moon, but also about 400 times larger — this is why they appear the same size in the sky, making total solar eclipses possible.
Common mistakes
Students often think Earth is closer to the Sun in summer. This is incorrect — seasons are caused by Earth's axial tilt, not distance from the Sun. In fact Earth is slightly farther from the Sun during Northern Hemisphere summer. Also, the Moon does not produce its own light; it reflects sunlight.
Summary
Earth rotates (24 h) causing day and night, and revolves around the Sun (365.25 days) with axial tilt (23.5°) causing seasons. The Moon orbits Earth (~27.3 days) and its phases result from the changing angle between the Sun, Moon, and Earth. Solar eclipses occur at new Moon; lunar eclipses at full Moon. Tides are caused by the Moon's (and Sun's) gravitational pull on Earth's oceans.