NCERT Class 8 Science Chapter 11

Ancient humans used the regular movements of celestial bodies to measure time. The rising and setting of the Sun defined a day. The complete cycle of Moon phases defined a month. The return of stars to the same position in the sky after a year defined the year. Sundials and water clocks were early instruments used to divide the day into smaller units.

The phases of the Moon are caused by the changing relative positions of the Earth, Moon, and Sun. As the Moon orbits Earth, different proportions of its sunlit half face toward Earth. When the Moon is between Earth and Sun, we see no illuminated surface (new moon). When Earth is between Moon and Sun, we see the full illuminated face (full moon).

A solar calendar is based on Earth's revolution around the Sun (365.25 days per year) — the Gregorian calendar is an example. A lunar calendar is based on Moon phases (29.5 days per month, ~354 days per year) — the Islamic calendar is purely lunar. The Hindu calendar is lunisolar, combining both systems.

Earth takes approximately 365.25 days to orbit the Sun, not exactly 365. To account for this quarter-day, an extra day (February 29) is added every four years, creating a leap year of 366 days. Without leap years, our calendar would drift out of alignment with seasons over centuries.

Solstices occur twice a year when the Sun is at its maximum tilt relative to Earth's equator — June solstice gives the longest day in the Northern Hemisphere; December solstice gives the shortest day. Equinoxes occur twice a year (March and September) when day and night are approximately equal in length everywhere on Earth.

The Sun appears to move from east to west across the sky each day because the Earth rotates on its axis from west to east. It is Earth's rotation, not the Sun's movement, that creates this apparent motion. The Sun also appears to follow a different path in the sky across seasons due to Earth's revolution and axial tilt.