Fundamentals of Geography: The Earth and its motions

Because the Earth is a sphere, only half of it faces the Sun at any time. The boundary between the lit half and the dark half is the circle of illumination. As the Earth rotates, places move across this circle, producing day and night. NCERT Class VI uses this term in the chapter on the motions of the Earth.

Both statements are correct. The four inner (terrestrial) planets are small, rocky and dense, while the four outer Jovian planets (Jupiter, Saturn, Uranus, Neptune) are giant planets made largely of gases and ices, with low density. The asteroid belt, a zone of numerous rocky bodies, lies between Mars and Jupiter and separates the two groups.

Three phenomena (1, 2 and 4) result from rotation. Rotation brings places alternately into the lit and dark halves, causing day and night; it produces the Coriolis effect, which deflects winds and currents; and because places on different meridians face the Sun at different moments, local times differ. The change of seasons results from revolution around the Sun with a tilted axis, not from rotation.

Both statements are correct and Statement-II explains Statement-I. The Earth is not a perfect sphere but a geoid, slightly flattened at the poles and bulging at the Equator: the equatorial diameter (about 12,756 km) exceeds the polar diameter by about 43 km. Points on the Equator are farthest from the axis and move fastest (about 1,670 km/h), so the centrifugal effect of rotation is greatest there and zero at the poles; over geological time this produced the equatorial bulge.

Statements 1 and 2 are correct. The Earth's orbit is slightly elliptical: it reaches perihelion (about 147 million km) around 3 January and aphelion (about 152 million km) around 4 July. The difference is small, and the Earth is actually nearest the Sun during the northern winter. Seasons are caused by the tilt of the Earth's axis as it revolves, which changes the angle of the Sun's rays and the length of day.

All three statements are correct. Venus shows retrograde rotation: it spins from east to west, so on Venus the Sun would rise in the west. Its rotation is extremely slow: one rotation takes about 243 Earth days, while one revolution around the Sun takes only about 225 Earth days, so its 'day' (sidereal rotation) is longer than its 'year'. Venus and Mercury are the only two planets with no moon.

Both statements are correct. Tidal friction transfers angular momentum from the Earth's spin to the Moon's orbit, so the day lengthens by roughly 2 milliseconds per century and the Moon slowly recedes. When mass moves away from the axis, for example meltwater from polar ice spreading towards the Equator, the Earth's moment of inertia rises and, by conservation of angular momentum, its spin slows slightly, much as a skater slows by stretching out the arms. Recent studies link part of the measured change in day length to such ice melt.

One rotation relative to the stars, the sidereal day, takes about 23 h 56 min. During that time the Earth also moves about 1° along its orbit (360° in about 365 days), so it must turn roughly one more degree before the Sun is again on the local meridian. One degree of rotation takes about 4 minutes, which makes the solar day 24 hours. The cause is the Earth's revolution, not any daily slowing.