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Quick recap: the Earth, its motions and interior on one page

The Earth: its place, motions and interior · Quick recap · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

All three parts condensed for quick revision: the solar system and the Earth's shape, rotation, revolution and seasons, latitude, longitude and time, IST and the Date Line, seismic waves and the shadow zone, the Earth's layers, and the three families of rock, with a one-table overview and eight mixed questions.

This chapter follows the Earth from the outside in. Part 1 places the planet in the solar system and explains its shape and its two motions, which give day and night and the seasons. Part 2 draws the grid of latitude and longitude and turns longitude into time, ending with Indian Standard Time and the International Date Line. Part 3 goes below the surface: how the interior is known, the layers that earthquake waves reveal, and the rocks of the crust. The figures in this recap are the ones Prelims tests most often.

The solar system has the Sun and eight planets: four small rocky inner planets (Mercury, Venus, Earth and Mars) and four outer giants (Jupiter, Saturn, Uranus and Neptune), with the asteroid belt between Mars and Jupiter. Under the International Astronomical Union's definition of August 2006, Pluto is a dwarf planet, because it has not cleared its orbit of other bodies. The Earth is a geoid that bulges at the equator: its equatorial radius is about 6,378 km and its polar radius about 6,357 km. Its average distance from the Sun is about 150 million km.

The Earth rotates from west to east; one turn measured against the Sun is a solar day of 24 hours, a little longer than one turn measured against the stars, about 23 hours 56 minutes. It revolves round the Sun in about 365¼ days, so a leap year adds a day every fourth year, with century years counting only when divisible by 400. The Earth is nearest the Sun (perihelion) in early January and farthest (aphelion) in early July, so distance does not cause the seasons. The tilt of the axis does: about 23½° from the perpendicular, or 66½° with the orbital plane. At the June solstice (about 21 June) the overhead Sun reaches the Tropic of Cancer and at the December solstice (about 22 December) the Tropic of Capricorn; at each equinox (about 21 March and 23 September) it is over the equator.

latitude measures distance north or south of the equator, from 0° to 90°. The tropics lie at about 23½° and the polar circles at about 66½°, dividing the globe into the torrid, temperate and frigid zones, and a degree of latitude is about 111 km everywhere. India's mainland stretches from about 8°4′N to 37°6′N. The Tropic of Cancer crosses eight states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.

longitude is counted east and west from the Prime Meridian at Greenwich, chosen by the International Meridian Conference in Washington in 1884. Because the Earth turns 360° in 24 hours, 15° of longitude equals one hour and 1° equals 4 minutes, and places to the east are ahead in time. Indian Standard Time is the local time of 82°30′E, which passes near Mirzapur, making it UTC + 5:30; the CSIR-National Physical Laboratory keeps it. India spans about 30° of longitude, so sunrise in the far east comes nearly two hours before the far west. The International Date Line runs roughly along 180° and bends around land; crossing it westward adds a day, while crossing it eastward repeats one.

Part 3 starts from evidence. Mines, deep drilling (the Kola borehole reached about 12.2 km) and volcanic material are direct sources; temperature, pressure and density, meteorites, gravity and magnetism, and above all seismic waves are indirect ones. P waves are the fastest and pass through solids, liquids and gases; S waves pass only through solids; surface waves are the slowest and most destructive. Direct P waves are missing between about 105° and 145° from the epicentre, and S waves everywhere beyond 105°; this shadow zone revealed a liquid outer core.

The crust is thin: continental crust, granitic and once called SIAL, is about 30–40 km thick and up to about 70 km under the Himalaya, while oceanic crust, basaltic and called SIMA, is about 5–10 km; the Moho, identified in 1909, marks its base. The mantle reaches about 2,900 km. Its weak, partly molten asthenosphere lies beneath the rigid lithosphere, the crust plus the uppermost mantle, which is broken into plates. The core, mainly iron and nickel, begins at the Gutenberg discontinuity; its liquid outer part generates the magnetic field, and its solid inner part lies beyond the Lehmann discontinuity at about 5,150 km.

By weight the crust is mostly oxygen, followed by silicon, aluminium and iron, while iron leads for the Earth as a whole. Igneous rocks form from cooling magma: granite is intrusive, basalt is extrusive, and the Deccan Traps are basalt. Sedimentary rocks are layered, hold fossils and include sandstone, shale, limestone and coal. Metamorphic rocks are changed by heat and pressure without melting: limestone becomes marble, sandstone quartzite, shale slate and granite gneiss. The rock cycle links all three families, driven by the Sun's energy at the surface and the Earth's heat within.

Exam traps

• The Earth is nearest the Sun (perihelion) in early January and farthest (aphelion) in early July; the tilt of the axis, not distance, causes the seasons.
• The axis is tilted about 23½° from the perpendicular, which equals about 66½° with the orbital plane; questions swap the two.
• IST is the local time of 82°30′E, which passes near Mirzapur, not through New Delhi; IST = UTC + 5 hours 30 minutes.
• The Tropic of Cancer crosses eight states; it does not pass through Odisha, Bihar or Uttar Pradesh.
• Crossing the International Date Line westward adds a day; crossing eastward repeats a day. The line is not perfectly straight.
• P waves pass through all states of matter; S waves only through solids. The S-wave shadow (all beyond about 105°) is larger than the P-wave shadow (about 105°–145°).
• Moho = crust and mantle; Gutenberg = mantle and core (about 2,900 km); Lehmann = outer and inner core (about 5,150 km).
• Oxygen is the most abundant element of the crust, iron of the Earth as a whole; fossils occur in sedimentary rocks; marble comes from limestone, slate from shale.

Recap

• Eight planets; Pluto a dwarf planet since the IAU's 2006 definition; the Earth a geoid about 150 million km from the Sun.
• Rotation west to east (solar day 24 hours); revolution about 365¼ days; perihelion early January, aphelion early July.
• Tilt of 23½° causes the seasons; solstices 21 June and 22 December; equinoxes 21 March and 23 September.
• Latitude: tropics 23½°, polar circles 66½°, three heat zones; India about 8°4′N to 37°6′N; Tropic of Cancer through 8 states.
• Longitude from Greenwich (1884); 15° = 1 hour; IST from 82°30′E = UTC + 5:30; Date Line near 180°.
• Interior known mostly from seismic waves; P through all states, S through solids only; shadow zone reveals a liquid outer core.
• Crust (Moho at base), mantle to about 2,900 km with the asthenosphere, liquid outer and solid inner core of iron and nickel.
• Crust: oxygen first; rocks are igneous, sedimentary or metamorphic, linked by the rock cycle.

Practice questions

Sources

• NASA Science: Earth facts, the Sun, planetary fact sheets, seasons, and the Earth's interior (National Aeronautics and Space Administration, US government) — https://science.nasa.gov/earth/facts/
• International Astronomical Union, Resolutions B5 and B6 of the XXVI General Assembly, Prague, August 2006 (definition of a planet; Pluto) — https://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf
• International Conference held at Washington for the purpose of fixing a Prime Meridian and a Universal Day, October 1884: Protocols of the Proceedings — https://www.gutenberg.org/ebooks/17759
• Survey of India, Department of Science and Technology: maps and geographical extent of India — https://surveyofindia.gov.in
• CSIR-National Physical Laboratory, New Delhi: Indian Standard Time and the time and frequency standards — https://www.nplindia.org
• USGS Earthquake Hazards Program: seismic waves, focus and epicentre, and the structure of the Earth from seismology — https://www.usgs.gov/programs/earthquake-hazards
• United States Geological Survey: 'This Dynamic Earth' and educational material on minerals and the rock cycle (US government publication) — https://pubs.usgs.gov/publication/7000097