CDS Geography · Formula sheet
Earth in Space, Maps and Coordinates formulas
1 formula, 7 reference tables and 14 common traps for CDS Geography Earth in Space, Maps and Coordinates, grouped by subtopic.
The Solar System
Learn this subtopic in the notesTheories of the origin of the Earth
| Theory | Proposed by |
|---|---|
| Gaseous (nebular) hypothesis | Immanuel Kant; revised by Laplace |
| Planetesimal hypothesis | Chamberlin and Moulton |
| Tidal hypothesis | James Jeans and Harold Jeffreys |
| Binary star hypothesis | H. N. RussellQ CDS 2026 (I): Kant gaseous, Russell binary star, Chamberlin planetesimal are all correctly matched. |
| Expanding universe (Big Bang) | Edwin Hubble showed the expansion |
| Tetrahedral hypothesis of continents and oceans | Lowthian GreenQ CDS 2026 (II): Lowthian Green, not Taylor, Wegener or Davis. |
| Continental drift | Alfred Wegener |
The two families of planets
| Family | Planets | Made of |
|---|---|---|
| Terrestrial (inner) | Mercury, Venus, Earth, Mars | Rock and metal; solid surfaceQ CDS 2020 (II): the four planets closest to the Sun are the terrestrial planets. |
| Giant or gas (outer) | Jupiter, Saturn, Uranus, Neptune | Hydrogen, helium and ices; no solid surfaceQ CDS 2023 (I): Mars is rocky, so any group of gas planets that includes Mars is wrong. |
| Dwarf planet | Pluto (since 2006) | Rock and ice; too small to clear its orbit |
| Asteroid belt | Between Mars and Jupiter | Rocky and metal fragments |
Planet facts CDS asks
| Planet | Fact to remember |
|---|---|
| Mercury | Smallest and least massive; almost no atmosphereQ CDS 2019 (I): Mercury has the least mass, less than Mars. |
| Venus | Hottest; thick clouds of sulphuric acid; no moonQ CDS 2022 (II): the yellowish clouds of Venus are sulphuric acid. |
| Mars | Red from iron oxide; two moons, Phobos and DeimosQ CDS 2023 (II): carbon dioxide is about 95 to 97% of the air of both Venus and Mars. |
| Jupiter | Largest planet; a gas giant with no solid surface |
| Saturn | Rings of ice and rocky dust; most known moonsQ CDS 2023 (II): Mercury has no atmosphere, Jupiter no land, Saturn's rings are ice and dust; Venus has no moons. |
| Uranus | Spins on its side |
| Neptune | Farthest planet from the Sun |
Syzygy, perihelion and aphelion
| Term | Meaning | When |
|---|---|---|
| Syzygy | Sun, Earth and Moon in a straight line | Every new moon and full moonQ CDS 2016 (II): 'syzygy conjunction occurs at the time of perihelion only' is the wrong statement. |
| Conjunction | Moon between the Sun and Earth | New moon |
| Opposition | Earth between the Sun and Moon | Full moon |
| Perihelion | Earth nearest the Sun | About 3 January |
| Aphelion | Earth farthest from the Sun | About 4 July |
Common traps
Davis and Taylor are distractors for origin theories
W. M. Davis gave the cycle of erosion and F. B. Taylor argued for moving continents. Neither proposed the tetrahedral hypothesis, which is Lowthian Green's.
Planetesimal is Chamberlin, tidal is Jeans
Both theories use a passing star. The one about small solid bodies (planetesimals) is Chamberlin and Moulton's; the one about a tide of matter is Jeans and Jeffreys'.
Mars is the last rocky planet
Mars is red and far out, but it is a terrestrial planet. The gas planets begin after the asteroid belt, with Jupiter.
Nearest is not hottest
Mercury is nearest the Sun, but Venus is the hottest planet, because its thick carbon dioxide air traps heat.
Moon counts change
Older books say Jupiter has the most moons. New moons of Saturn were confirmed in 2019 and again later, and Saturn now has the most known moons. Read moon-count options with the date of the paper in mind.
The Earth is nearest the Sun in January
Perihelion comes about 3 January, in the northern winter. Seasons come from the tilt of the axis, not from the distance to the Sun.
Latitude, Longitude, Time and Maps
Learn this subtopic in the notesLocal time and longitude
Time difference from longitude
- 4 minutes of time
- 1 hour of time
The Earth's shape and rotation
| Fact | Value |
|---|---|
| Shape | Oblate spheroid, flattened at the poles |
| Circumference at the equator | About 40,000 km |
| Eratosthenes' two cities | Syene (Aswan) and AlexandriaQ CDS 2024 (II): Alexandria and Syene, not Troy or Thebes. |
| Direction of rotation | West to east |
| Angular velocity | 15° an hour, the same at every placeQ CDS 2024 (I): angular velocity is the same everywhere; linear velocity is greatest at the equator, least at the poles. |
| Linear velocity | About 1,670 km an hour at the equator; zero at the poles |
| Tilt of the axis | 23½° from the upright |
Latitude and what it tells you
| Line or place | Latitude or fact |
|---|---|
| Equator | 0° |
| Indira Point, Great Nicobar | About 6°45′ N, India's southernmost pointQ CDS 2017 (II): all three statements about latitude were true, including Indira Point at about 6°45′ N. |
| Panaji | About 15½° NQ CDS 2023 (II): Panaji is the closest of these four capitals to the equator. |
| Hyderabad | About 17½° N |
| Mumbai | About 19° N |
| Bhubaneswar | About 20° N |
| Tropic of Cancer | 23½° N |
| Arctic Circle | 66½° N |
| Parallels drawn every 1° | 89 in each hemisphere plus the equator: 179Q CDS 2025 (II): both statements true; 179 parallels, and a degree of latitude changes slightly in length. |
Isolines and diagrams
| Line or diagram | What it shows |
|---|---|
| Isotherm | Equal temperature |
| Isobar | Equal pressure |
| Isohyet | Equal rainfall |
| Isohel | Equal sunshine |
| Isonif | Equal snowfall |
| Isoneph | Equal cloudiness |
| Isohypse (contour) | Equal height above sea levelQ CDS 2023 (II): an isohypse joins equal height, not cloudiness. |
| Isobath | Equal depth of water |
| Scatter graph | Whether two sets of data are relatedQ CDS 2023 (I): population size against number of services needs a scatter graph. |
| Triangular graph | Three parts adding up to 100% |
Common traps
Angular speed is the same, linear speed is not
Every place turns through 15° an hour. The distance covered in that hour is greatest at the equator and falls to zero at the poles. A statement that mixes the two up is false.
Closest to the equator means lowest latitude
For places north of the equator, the one with the smallest latitude is nearest the equator. Compare the numbers; do not judge by how far south a city feels on the map.
The poles are not parallels
Counting parallels at 1° gives 179, not 181: the two poles are points.
IST is ahead, not behind
India is east of Greenwich, so Indian Standard Time is 5 hours 30 minutes ahead of GMT. A statement saying it is behind is false.
Opposite sides of Greenwich: add the longitudes
Between 30° E and 45° W the difference is 75°, not 15°. Subtract only when both places are on the same side of Greenwich.
One time zone for all of India
The Andaman and Nicobar Islands and Lakshadweep keep Indian Standard Time like the mainland. India has no second official time zone.
Isoneph, not isohypse, is cloudiness
Isoneph joins equal cloudiness. Isohypse is another name for a contour, joining equal height.
Two variables need a scatter graph
A pie chart or bar chart shows one set of data. To see whether two sets rise and fall together, plot them as points on a scatter graph.