The Earth, Its Motions and Time

What you need to know

• The earth is an oblate spheroid: a sphere flattened at the poles and bulging at the equator. Evidence includes circumnavigation, a ship disappearing hull first over the horizon, the circular shadow the earth casts on the moon during a lunar eclipse, the fact that sunrise reaches eastern places before western ones, and satellite photographs.
• Lines of latitude are parallels running east to west, numbered 0 degrees at the equator to 90 degrees at each pole. They are true circles of decreasing size towards the poles, they never meet, and one degree of latitude is roughly 111 km everywhere on the globe.
• Key parallels are the equator at 0 degrees, the Tropic of Cancer at 23 and a half degrees north, the Tropic of Capricorn at 23 and a half degrees south, the Arctic Circle at 66 and a half degrees north and the Antarctic Circle at 66 and a half degrees south.
• Lines of longitude, or meridians, run from pole to pole, numbered 0 degrees at Greenwich to 180 degrees east and west. Unlike parallels they are all the same length, they converge at the poles, and the distance between them shrinks from about 111 km at the equator to zero at the poles.
• Rotation is the spin of the earth on its own axis from west to east, completed once in 24 hours. It causes day and night, the apparent east to west movement of the sun, the difference in time between meridians, the deflection of winds and ocean currents, and the daily rhythm of tides.
• Revolution is the movement of the earth round the sun in an elliptical orbit, completed in 365 and a quarter days. Because the axis is tilted at 23 and a half degrees from the vertical, or 66 and a half degrees to the plane of the orbit, and because that tilt stays pointing the same way all year, different hemispheres lean towards the sun at different times. That is what produces the seasons.
• On 21 March and 23 September the sun is overhead at the equator, day and night are equal everywhere, and these are the equinoxes. On 21 June the sun is overhead at the Tropic of Cancer, giving summer in the northern hemisphere, and on 22 December it is overhead at the Tropic of Capricorn, giving summer in the southern hemisphere. These are the solstices.
• Because the earth turns 360 degrees in 24 hours, it turns 15 degrees in one hour and 1 degree in 4 minutes. Places east of you see the sun earlier, so their time is ahead; places west of you are behind. East add, west subtract.
• Local or sun time is the time at a particular meridian, and it changes with every degree of longitude. Standard time is the single time adopted by a whole country or zone, usually based on a convenient central meridian. Nigeria uses the 15 degrees east meridian, so Nigerian standard time is GMT plus one hour.
• The International Date Line roughly follows the 180 degree meridian but zigzags to keep island groups and countries on the same date. Crossing it travelling westward you add one day; crossing it travelling eastward you subtract one day.
• A solar eclipse happens when the moon comes between the sun and the earth and throws its shadow on the earth, and it can only occur at new moon. The dark inner shadow is the umbra, where the eclipse is total, and the lighter outer shadow is the penumbra, where it is partial.
• A lunar eclipse happens when the earth comes between the sun and the moon so that the earth's shadow falls on the moon, and it can only occur at full moon. A lunar eclipse lasts longer and is seen from a much larger part of the earth than a solar eclipse.
• Day length varies with latitude and season: at the equator day and night stay close to 12 hours all year, while beyond the Arctic and Antarctic Circles there are periods of continuous daylight and continuous darkness. Nigeria, lying between about 4 degrees and 14 degrees north, has only a small variation in day length through the year.