Motions of the Earth

Rotation The Earth rotates on its axis. ‘Rotation’ is the spinning of the Earth, from west to east, on its axis. It makes a complete 3600 turn in 24 hours at a speed of 1600 km/h at the equator. Most places on earth experience a period of daylight and one of darkness during this 24 hour period.

The most important effects of rotation are related to changes in light, heat and humidity that occur between night and day. By imposing a daily rhythm of day and night on many things, this motion has had a profound effect on life and development of all forms of plants and animals. The earth’s rotation also has an important influence on the movement of air and water on the earth’s surface.

Revolution Earth ‘revolves’ around the sun once in 365.242 days. (Leap Year: add one (1) day to February every four (4) years to make up the four 1/4 days). It travels an average distance of 150,000,000km. The average velocity of the earth is 106,300 km/h. The path or orbit made by the earth as it revolves around the sun is elliptical in shape rather than a circle. The distance from the sun at aphelion (apogee) is 152 million km (July 4) and at perihelion (perigee) the distance is 147 million km (Jan. 3). This variation has little effect on the temperature of the earth.

The path followed by the earth around the sun can be thought of as forming a flat surface or plane. This is known as the plane of orbit or the plane of the ecliptic. The earth’s axis is tilted 23.5 degrees from the perpendicular to the plane of orbit, in other words the angle between the axis and the plane of orbit is 66.5 (90 - 23.5).

The axis always maintains this degree of tilt, remaining parallel to all its previous positions. This is known as the ‘parallelism of the axis’. The revolution of the earth about the sun on an inclined axis has two very important effects: seasons (seasonal temperature differences) and variations in the length of day and night. Neither would happen if the axis was not inclined.

Four important dates should be noted and are related to five important lines of latitude. Time to review Latitude and Longitude! 1. June 21st: The sun’s direct rays are on the Tropic of Cancer. This day is called Summer Solstice in the Northern Hemisphere. The days are longer because on this day the Northern Hemisphere is inclined towards the sun. During one complete rotation, all places in the Northern Hemisphere spend a greater proportion of the 24 hours in daylight than they do in darkness. As you travel further north the day becomes longer. Any point north of the Arctic Circle receives 24 hours of daylight and any point south of the Antarctic Circle receives 24 hours of night.

This is an important slide:

2. Sept. 21st: The sun’s direct rays are over the Equator 2. Sept. 21st: The sun’s direct rays are over the Equator. This day is called Fall (autumnal) Equinox. The days are very close to being 12 hours day and 12 hours night. 3. Dec. 21st: The sun’s direct rays are on the Tropic of Capricorn. This time is called Winter Solstice in the Northern Hemisphere. Days are shorter. Nights are longer. 4. March 21st: The sun’s direct rays are over the Equator again. This day is called Spring (vernal) Equinox. The days are very close to being 12 hours day and 12 hours night.

Equinox

Solstice

There are two principal causes for the seasonal differences in temperature created by the tilt of the earth and the curvature of the earth. INTENSITY and DURATION. Duration Duration refers to the amount of daylight any place receives. The longer the length of daylight a place experiences (ie. the greater the duration), the greater the amount of heat that place will receive. If you study the diagram on the next slide you will notice how the position of the sun overhead and its movement in the sky during the day, determines the length of sunlight. The length of day or the arch that the sun travels in the sky depends on the solstices and equinoxes.

Follow the Arches!

Intensity Variations in intensity are caused by the difference in the angle at which the sun’s rays strike the earth’s surface. If you study the diagram on the next slide, you will notice that in Fig. 2 the curvature of the earth forces the sun’s rays to cover a larger area than in Fig. 1. It is colder in Fig. 2 than in Fig. 1. During winter solstice when the northern hemisphere is tilted away from the sun the intensity of the sun is even less therefore it is even colder in Fig. 2.

This is another way of looking at the concept of Intensity.

Time Zones There is one last effect caused by the rotation of the earth. Not all points on the earth begin their day at the same time. All places lying on the same line of longitude have the same time. In theory, this means that the place you live in actually has a different solar time of day from any other place located east or west. Depending on the distance the difference could be seconds, minutes or hours.

Before high speed travel and worldwide communications differences in time were not that important. Today we need a system of time zones. Sir Sanford Fleming, a Canadian, developed a standardized time zone system for the earth in 1886. The earth is divided into 24 time zones. Since the earth is a 360 degree circle. The sun movers through 15 degrees of longitude every hour. (360/24=15)

Some modifications to the time zone boundaries were made to ensure that political regions remained in the same time zone. Since the Prime Meridian (0 degrees longitude) is located at Greenwich, England, it was decided that the time zones would start there. The 180 degree line (on the opposite side of the earth) became the International Date Line. (If you cross it going from west to east you would go back one day on your calendar, in a sense you are gaining a day).

Think of the economics behind daylight saving time. Especially in urban areas, many human activities begin well after sunrise and continue after sunset. Daylight saving time is obtained by setting ahead all clocks by one hour the first Sunday in April and is discontinued on the last Sunday in October. Think of the economics behind daylight saving time.

The End