EV203 Part II Lesson 3

Describe the changes in the patterns of sunlight around Earth during the year, aside from the solstices and equinoxes. Explain how annual variations in day length and Sun angle change with latitude.

The vertical rays of the Sun strike Earth only between the Tropic of Cancer and the Tropic of Capricorn. After the March Equinox, the vertical rays of the Sun migrate north from the equator, striking the Tropic of Cancer on the June solstice. After the June Solstice, the vertical rays migrate south, striking the equator again on the September equinox and reaching the Tropic of Capricorn on the December solstice. After the December solstice, the vertical rays migrate northward, reaching the equator once again on the March equinox. Day length is only constant throughout the year at the equator-12 hours of daylight every day of the year. For all regions in the Northern Hemisphere up to the latitude of the Arctic Circle, after the shortest day of the year on the December solstice, the number of hours of daylight gradually increases, reaching 12 hours on the March equinox. After the equinox, day length continues to increase until the longest day of the year, on the June solstice. After the longest day of the year in the Northern Hemisphere, the June solstice, the pattern is reversed: the days get shorter in the Northern Hemisphere-reaching 12 hours on the September equinox. Day length decreases until the shortest day of the year, the December solstice. The annual variation in day length is the least in the tropics and greatest at high latitudes.

Universal Time Coordinated

aka Greenwich Mean Time. the mean solar time at the Greenwich meridian, adopted as the standard time in a zone that includes the British Isles.

circle of illumination

the dividing line between the daylight half of Earth and nighttime half of Earth

solar altitude

the height of the noon Sun above the horizon

declination of the sun (subsolar point)

the latitude receiving the vertical rays of the sun

Describe the changes in the patterns of sunlight around Earth on the June solstice, the September equinox, the December solstice, and the March equinox. In particular, explain how the changing relationships of the Earth to the Sun cause variations in day length and in the angle at which the Sun's rays strike the surface of the Earth.

On the June solstice, Earth reaches the position in its orbit where the North Pole is oriented most directly toward the Sun. The vertical rays of the Sun strike the Tropic of Cancer 23.5 degrees north of the equator. The circle of illumination bisects the equator so the equator receives equal day and night-12 hours of daylight and 12 hours of darkness. On the September equinox, the vertical rays of the Sun strike the equator. The circle of illumination just touches both poles, bisecting all other parallels- 12 hours of daylight and 12 hours of darkness. On the December solstice, the vertical rays of the Sun strike 23.5 degrees South, the Tropic of Capricorn. The circle of illumination reaches to the far side of one pole and falls short on the near side of the other pole, so areas north of the Arctic Circle are in continuous darkness, whereas areas south of the Antarctic Circle are in daylight for 24 hours. On the March Equinox is the same as the September Equinox.

Summarize the factors that cause the annual change of seasons.

1. The declination of the Sun (the latitude receiving the vertical rays of the sun-rays striking the surface at a right angle) 2.The solar altitude(the height of the noon Sun above the horizon) at different latitudes 3. The length of day(number of daylight hours) at different latitudes.

International Date Line

An arc that for the most part follows 180° longitude, although it deviates in several places to avoid dividing land areas. When you cross the International Date Line is crossed heading east (toward America), the clock moves back 24 hours, or one entire day. When it is crossed heading west (toward Asia), the calendar moves ahead one day.

Describe how time zones are used to establish actual times around the world. Discuss the relationship between time zones and the International Date Line.

Countries established 24 central meridians, 15 degrees of longitude apart, to divide the world into standard time zones. The mean (averaged) local solar time of the Greenwich prime meridian was chosen as the standard for the entire system. The prime meridian became the center of a time zone that extends 7.5 degrees of longitude to the west and 7.5 degrees to the east of the prime meridian were set as the central meridians for the other time zones. When you cross into the next time zone from west to east, the time becomes 1 hour later. The International Date Line is in the middle of the time zone defined by the 180 degrees meridian. There is no time change when you cross the International Date Line, only the calendar day changes, not the clock. When you cross the International Date Line from west to east, it becomes 1 day earlier; when you move across the line from east to west, it becomes 1 day later.