Chapter 2, Problem 1E

The Earth–Sun orientation will change throughout the year as Earth revolves around the Sun. Using Figures 2-3 and 2-4 as models, sketch two similar diagrams for each date given to the right and below. First draw Earth’s axis and equator on the globe. Then, on the “sunny side” of the globe, draw a short line representing a flat surface at 66.5° N, 30° N, 0°, and 23.5° S, a stick figure at each site with the Sun’s rays striking the flat surface at the feet of the stick figure. On the profile view, draw the Sun’s rays striking the flat surface transcribing the angles that you drew on the globe.

Sun’s rays striking Earth on March 21.

Profile view at Earth’s surface: solar noon on March 21.

Sun’s rays striking Earth on June 21.

Profile view at Earth’s surface: solar noon on June 21.

To determine

Sketch representation of the incidence of the Sun’s rays on the images given in the question, on March 21 and June 21.

Answer to Problem 1E

On March 21, which is the time of vernal equinox on the Northern Hemisphere and autumn equinox in the Southern Hemisphere, there will be direct incidence of sunlight on the equator as well as on the other latitudes north and south of the equator. The figures given below represent the angle of incidence of insolation on various latitudes.

On June 21, which is the time of Summer Solstice in the Northern Hemisphere, the Sun will be more direct over the Northern Hemisphere, indicating the beginning of Summer season. The North Pole will be inclined at an angle of ${23.5}^{\circ }$ toward the sun during June Solstice. Therefore, the angle of ${23.5}^{\circ }$ has to be added to or subtracted from the March 21 profile to obtain the June 21 profile view, at each position. The figures given below show the angle of incidence of insolation over different latitudes of the Earth during the June Solstice.

Explanation of Solution

On March 21, the Earth exhibits an equinox time. It is generally known as vernal equinox in the Northern hemisphere, while in the Southern Hemisphere, it is known as autumn equinox. During equinox, neither of the poles would be specifically inclined toward the Sun nor would any of the poles be receiving more insolation. Instead, both the hemispheres will experience equal duration of day and night. The insolation might fall on the Earth vertically, which means the equator along with latitudes above and below it receives vertical rays of the Sun.

June 21 is referred to as the June Solstice or Summer Solstice in the Northern Hemisphere. Solstice is the time during which one of the hemispheres will be facing the Sun more directly. During June Solstice, the Earth’s rotation axis over the North Pole is most directed or inclined (by nearly ${23.5}^{\circ }$) toward the Sun. Simultaneously, the South Pole and the Southern Hemisphere will be facing away from the Sun, and hence will start experiencing winter. The Northern Hemisphere will have the longest day and shortest night on this day.