In December, the situation is reversed: the Southern Hemisphere leans into the Sun, and the Northern Hemisphere leans away. We see Earth at different seasons as it circles the Sun. In spring and autumn, the two hemispheres receive more equal shares of sunlight. There are two effects we need to consider.
You can get a similar effect by shining a flashlight onto a wall. If you shine the flashlight straight on, you get an intense spot of light on the wall. Like the straight-on light, the sunlight in June is more direct and intense in the Northern Hemisphere, and hence more effective at heating.
The second effect has to do with the length of time the Sun spends above the horizon [link]. On June 21, the Sun rises north of east and sets north of west. For observers in the Northern Hemisphere of Earth, the Sun spends about 15 hours above the horizon in the United States, meaning more hours of daylight. On December 21, the Sun rises south of east and sets south of west. It spends 9 hours above the horizon in the United States, which means fewer hours of daylight and more hours of night in northern lands and a strong need for people to hold celebrations to cheer themselves up.
On March 21 and September 21, the Sun spends equal amounts of time above and below the horizon in both hemispheres. As we saw in Observing the Sky: The Birth of Astronomy , an equivalent way to look at our path around the Sun each year is to pretend that the Sun moves around Earth on a circle called the ecliptic. As a result, where we see the Sun in the sky changes as the year wears on.
In June, the Sun is north of the celestial equator and spends more time with those who live in the Northern Hemisphere. It rises high in the sky and is above the horizon in the United States for as long as 15 hours. Thus, the Sun not only heats us with more direct rays, but it also has more time to do it each day. There the June Sun is low in the sky, meaning fewer daylight hours.
In Chile, for example, June is a colder, darker time of year. In December, when the Sun is south of the celestial equator, the situation is reversed. On or about June 21 1 the date we who live in the Northern Hemisphere call the summer solstice or sometimes the first day of summer , the Sun shines down most directly upon the Northern Hemisphere of Earth.
The situation is shown in detail in [link]. This latitude, where the Sun can appear at the zenith at noon on the first day of summer, is called the Tropic of Cancer. That circle of latitude is called the Arctic Circle. Earth on June This is the date of the summer solstice in the Northern Hemisphere. Note that as Earth turns on its axis the line connecting the North and South Poles , the North Pole is in constant sunlight while the South Pole is veiled in 24 hours of darkness. The Sun is at the zenith for observers on the Tropic of Cancer.
Many early cultures scheduled special events around the summer solstice to celebrate the longest days and thank their gods for making the weather warm. In spring, the Sun will rise farther and farther north of east, and set farther and farther north of west, reaching the maximum around the summer solstice.
Now look at the South Pole in [link]. At the equator, all days of the year have the same number of hours of light and dark.
Between the two tropics zones, which includes the equator, the Sun is directly overhead twice per year. Outside the tropic zones, whether to the south or north, the Sun is never directly overhead. These circles are as far from the poles as the Tropic of Cancer and the Tropic of Capricorn are from the equator. On that one day, the Sun traces a complete circle just above the horizon as the Earth rotates.
As you go closer to the poles, you have more and more days when the Sun does not set or rise , until, at the poles, the Sun remains above or below the horizon for six months at a time. A webcam at the South Pole captured this picture on June 18, , at high noon. They orbit exactly above the equator, at a very great distance 22, miles , which allows them to make just one orbit per day. They "hover" over one point on Earth's equator.
That way, they have a full view of almost one-half of Earth and can keep a continual watch on developing weather. Assume the local surface of the Earth is flat, as is the thickness H of the atmosphere above the surface. The solar intensity at the top of the atmosphere is the amount of sunlight per unit area that is incident on the top of the atmosphere I top. But some amount of that sunlight is absorbed in the atmosphere and doesn't make it to the surface; thus, the amount of heat that reaches us depends on what happens to the sunlight as it passes through the atmosphere.
What happens in the atmosphere is that a great amount of sunlight is scattered and absorbed we call this extinction. If the Sun is directly overhead, sunlight passes through this atmosphere with a minimum of attenuation. The total amount of sunlight that is removed is proportional to s and depends on the amount of extinction k per unit distance along the light path through the atmosphere.
Mathematically, the sunlight that reaches the Earth is found as:. The changing distance from the Sun, caused by the Earth moving from perihelion closest approach to aphelion furthest distance in it's elliptical orbit around the Sun is not a primary contributor to seasonal change. The dates of perihelia and aphelia change each year. Exact dates can be found here. Arctic circle:
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