The following is a specific question about the seasons in regards to axis vs. orbit:If the earth's perihelion in orbit is on January 2 (the closest point to the sun at 91,400,005 miles) and the...

The following is a specific question about the seasons in regards to axis vs. orbit:

If the earth's perihelion in orbit is on January 2 (the closest point to the sun at 91,400,005 miles) and the earth's aphelion in orbit is on July 6 (the furthest point from the sun at 94,512,258 miles), then there is a difference of more than three million miles during the course of our year.  Why then, if the earth's equatorial diameter is a measly 7925 miles, . . . WHY does the earth's axis determine the seasons while the three million mile difference in orbit matters not?

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krishna-agrawala | College Teacher | (Level 3) Valedictorian

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The distance of earth from sun due to elliptical orbit of the earth affects the difference in heating and cooling, or the temperature, of the whole earth to almost same degree. Therefor it does not explain the types of weather on the earth at the same time. For example, countries in northern hemisphere experience winter at Christmas time while those in southern hemisphere experience summer. On the other hand when countries in northern hemisphere have summer, those in southern hemispheres experience summer.  Also this variation in distance of earth from sun is just about 1.7 percent  of the average distance.which is  not significant enough to account for the extent of variation in temperature actually experienced during different seasons.

The tilt of the earth does affect the distance from sun of places at different altitudes on earth, and this does have some effect on temperature of the place. But much more than the distance, it is the angle at which the sun rays fall on a place on earth determine the amount of heat in form of sun rays received per unit area of the earth. When the sun rays fall directly at right angles to surface of earth, the area covered by same amount of sun rays is minimum, resulting in maximum heat received per unit are. As the angle of incidence of sun rays deviates from the perpendicular to surface of earth the area covered by the sun rays increases, and with that heat received per unit area decreases. The effect of the angle of incidence of sun rays on heat of sun received a a place is also affects the intensity of sunlight at a place during the day. The intensity of sunlight is maximum at noon when sun is at the highest point in the sky, and therefor closest to the perpendicular direction. As we move away from the noon time, either before it towards sunrise, or after it towards sunset, the intensity of sunlight decreases. This intensity is minimum at sunrise and sun set when the sun rays fall almost parallel to the earth's surface.

The spherical shape of the earth causes the sun rays to fall at different latitudes on earth at different angles. Thus when the sun rays are falling at right angles at the equator, these are almost parallel at the two poles. The tilt of the earth, combined with its orbit round the sun, cause the place at which the sun rays fall at right angles to keep on shifting between 23.5 north (Tropic of Cancer) and 23.5 south (Tropic of Capricorn) latitudes. This is the reason why we experience changes in weather in an annual cycle.

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