earth turns on an imaginary line called its axis the north and south poles are at the ends of this axis the motion is called rotation it takes just about 24 hours for earth to make one rotation earth travels in a slightly elliptical orbit around the sun with one revolution taking 365 and a quarter days the average distance between the sun and earth is 149.6 million kilometers but because earth's orbit is not quite round earth is predictably farther from and closer to the sun during its annual circumnavigation perihelion which is the closest point occurs about january
third and the distance is 147 million kilometers aphelion the furthest point is around july 4th the distance is 152 million kilometers because earth is closest to the sun around january 3rd common sense might suggest that is when earth would be warmest people in the southern hemisphere would not in agreement because january is summer down there in the northern hemisphere however we know this is not the case earth as a whole does receive more energy from the sun in january at any other time of the year but that is not the reason for the seasons proximity
to the sun is not the variable that determines summer and winter it's the tilt of earth's axis earth's axis is tipped by 23 and a half degrees from vertical to with respect to the plane of its orbit around the sun as earth revolves around the sun in its elliptical orbit it also rotates on its tipped axis one rotation every 24 hours by coincidence the north pole aims towards a star called polaris or the north star at this time in history no matter where earth is in its orbit the north pole always points towards the north
star during perihelion the north pole is leaning away from the sun this is the time of darkness for earthlings living north of the arctic circle days are shortest and nights are longest for the northern hemisphere the season is winter half a revolution later the north pole still pointing at the north star is leaning towards the sun the days in the northern hemisphere are longer at the same time the south pole angles away from the sun and the southern hemisphere experiences shorter days and longer nights think about the last time you spent the whole day outside
in the yard playing riding your bike or going to your favorite park you start walking and playing as the sun rises over the horizon you see the rays of sun angle their way over the hills or through the trees and feel slight warmth on your face the sun moves steadily higher in the sky until at noon it's beating nearly straight down on your sweaty face and is boiling hot time to rest in the shade over six hours or so you experience the effect of changing light intensity on the ground as the angle of the sun's
rays changed early in the morning earth was illuminated by light that hit the surface at a very small angle perhaps only 10 degrees the consequence was that the beam was spread over a large area the concentration of radiation was reduced because it was distributed across a large area the lower the solar angle is the less intense the light is on earth's surface because energy in the light beams spread over a larger area you can observe this effect when you shine a flashlight on the floor across the room as compared to shining it straight down between
your feet the beam has exactly the same amount of light in both instances but the beam at spot 2 spreads out over a larger area the intensity of light per unit is less than spot 2 than it is in spot 1 which is illuminated from directly above 90 degrees solar angle affects the intensity of light on an area which in turn affects the amount of energy which in turn affects the amount of heat transferred to an area of earth during the day and affects seasonal heating near the equator the sun always transits high in the
sky solar angle is close to 90 degrees and light intensity on the ground is high the farther north or south you go from the equator the more significant the change in solar angle becomes during part of the year the reduced energy concentration caused by decreasing solar angle is the major factor responsible for winter weather earth's atmosphere also affects how much radiation reaches earth's surface light rays striking earth's surface at a small angle between 20 degrees or 30 degrees pass through more of earth's atmosphere than rays striking the surface at larger angles 80 or 90 degrees
as small angle rays travel through the atmosphere it absorbs more energy leaving less energy to reach the surface large angle rays pass through less atmosphere so more energy reaches earth's surface we take night and day for granted the sun comes up the sun goes down because earth is tilted on its axis however the lengths of days and nights change as the year progresses remember when it's summer in the northern hemisphere the north pole angles towards the sun resulting in more hours of daylight in fact the sun does not set at the north pole at any
location north of latitude 62 and a half degrees north which is the arctic circle daylight lasts 24 hours on the summer solstice at the north pole a day lasts six months followed by six months of night
