Understanding the Earth’s Seasons
UNDERSTANDING THE MOVEMENT OF THE EARTH AROUND THE SUN
Generally your Sundial should give you pretty good indication of the correct time.
Understanding the earth’s seasons is difficult for not only high school students but for a lot of adults also.
The main reasons for this is-
That the earth’s orbit is elliptical ……….(as seen in images of the suns revolution around the sun).
This causes the misunderstanding that this elliptical orbit takes the earth closer to the sun therefore being the reason for producing summer.
Six months later when the earth is at the opposite end of its orbit around the sun it is further away from the sun therefore causing winter.
The earth’s orbit is in fact almost circular so the variations in distance would have little or no effect.
The reason we have the differing seasons is because the earth is tilted on its axis at an angle of 23°.5.
This causes the suns rays to strike the earth’s surface at different angles as it orbits the sun.
During the summer in the Southern Hemisphere the earth is tilted towards the sun.
The suns rays strike the earth’s surface at a more direct angle which is the reason that causes the atmosphere to heat up and the days to get longer days.
During the Southern Hemisphere’s winter the suns rays strike the earth at a more oblique angle giving less heat and the days to get shorter..
The Northern Hemisphere experiences the opposite seasons.
‘A little known fact is that when the earth’s Northern Hemisphere is tilted towards the sun and is at its closest approach to the sun it is the Northern Hemisphere’s winter.’
Kepler’s 2nd Law: The Speeds of Planets
Kepler’s second law, he discovered by trial and error that the line connecting the planet and the Sun sweeps out equal area in equal time.
Study the animation and you will see what Kepler found is that it takes the same amount of time for the blue planet to go from A to B as it does to go from C to D. But the distance from C to D is much larger than that from A to B. the brown coloured regions have the same area.
So the planet must be moving faster between C and D than it is between A and B. This means that when the earth is near the Sun in its orbit, it move faster than when it is further away.
Here is a little more information for you which may be of some help to the school students to understand the movement of the earth around the sun.
Generally, your Sundial should give you pretty good indication of the correct time. However, at certain times of the year you may notice a slight difference of it being a little fast or slow.
The differences reach a peak of around 14 minutes mid-February (when the” sun time” is slow to ‘clock time”). And around 16 minutes at the beginning of November (when the ‘Sun time” is fast to “clock time”).
However, at other times these time differences reduce considerably, depending on the position of the earth as it revolves around the sun.
The seasonal time differences are caused by the differing speed of the earth as it travels around the sun being faster on its closest approach to the sun. And slower when the earth is at its furthest distance from the sun.
The earth travels around the sun in a slightly elliptical orbit. To tell the correct time it is very important that the person using the Human sundial to stand with one foot either side of the central line of the date scale facing South. And not to one side on the actual month itself which is a common error as seen in some pictures.
Obviously, the correct position of where to stand must be estimated according to the actual day of the month.
The Seasonal changes are due to the 23.5 degree tilt of the Earth’s rotational axis.
FOR MORE INFORMATION
CONTACT: George Marshall
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