The Relations of the Sun to the Earth
All this murmuring and glistening life on our earth planet has its source in the great sun which swings through our skies daily, sending to us through the friendly ether his messages of light and warmth—messages that kindle life in the seed and perfect the existence of every living organism, whether it be the weed in the field or the king on his throne.
At sunrise this heat which the sun sends out equally at all times of day and night, is tempered when it reaches us because it passes obliquely through our atmosphere-blanket, and thus traverses a greater distance in the cooling air. The same is true at sunset; but at noon, when the sun is most directly over our heads its rays pass through the least possible distance of the atmosphere-blanket and, therefore, lose less heat on the way. It is true that often about three o'clock in the afternoon is the hottest period of the day, but this is because the air-blanket has become thoroughly heated; but we receive the most heat directly from the sun at noon.
The variations in the time of the rising and the setting of the sun may be made a most interesting investigation on the part of the pupils. They should keep a record for a month in the winter; and with this as a basis, use the almanac to complete the lesson. Thus, each one may learn for himself which is the shortest and which the longest day of the year. There is a slight variation in different years; the shortest day of the year when this lesson was written, as computed from a current almanac, was the 22d of December; it was nine hours and fourteen minutes long. The longest day of the year was the 22d of June, and it was fifteen hours and six minutes in duration. On the longest day of the year the sun reaches its farthest point north and is, therefore, most nearly above us at mid-day. On the shortest day of the year, the sun reaches its farthest point south and is, therefore, farther from the point directly above us at mid-day than during any other day of the year.
Also the movement of the sun north and south is an interesting subject for personal investigation, as suggested in the lesson. Through quite involuntary observation, I have become so accustomed to the arc traversed by the points of sunrise as seen from my home, that I can tell what month of the year it is, by simply noting the place where the sun rises. When it first peeps at us over a certain pine tree far to the south, it is December; when it rises over the reservoir it is February or October; and when it rises over Beebe pond it is July. Only at the equinox of spring and fall does it rise exactly in the east and set directly in the west. Equinox means equal nights, that is, the length of the night is equal to that of the day.
So vast is the weight of the sun that the force of gravity upon its surface is so great that even if it were not for the white-hot fireworks there so constantly active, we could not live upon it, for our own weight would crush us to death. But this multiplying the weight of common objects by twenty-seven and two-thirds to find how much they would weigh on the sun is an interesting diversion for the pupils, and incidentally teaches them how to weigh objects, and something about that mysterious force called gravity; and it is also an excellent lesson in fractions.
The Relation of the Sun to the Earth
Leading thought—The sun which is the source of all our light and heat and, therefore, of all life on our globe travels a path that is higher across the sky in June than the path which it follows in December, and hence we experience changes of seasons. The lesson should be given to the pupils of the upper grades and should be correlated with reading, arithmetic and thinking.
1. What does the sun do for us?
2. At what time of the day after the sun rises do we get the least heat from it? What hour of the day do we get the most heat from it?
3. Is the sun equally hot all day? Why does it seem hotter to us at one time of the day than at another?
4. At what hour does the sun rise and set on the first of the following months; February, March, April, May and June?
5. Which is the shortest day of the year, and how long is it?
6. Which is the longest day of the year, and how many hours and minutes are there in it?
7. What day of the year is the sun nearest a point directly over our heads at mid-day?
8. Which day of the year is the sun at mid-day farthest from the point directly above our heads? Explain why this is so.
9. Standing in a certain place, mark by some building, tree or other object just where the sun rises in the east and sets in the west on the first of February. Observe the rising and setting of the sun from the same place on the first day of March and again on the first of April. Does it rise and set in the same place always or does it move northward or southward?
10. Is the sun farthest south on the shortest day of the year? If so, is it farthest north on the longest day of the year?
11. At what time of the year does the sun rise due east and set due west?
12. The sun is so much larger than the earth that its force of gravity is twenty-seven and two-thirds times that of the earth. How much would your watch weigh if you were living on the sun? How much would you yourself weigh if you were there?
13. Experiment. A shadow stick—Place a peg two or three inches high upright in a board and place the board lengthwise on the sill of a south window or where it will get the south light. Note the length cast by the shadow of the peg during a sunny day and draw a line with pencil or chalk outlining the tip of the shadow of the stick from 9 a.m. to 4 p.m. Make a similar outline a month later and again a month later and note whether the shadow traces the same line during each of these days of observation. Note especially the length of the shadow at noon.
Another excellent observation lesson for teaching the fact that the sun travels farther south in the winter, is to measure the shadow of a tree on the school grounds at noonday once a month during the school year. The length of the tree shadow can be measured from the base of the tree trunk, a memorandum being made of it.
14. When does the stick or tree cast its longest shadow at noon—in December or February? February or April? April or June? Why?
Topics for English themes—The size and distance of the sun. The heat of the sun and its effect upon the earth. What we know about the sun spots. Our path around the sun.
Supplementary reading—Starland, Ball; The Earth and Sky, Holden.