Suzanne Petersen, Lamprey River Advisory Committee
Winter solstice is significant to people and nature as the shortest day of the year. It marks the official start of winter on the calendar. Culturally, it is significant as a turning point… the longest night yields to lengthening daylight. A solstice, however, is most important astronomically. Solstices occur because of several astronomical realities:
The amount of heat the Earth receives from the sun in the form of solar radiation is not determined by the length of day, but length of day is a contributing factor.
Seasons as we know them are caused not so much by changes in the number of hours of sunlight but in the intensity of sunlight. When the sun shines most directly on a certain place on Earth, its intensity and ability to warm are greatest. Sunshine on your face in the summer is more intense than sunshine on your face in winter, regardless of the temperature outside. Sunshine at the Equator and in tropical areas is more intense than in areas that lie between the tropics and the poles. A quick rule is that when a shadow at noon is short, the sun is more intense. When a shadow at noon is long, the sun is less intense.
(Winter solstice. The sun in this illustration is much smaller and closer than reality, but it does show that at the Tropic of Capricorn, the sun’s rays hit the earth at a perpendicular angle during winter solstice. Note that points north of the Arctic Circle get no sun at all.)
flashlight with intense beam
graph paper with fine grid
mechanism to suspend flashlight so it shines vertically onto a desk’s edge
angle of tilt
# of squares lit
average # of squares of 3 trials
% solar radiation per square
formula: % solar radiation per square = # of squares lit at zero degrees / # of squares lit at (x) degrees X 100
As the angle of tilt increases, the number of lit squares ___________________.
As the angle of tilt increases, the percentage of solar radiation per square ___________________.
Place a sheet of graph paper onto the clipboard.
Suspend flashlight about one foot above the desk so that it shines straight down onto the desk’s edge, with half the center of the beam on the desk.
Place the clipboard flat on the edge of a desk so that the light shines completely onto the graph paper.
Outline the area covered by light on the graph paper. Count the number of lit squares and note on data sheet. Be sure to be consistent as to what is considered to be “lit”. Repeat two times using fresh areas of the graph paper each time.
Move the clipboard so that the light shines on a new area. Using a protractor, tilt the clipboard by 5 degrees. Count the number of lit squares and record on data sheet. Repeat two times.
Repeat this process, using fresh graph paper when necessary, for 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, and 60 degrees.
Does the graph make a straight line? __________ Does the graph do something else? ________________________________________________
Summarize the graph by filling in the blank: (choose “increases” or “decreases”
”As the angle of tilt increases, the area lit __________________________.”
Based on the graph, predict how many squares will be lit with a 65 degree angle. _______________
Test your hypothesis by measuring the lit area at 65 degrees. _______________ Predict how many squares will be lit at 70 degrees. _______________
Test your hypothesis. _______________
Complete the fourth column on the data sheet, % solar radiation per square:
Create a new graph. Mark the x axis as “degrees of tilt” and mark the y axis as “% solar radiation per square”. Graph these two sets of data.
Connect the plot points with a smooth line. Describe the relationship by filling in the blank: (choose “increases” or “decreases”)
“As the degree of tilt increases, the percentage of light per square of area ____________________.”
According to the results of this experiment, you know that the area of light hitting a surface at an angle is not the same as light hitting a surface straight on. Assuming the amount of energy contained in a beam of light is constant, and the ability of a surface to absorb energy is constant, is the energy absorbed by a square of surface going to be the same if the light strikes at an angle? Explain. ___________________________________________________________________ ___________________________________________________________________
Based on your data, **what percentage of light is absorbed by a square at 45 degrees versus straight on?** _______________
Here in New England, we live approximately half way between the Equator and the North Pole, or 45 degrees north latitude. Because of Earth’s tilt, New England receives little sun in the winter and lots of sun in the summer. During the spring and fall equinoxes, however, the Earth’s tilt does not have an impact on the amount of light energy. Forty five degrees north latitude is 45 degrees tilt. During the equinoxes, New England receives as much day light (twelve hours) as the Equator, but clearly the temperature is very different in these two areas. The Equator is hot, and New England is warm to cool. Based on your answer to the question** above, what percentage of the Equator’s sun does New England receive during an equinox? _______________
During the winter solstice, when the north side of the Earth is pointing its maximum angle away from the sun, what is the approximate angle of solar radiation hitting New England? (latitude plus Earth’s tilt) _________________ Based on your graph, what is the approximate percentage of solar radiation per square in New England during winter solstice? _______________
The Arctic Circle lies at approximately 66.5 degrees from the Equator.
What percentage of the sun’s energy would a square located there receive during an equinox? _______________
What angle is the Arctic Circle during winter solstice? _______________ Approximately what percentage of solar radiation per square is being received at that time? _______________
During summer solstice, the North Pole receives 24 hours of daylight.
What is the angle of solar radiation at that time? _______________
What is the percentage of solar radiation per square area? _______________
Why isn’t the North Pole hot during summer solstice? ___________________________________________________________________
The data you collected were based on a flat surface, but the Earth is not flat. If you were to shine the flashlight’s beam onto the globe at the same distance as from your paper, would it cover more, less, or the same amount of area as it did on flat paper? _______________
Scientists often choose to study solar radiation at noon. Why would they choose this time versus some other time?___________________________________________ ___________________________________________________________________
What might be meant by “solar noon”? ____________________________________
Is solar noon the same time for all places in a time zone? _______________
Scientists can determine very precisely when winter solstice occurs. If they say that the winter solstice will occur at, for example, 3:17 A.M. EST, what does that mean? On your globe, locate New England and the Eastern Standard Time zone. If the time in New England is 3:17 A.M., go east 9 longitude zones so that the time is 12:17 P.M.. Trace the time zone down the globe until it reaches the Tropic of Capricorn. At exactly 12:17 in that time zone, the sun’s rays will strike the Earth straight on somewhere on the Tropic of Capricorn and there will be no shadows there. That will indicate the northern-most point of Earth’s yearly revolution around the sun. Shortly afterward, the Earth’s continuing journey will reveal that the sun has begun to cast shadows.
What astronomical factors contribute to seasons in New England? Based on your data, what factor is the most important? Why?How does the general public define winter solstice? How would an astronomer define winter solstice? Which definition better explains why winter solstice is cold?
Why are the tropics and Equator so warm? Why are areas north and south of the tropics cooler?
Why are the polar regions so cold, even though they sometimes receive 24 hours of day light?