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Shadows on the Earth

When the sunlight reaches an object on Earth, it creates a shadow behind it. If you begin to watch the shadow more closely you will notice that it moves. Likewise, if the shadow of the same object is observed at various times of the day or the year, it is easy to notice the change in the length of the shadow. To the astronomer exploring the shadows is a part of scientific research. It helps to determine for example an object’s location on Earth and the current season. In the picture, there is a branch of a pine in the snow. The photo was taken at noon in February in Enontekiö, Lapland (picture IH, Finland).



Aim: Study how to determine time and place by means of shadows.


Explorations: The students know the concept shadow from everyday life. They know it has to do with the sunlight and they are even able to exploit it. Every one of them has certainly sought shelter in a shadow of a tree or some opaque object on sultry days. In the games of childhood, a child’s own shadow may have been a funny, mysterious playmate. Discuss these everyday shadows with the students. Explore the formation of shadows by using a bright lamp, a small object and a screen in a darkened classroom.


Take an excursion outdoors and use discussion to explore how the shadows can be utilized in astronomical measurements.


1)  Explore on a sunny day on the schoolyard shadows formed by various kinds of objects. Discuss the direction, length and width of the shadows. Stop for a moment at some shadow and notice how it changes. Each student can write down notes of his observations, draw a picture of them and later in the classroom make an artistic drawing.


2)  Give students, working in pairs or as a team, a task to explore as function of time the motion of a shadow produced by a stick vertically fixed on the ground. The students can fix the stick on the ground by themselves or they can choose any existing vertical thin bar for the purpose. At even intervals the students mark the location of the shadow on the ground by using small stones or balls. To ensure a valid result, several measurements need to be made. Two or three are not enough.  Discuss the speed of the motion of the shadow and how it is related to the motion of the Sun. This will later support the understanding of a sundial.


3)  Plan together a simple sundial on the schoolyard. Since we have a schedule for building a proper sundial, let’s carry out this quick proto model as a flat sundial. On the schoolyard, choose a place peaceful and without distractions. Fix a thick stick on the ground with enough space on the northern side of the stick. Draw on that side a semicircle with the stick as a centre. The semicircle can be drawn either on the ground or on a cardboard or cloth spread on the ground. Mark the locations of the shadow once an hour at the same intervals on the circle line. It is especially important to make sure that the location of the noon on the line is exact. If it is possible, draw lines from the stick to the location signs. The result compares to a clock face. Discuss the results and measuring methods as well as possible bias factors.


4)  Explore the length of a shadow on various areas of the globe. The students, working in pairs, prepare a big styrox ball to represent the Earth. On the ball they draw with a marker pen the equator, the polar circles, the tropics and the poles. Fix pieces of thin wooden sticks of the same size into the ball both on the northern and the southern hemisphere as well as on the equator and the circle lines. Explore the shadows in a darkened classroom using a bright lamp representing the Sun. Change the position of the Sun up and down indicating seasons on both hemispheres. Find the connection between the length of the shadow and the latitude. Discuss the discoveries non-stop. Finally, as a repetition, present the results of the experiment to all. In the picture below there are the shadows of two people at the same time and place as a branch of a pine in the picture above (picture IH, Finland). Since the timing of the photos is the same, the declination of the Sun calculated from both pictures is also the same.





5) * The shape of the path of the Sun may be explored by means of a shadow. It is possible to carry out the measurement either in the classroom or out on the schoolyard. A pencil or some other pencil-like object can be used as a stick producing a shadow in the classroom. A stick mentioned in exercise 2 can be used in measurements outdoors. The location of the tip of the stick is carefully marked at fairly short intervals on the ground or cardboard. The intervals need not be equal since the marks are finally connected by drawing a line indicating the path. For further processing it is useful to write the time of observation down to every observed location of the shadow.  Use a paper sheet when discussing with the students the shaping of the path. The Sun is at its highest in the sky at about noon. The orbit line, on the contrary, is at noon at its lowest because of the shortening of the shadow.  


6) * The most enthusiastic students may be eager to try and explore how the shades of the eclipses take shape. This exploration can be carried out by using sensitive meters. An object creating a shadow needs to be high and relatively narrow, e.g. a water tower or a beam of a bridge. Measure the amount of light with a very sensitive light gauge from one edge of the shadow via the centre to the other edge. This series of measurements will be made at various distances from the object. Draw the discoveries to graphs on a paper.  Try to distinguish the areas of an umbra and a penumbra just as they show up during the eclipses.


7) * If possible and if there is enough interest, explore the shadows formed by the Moon. Compare the results to the explorations of the shadows formed by the Sun. Discuss the benefits and the obstacles.


Methods: Explore the phenomena outdoors by using simple instruments. The most usual method is to use a wooden stick to produce shadows. Explore the motion of a shadow as a function of time. Describe the measured results as graphic drawings.


Materials: Rods, sticks, styrox balls, craft materials, cardboard, cloth, lighting meter, watch.


Pondering: There are many topics that may be discussed with the students, e.g. the meaning of the zero shadow in positioning, the meaning of the longitude in defining the path of the Sun as well as the question: how the shadow formed by the Moon differs from that of the Sun.


Evaluation of the results: Discuss the following topics: reliability of measurements using a shadow stick, veracity of the shape of the Sun’s path, finding the location of noon and making conclusions of it, comparison with the zero meridian. Compare the drawings of the students and try to find explanations to the differences found. Recall the basic concepts of measurement theory.


Hints: When measuring the shape of the Sun’s path outdoors it is useful, in addition to natural objects, to let the students draw in teams models on paper for further use in the classroom. It is easy to continue working with the paper models later in the classroom and they can even be copied to the other groups for further exercises.


Keywords: Shadow, path of the Sun, flat sundial, declination, lighting meter.


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