ARCturus

Eclipses occur in the sky when some celestial body on its orbit seen from the Earth happens to cover some other celestial body so that it cannot be seen.  The most well-known eclipses to us are the solar eclipse and lunar eclipse. A solar eclipse happens when the Moon is between the Sun and the Earth. Since the diametre of the Moon seen from the Earth is the same as that of the Sun, the Moon usually covers the Sun completely. On rare occasions being a little farther from the Earth the Moon seems to be smaller and not large enough to cover the Sun. This is called an annular eclipse. Likewise when the Moon is on the same line as the Earth and the Sun, but seen from the Sun behind the Earth, there will be lunar eclipse. In the picture, the black Moon is between the Earth and the Sun, illustrating the occasion of the solar eclipse. The model was introduced in the summer school of astronomy in Briey, France in 1999 (picture IH, Finland). The participants there had a great opportunity to see also a total solar eclipse during the summer school.

 

 

Aim: Illustrate solar and lunar eclipses by using graphic presentations. Use source material in literature and Internet to make sure that the phenomena are understood.

Explorations: Discuss with students the locations of Earth, Moon and the Sun in relation to each other on the basis of the observations made in nature. Recall the basic concepts of this topic, e.g. year, day and night, season and phases of the Moon.

Eclipses are rare phenomena that can be observed in nature without optical instruments (except for the filter which is compulsory!) The eclipse may occur when the Moon’s orbit intersects the line between the Sun and the Earth.  It happens every now and then. If the Moon’s orbital plane were the same as that of the ecliptic, the eclipse would always occur during the phase of a new moon or a full moon.  The orbit plane of the Moon deviates from that of the Earth by 5 degrees. Therefore these three celestial bodies are on the same line at the intersections of the orbit planes, i.e. at the knots.

During the solar eclipse the Moon is at the knot point between the Sun and the Earth. Because the bright disk of the Sun is a large surface, the shadow it creates behind the Moon is not coherent. Behind the Moon, in the middle, there is a deep conical shadow the top of which causes a belt of a total eclipse when hitting to the ground. The location of the belt varies depending on where on Earth the eclipse is seen. Around the deep shadow there is a semi-shadow area where a partial eclipse can be seen. A third type of an eclipse is an annular eclipse. The distance between Moon and Earth varies a little. That is why the Moon, during the eclipse, may sometimes be farther from Earth and its size is not large enough to give a total cover to the Sun disk.  Man can then see on Earth a bright ring of the Sun and a black disk of Moon in the middle of the ring. Again and again remember to remind students not to look at the Sun during the eclipse with the naked eye without filters.

During the lunar eclipse the Moon is behind the Earth at the knot point. Observation of lunar eclipse can well be done with the naked eye whereas observing the eclipse of the Sun needs filters.  There are also filters for observing the Moon. They are needed when a bright full moon is observed by using binoculars or a telescope. No filters are needed for observing a darkened Moon. On the back side of Earth (seen from the Sun) there will be an umbra in the middle and around it a penumbra. The Moon does not always intersect the umbra area. That is why the eclipse on the penumbra area is called a partial lunar eclipse or a penumbra eclipse. A total lunar eclipse changes the colour of the Moon into a beautiful reddening.

1)  In order to make it easier to comprehend the related locations of Earth, Moon and the Sun, prepare a scale model of about one square metre using stiff cardboard or a thin plywood panel. The surface of the panel indicates the ecliptic of the Earth. The Sun is positioned and fixed in a hole in the middle of the panel. Paint or groove the orbit of Earth on the panel. The Earth is movable along its orbit. For observing eclipses fix circle panels indicating orbit of the Moon at the knot points in an angle of 5 degrees. In the picture, there is an orbit plane of the Moon illustrating the path of the Moon around  the Earth. Also this model was introduced in Briey (picture IH, Finland).

2)  If possible, observe a lunar eclipse with students outdoors. While waiting for the eclipse and during it, the students can make exercises. Descriptions of the eclipse process are made together at the time orally or perhaps by being written  down. Notes of the eclipse time and extent may be made in tables. It is fun to draw pictures of the observation results and to compare them with other students’ pictures. Make a poster of the results or use a computer to present them to all students of the school.

3) Observing a solar eclipse needs proper preparations. This beautiful natural phenomenon is a once-in-a-lifetime phenomenon for most people. It certainly is a memorable moment for everyone.  Preparations begin by acquiring film made for this purpose from a specialized shop. Every objective, whether binoculars or telescopes, will be covered by this film. In the picture there are binoculars covered with special film for observing the Sun. It was the purpose to observe Venus transit in summer 2012 in Enontekiö, Lapland (picture IH, Finland).

 

 

 

 

It would be good to acquire cheap filter glasses for everyone. They act like sunglasses. Using them makes it safe to look at the Sun. If there is a telescope at school the picture through the eyepiece can be reflected round a corner on a cardboard. All in the classroom can simultaneously explore the picture.

Students are asked to pay attention to the exceptional circumstances they notice in nature during a solar eclipse and report their remarks carefully. For example, the kind of things worth noticing are temperature, amount of light and the  behaviour of birds. An unusual opportunity to see is a flash of corona during the deepest darkness. It cannot be seen at any other moment because of the brightness of the Sun. One of the rarest moments is to notice a quick approach of the  Moon’s shadow in the clouds or misty layers of air.

4)  A solar eclipse may be observed together as a group by using a pinhole camera. Take a big cardboard box to the schoolyard and make a small hole on one side of it. A picture of the Sun can be seen on the side of the box opposite the hole. Phases of the eclipse can easily be seen as the small bright disk darkens. The same idea of a pinhole camera works also in a small shed if there is a hole on the wall towards the Sun and if the shed itself is dark. Students can see the picture of the Sun on the back wall of the shed. It is easy to take photos of it.

5)  Every student can observe a solar eclipse either independently or in a team of two by using simple equipment. The student stands outdoors in sunlight with his back to the Sun and holds in his hand a cardboard with a small hole in it. The picture of the Sun can be seen as a light disk on the screen behind the first cardboard. Also phases of the eclipse can well be seen like this.

6) *The teacher can ask students to suggest other ways of observing an eclipse. During warm months tens of suns can be seen in the shadow of a deciduous tree from gaps between the leaves. Any gap, on some opaque surface the sunlight may pierce, is a case the result of which is a picture of the Sun behind the surface. These are objects that students may try to find and suggest outdoors in sunlight before the eclipse.

7) *Both solar and lunar eclipses can be observed at the moment of their occurrence with all the students of the school present. If it is possible at school, connect the telescope with the computer. Observations are reflected on the wall in a hall and results are saved for later use. Discuss the findings together. They are processed and exploited in the classrooms in various projects. The finished results are presented at school or in the Internet.

8) *It is a valuable bonus to the observational results if the students have managed to take photos of eclipses. Image processing may happen in special courses under many subjects.

Methods: Observation outdoors, drawing, description and saving of findings, taking photos and image processing.

Materials: Special film, cardboard, binoculars, camera, craft materials.

 

Pondering: It is important to emphasize the meaning of the film as a prerequisite for the observation activity. Also the need for special spectacles and awareness of possible dangers cannot be underestimated. Ponder together how the occurrence of solar eclipses could change in the long term considering the Moon’s predicted motions on her orbit.

 

Evaluation of the results: Compare observational findings and drawings of the students and discuss good observations and misunderstandings if there are any. Use reflection pictures saved to explore the surface of the Sun and sunspots. Discuss the process of the Moon disk covering the Sun, i.e. from what direction, at what time and how quickly.

 

Hints: In order to have safe instruments for observation, arrange in the classroom a project during which the objective of every observational tool is covered with a special film. The teacher ensures that the film is intact and properly fixed. Using equipment for simultaneous observation is safe and results are comparable. Examples of this kind of equipment are reflection of a telescope picture on the wall or a cardboard as well as a projector used to show a telescope picture from  computer to the wall.

 

Keywords: Solar eclipse, lunar eclipse, umbra, penumbra, corona, rising knot, descending knot.

 

 

 

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