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Wednesday, 28 June 2017 20:54

Irma Hannula

"EAAE Summerschools" Working Group

University of Helsinki (Finland)

Abstract

In this very practical workshop you can make the model either as a painted picture onto the wall in a 2-dimensional form or use pupils presenting "constellations" in a 3-dimensional form in the middle of the classroom or on the yard of the school. We will use now the latter method.

In the middle of the construction there will be the Sun and the Earth with its orbit. The zodiacal constellations are around this system in the right positions due to the seasons. If you imagine yourself being on the surface of the Earth and looking at the direction of the Sun, you know that there is a constellation behind the Sun. The constellations through which the Sun "goes" during the year form the Zodiac.

When we construct the model, we have to think of the positions of the zodiacal constellations compared with the positions of the Earth in its orbit. We have to take into account the summer and the winter, i.e. the positions where the Earth is at the nearest or farthest away, in perihelion or aphelion.

We will use the overhead projector for drawing the constellations (only the brightest stars) on the cardboard in an as so enlarged size as we want. For making this model more understandable we could paint the mystic figures onto the constellations freehand or using transparencies. The model could work as a two-dimensional version on the wall or it could be more illustrative as a three-dimensional version in the middle of the class. Also we can demonstrate these things by letting the pupils be the parts of this model, each carrying a cardboard of one constellation. So we need twelve "constellations", one "Earth" and one "Sun". There are many astronomical subjects you can teach with this model.

The main theme in our summer school this year is "Astronomy and Navigation". In our workshop we will have some discussions on how to use using stars in the navigation like in the "old" times and also how to use sky photographs in teaching constellations. Our main subject is still the construction of the zodiacal model.

Using the stars in the navigation

In the college of navigation the students have to learn to navigate also like in the "old" times. It means that they have to learn to know how to find the necessary information for navigation on the Sun, the Moon, the Zodiac and the biggest stars as well as the eclipses of the Sun and the Moon. Usually only 20-30 brightest stars are used in the astronomical observations of the navigation.

As an example we can learn to find some stars by using constellations we know, e.g. Ursa Major and Ursa Minor. (Fig. 1 and 2)

The path of the Sun during one year

The Earth is circulating the Sun in the plane of the Ecliptic. We mean by the Ecliptic the big circle, which forms as a section line of the plane of the orbit of the Earth and the celestial sphere. If you imagine yourself looking at the direction of the Sun, it seems to be in some constellation behind the Sun (fig. 3). These constellations are located near the Ecliptic and this combination is called the Zodiac. The angle between the Ecliptic and Equator is 23º 27' and the intersectional points are the equinox and autumnal equinox. The time which the Sun needs to move from the equinox to the equinox again is called the tropical year. Its lenght is 365 ¼ days, which means that the Sun moves on the celestial sphere about 1 º = 4 m during one day.

The Zodiac's breadth is a 16 degree zone on both sides of the Ecliptic. The signs of the Zodiac devide the ecliptic into 12 part, each one 30 degrees. Because of the motion of the equinox (Aries earlier, now Pisces, one circle in 26000 years) the places of the signs of the Zodiac deviate by 30 degree from the zodiacal constellation with the same name. Only 30 stars of those listed in the Nautical Almanac (NA) is usually used in the navigation. In the almanac you can find very important information on the stars, the planets, the Moon and the Sun, e.g. SHA= Sideric Hour Angle, Dec= declination, conversions of arcs to time units and different corrections.

The names of the zodiacal constellations in order from the equinox are: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricornus, Aquarius and Pisces. Inside this zodiacal zone are the Moon and the planets also moving because the angles between the planes of their orbits and the Ecliptic are less than 8 º.

In the navigation only four planets are significant. They are Mars, Jupiter, Venus and Saturn. Information on these planets, which is important for the navigation, is available in the Nautical Almanac. You can recognize the planets by their even light, which differs from the twinkling light of the stars. Venus and Jupiter are much brighter than any star. The light of Mars is twinkling red. In the ancient times the stars were classified according to their brightness in the classes of the magnitudes The faintest stars, that could be observed with naked eyes, belonged to the sixth class. Nowadays the brightness is determined by photometric measures.

Construction of the zodiacal model

We are going to build a concrete model of the Zodiac, the Sun and the Earth. We will use all of you as living models to make those astronomical subjects perfectly understandable. You can verify the different situations, e.g. the summer, the winter, the periods of the year etc. "The Earth" may circulate around the Sun and "the Sun" is shining very brightly! "The zodiacal constellations" should be in the right positions but some of them might have difficulties in "hanging up" upside down!!!

You can work in groups of 2-3 persons. Each group will take one constellation to paint. First we will draw the zodiacal constellations to the cardboards. We'll use the overhead projectors for reflecting the pictures from the transparencies to the wall. The distance between the overhead projector and the wall should be the same all the time. (Fig. 4)

After drawing the constellation (only the brightest stars) you can change the transparency with the mystic figure and draw it onto the constellation in the right position and size. If you want, you can be an artist yourself and draw the mystic figure. You can use some colours, too.

After painting all the constellations we are going to collect all parts together to form a compact system of Zodiac-Sun-Earth. In the middle of the classroom we'll make an elliptic circle, in the middle of it there will be the Sun and the Earth. The orbit of the Earth can be marked on the floor, if you think it's necessary. Try to form the right order for the constellations and to take the seasons into account. (Fig. 5)

If you are on a mysterious mood, you can listen to the ancient stories concerning the constellations. The mysterious figures were very warlike, but some of them were also very romantic. Enjoy yourself!

References

  • Kaila, K. 1979. Tähtitaivaan opas, Tähtitieteellinen yhdistys URSA ry, Jyväskylä: Gummerus
  • Lovi, G., Tirion, W. 1989. Men, monsters and the modern universe
  • Löfgren, K-E. 1997. Merenkulkuopin perusteet III: Avomerinavigointi 1993, Helsinki: Suomen navigaatioliitto
  • Staal, J. D. 1988. The New Patterns in the Sky, Myths and Legends of the Stars, The McDonald and Woodward Publishing Company, Blacksburg, Virginia