Basically, astronomy is the perception of lights in motion with respect to our local horizon. The horizon is our open window towards the sky, our starting point for surfing in the vast cosmos.

For us, Earth-centred observers of the sky:

-                the sky above us is experienced as an hemisphere or a dome

-                the dome of the sky meets the plane of the earth surface in the skyline

This is the theatre for celestial events to happen!

For this reason a beginning in astronomy from “horizon activities”, with a geocentric approach, plays an important role into the understanding of celestial motions and phenomena.

1.     Telling a myth: the origin of the skyline

In the ancient Greece explanations of geographical and astronomical phenomena consist of poetical and mythological images. This metaphorical approach is "the first level " of scientific knowledge.

In a lot of myths, we can find the memory of the ancient union between the Earth and the Sky and of their separation along the skyline...

The Greek myth of the "Cosmic Egg"

At the beginning of the world there was only the dark sky, the Night. The Night liked dancing, and because of her dancing, the Wind, was born. He was Borea, from the North, a fertile wind. As Night was dancing, the Wind caught up with her and enveloped her: the Wind fertilised the Night. So, when the Night began dancing again, she carried an egg inside her: a giant egg, somebody says it was a silver egg. The night laid her egg in a deep abyss, and it stayed there for a long and incalculable time. Finally, when the silver egg opened, Fanete emerged. The God had light golden wings on his back, and he flew away as a light-wind-storm. He is also called Eros, the God who arouses every longing for love.

First of all, Eros made order in that primordial egg: heavy things went down to the low cavity: mountains, abyssal water, lands… and they became the Earth, Gaea; light things -air, clouds, stars, sun and moon - went up to form the sky, Uranus. So the Earth and the Sky, Gaea and Uranus, took different shapes inside the primordial egg.

A deep union lasted long between them: in fact Uranus fell in love with Gaea and his embrace was so deep and strong that she couldn't breathe, nor to give birth to the children she carried. The Earth stayed flat under the Sky, completely crushed, without any life. So she asked her children to help her, and Cronos, the youngest of all, came out from the Earth and wounded his father. Crying, the Sky went away from the Earth and they were separated for ever.

  The skyline marks their separation.

2.       Observing the skyline

Seen from the observation point in any one direction, the skyline appears as an uninterrupted line: at sea it is level, but usually trees, hills, mountains or buildings move it.

Firstly students (from the Astronomical Terrace,  "G.G.Belli" in Via Col di Lana, Rome) are invited to look around at the skyline, carefully, in different ways and positions: standing up and down, turning around at different speed, looking through their hands as a telescope, with their arm outstretched or bended.

Secondly they follow the skyline with a finger, as they make a drawing in the air.

3.            Measuring the skyline

The best way to measure phenomena in the sky is to use angles and a unit of measurement that is independent of distance: the degree. The skyline around us defines a circle that can be divided into 360 degrees. Holding our little finger up at arm’s length, its width is approximately 1 degree; a thumb held at arm’s length is 2 degrees; a fist, about 10 degrees, an outstretched hand 22 degrees.

These, of course are rough angular measures, but they are an excellent system to measure the skyline, to describe how large something appears in the sky, how high it is above the horizon… as well for communicating our observations with others.

4.               Making a drawing of the skyline

We can keep a record of our observations by making a drawing of the skyline: it is a simple instrument that will help us remember important details that we might otherwise forget.

We select a special location in the outdoors, in a large, open area:  it is the Terrace of our school, Via Col di Lana, in Rome.

Eight students stand in a small circle, facing at the horizon, and an observer is in the very centre of them. Firstly, the skyline is measured in hands, at arm’s length: 2 hands measure approximately 45 degrees (Figure 1), so 8 people cover the whole circle of horizon.

Then the skyline is sketched to scale on the cardboard, divided in 8 numerated parts, one for each student (or a pair of them).

The search for harmony is the heart of this activity: harmony between what you see and what you plot as well as between you and the other drawers. The agreement with the “skyline-drawer” next to you  has to be accurate and precise: where the one stops to plot his part of skyline, the other starts at the same position and level.

Finally, all parts have to be connected in order to create a large circular (or octagonal)  model of the local skyline with an internal point of view. People can enter inside it.

5.              Using the skyline-drawing as a horizon observatory

Observing the motions and changes in the sun and stars is the foundation for relating astronomy to the human life, to orientation and to the passage of time. This is a long-term activity.

-           Observing and measuring the daily motion of the sun and its position at midday, the North-South direction (local meridian) can be found. Besides that, students will observe and record on their model of skyline the sunset and sunrise positions on two or more days for each month of the year and measure azimuth (being sure to always make their observations from the same location!).

They will see the sunset and sunrise positions change with time, the fastest during autumn and spring and less rapidly around the winter and summer solstices.

Finally they will discuss and interpret their results:

-           How far to the North does the Sun rise on the Summer Solstice?

-          How far to the South does it rise on the Winter Solstice?

-          When does it rise due East? (Only on equinoxes)

The width of the arcs of sunrise amplitude and sunset amplitude on the horizon depends on the local latitude and increases with it. For 42°(latitude of Rome) the amplitude is about 70°.

It is interesting to explore the day to day change in the azimuth of sunrise, as well in the height of the Sun at midday, and plot a chart of the daily paths of the Sun for the local latitude, over the year.

6.            “Astronomical windows”

Instead of making a drawing of the whole skyline, you can select a window in your class where you can observe the setting (or rising) sun. Make a drawing or take a picture of the skyline in that area. Once or twice a week over several months, mark the point when the sun sets (or rises) and record the date and time.

7.            Horizon calendars

By observing and recording where the sun sets (or rises) over one year, the skyline-model becomes a horizon calendar.

Examples of horizon calendars – using sunrise and sunset positions on the horizon as well as moon or stars positions–exist in the history of many cultures in Europe and America. One of the best known examples is Stonehenge.

Bibliography:

L. Fucili (2003) “The horizon as an observatory” 7°EAAE Summer School Proceedings, Rosa M. Ross Editor  p. 199-209