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Wednesday, 28 June 2017 00:16

Francis Berthomieu

"EAAE Summerschools" Working Group

CLEA, France


Evidently, the Moon is the most known astronomical object, but its scientific and rigorous observation is not such an easy thing... Don't many people often say that the Moon can only be seen by night, that its phases can be due to the fact that it passes through the Earth shadow, that it has an influence on our destiny, on the climate, or on births?

This Workshop tries to provide a didactic material that allows to make a structured observation of our satellite and a rigorous study of its movement, and to make people realise that its aspect is only due to its position relative to the Sun and the Earth, and even to estimate the likelihood for an eclipse to happen at a given date.

And all of this using commun materials: cardboard, paper, paper fasteners, and "high performance" tools: scissors and a glue stick.

1) How to locate the Moon relative to the Sun

We shall construct a very simple measuring tool.

With this tool, a terrestrial observer can aim at the Moon accurately, and locate the position of the shadow of a pin end on a graduated circle. He or she can then determine, with great accuracy, the "elongation" of the Moon : that is to say the angle between the Observer-Sun and the Observer-Moon directions. By making this measurement several times during a month, and by representing the evolution of this angle against time, one can determine:

- the lunar month duration

- various angular velocities of the Moon while it rotates around the Earth.

One will discover that this movement is not uniform, and observe that the velocity of the Moon is high while at its perigee and slows down towards its apogee.

2) How to locate the Moon on the celestial vault

A mobile model (annex 1, annex 2, annex 3) allows you to represent the relative positions of the Earth, the Moon and Sun rays, at a given date.

You only have to adjust some parameters on the desired date (day, month, year), and to analyse rigorously the relative positions of the model elements.

This tool allows you to predict the aspect of the Moon (its phase: crescent, quarter, full Moon) at this given date, and also its place on the celestial vault, giving you the name of the zodiacal constellation where it lays.

A sky map (annex 4, annex 5, annex 6, annex 7), constructed with ecliptic coordinates, allows you to locate the Moon more precisely among the stars, while you learn to recognise the constellations and to look for the myths they are associated with.

3 ) How to predict eclipses

By adding up a new device to the anterior model, you will materialise the "Node Line". It gives an easy answer to a quite frequently asked question: "why don't a Sun eclipse and a Moon eclipse happen each month ?" Using this device in a reasoning way, you will be able to say without any doubt that an eclipse can't take place at certain dates.

You also will be able to affirm the likelihood for a Sun and a Moon eclipse to happen at other dates, but without a 100% guarantee: as the device is not sophisticated, it will not be possible to determine which zones on the Earth will be concerned by the predictable Sun eclipse.

To achieve that, more sophisticated tools have to be used, but this is a good approach to the scientific methodology.