The photo below has been taken with a telescope (that has focal distance = 18.56m).

Knowing distance from Earth to the Moon is 384444 km, find out Moon's diameter.

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Tabla de Contenidos

- A pen and a piece of paper.
- Some knowledge related with telescopes
- What do you know about radians?
- How to obtein the formula
- Programm
*SalsaJ*(you can free download it: http://www.houspain.com/gttp/salsaj)

If some dynamic worksheets are not available, java can be download from: http://www.houspain.com/gttp/salsaj

Knowing distance from Earth to the Moon is 384444 km, find out Moon's diameter.

- Focal length of the telescope,
- the distance from the Earth to the Moon is 384,444 km
- Two formulas: and

- We'd like to work out the Moon's diameter, D, this being twice the radius, R.

- With the programm SalsaJ, open the photos below and measure diameter's moon (mm). We'll call this value “u”. The scale of each photo is below them.

**Escala**: 0.06 píxel/Km. 1.15 píxel/Km.

- The focal lenght of the telescope is , so we can claculate s using the formula .

- Find out the angular diameter with , since we know u and s.

- Transform this value into radians (just multiply it by 0.0174

- Since we know that the distance from Earth to Moon is roughly 384,444 km, we can find out Moon's diameter, D, from y r (the distance from we to the object.