How to use the astrolabe

The astrolabe is used to measure the angular height (measured in degrees of arc) of the objects on the horizon. To measure it is focused through the sorbet of the object (for example a star or a planet), at that moment another person reads the number that the string indicates on the scale of the astrolabe. That is the height of the object. Already in 1550 Ptolemy used this device to measure the angular height.

one

To use the astrolabe, use the compass to determine the North and the South, imagine a line passing through the sky joining those cardinal points and passing over our heads (this imaginary line is called the meridian of the place).

two

With the astrolabe measure the height of the chosen star just when it is going through that line.

3

If it is difficult to determine where the imaginary line is, you can take the measurement like this: focus the star making sure that the sorbet is aligned with the compass needles. If this does not happen, it is because the star is not passing through the superior meridian of the place.

4

Another way to use the astrolabe and measure the angular height is when the star is passing through the meridian of the place is when its altitude above the horizon is maximum. Then: if the star is approaching the meridian (we do not know where it is but we intuit it from the cardinal points)

5

We begin to measure your altitude, and repeat the measurement every 5 minutes. The value of the altitude will be increasing in each new measurement until at a time it will decrease, then, we are left with the next-to-last value, which is the maximum.

• The formula to measure the altitude is: (measured height of the star) + (declination of the star - 90º if you are in the southern hemisphere
• The formula to measure the altitude is: 90º- (measured height of the star) + (declination of the star if you are in the southern hemisphere