NOVA

- [Narrator] An Astrolabe has many uses, from identifying stars, to finding daily time.

It may have been developed by the Greeks but it reached its zenith in the hands of Islamic scientists.

They wanted to make better, more accurate instruments to calculate time.

- [Taha] This is an Islamic astrolabe.

This instrument is actually a mechanical computer.

What you see here is the projection of the sky for a specific latitude.

This is for Istanbul.

- [Narrator] Etched on the base plate, is the horizon line.

Precise altitude circles marking the sun's height above the horizon and the meridian, showing midday and midnight.

On top of the baseplate is a movable plate showing stars and constellations, and a ring that represents the apparent movement of the sun throughout the year.

It's labeled with dates.

- [Taha] It starts with one single observation and we will actually try to maintain the position of this this piece exactly aligning with the sun.

I think it's now aligned.

This is a perfect alignment, and we just read the altitude from here to here is 54 degrees.

- [Narrator] That means the sun is 54 degrees above the horizon.

The user now turns the Astralabe over, to find the 54 degree circle on the bottom plate.

Next step, find and mark the date.

It's etched on the ring that represents the sun's path.

Then rotate the plate until the date aligns with the altitude mark.

If you take a piece of string from the center of the astrolabe through the aligned points, you can read the time from the rim.

The line is like the hand of a clock.

It's four minutes past two in the afternoon.

- [Taha] Once we reach that, we can calculate any time.

So that is not as simple to use instrument, but accurate enough for all timekeeping applications.

(upbeat music)