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The Way to the Stars: Build Your Own Astrolabe

How to Use Your Astrolabe: What's in the Sky Tonight?

You can use your astrolabe to find out what will be visible in the sky throughout any night. The instructions below explain how to find out what you can see on 20 March at 7.30pm GMT (19.30 on the 24-hour clock). You can easily adapt the instructions to apply to any date and time.

The time must always be set according to Greenwich Mean Time (GMT, also know as Coordinated Universal Time, UTC). For astrolabe users in the UK, this means subtracting one hour from the clock time during British Summer Time periods (from the end of March to the end of October).  For astrolabe users in other parts of the world, you will need to adjust your calculations accordingly.

Set the date

To set up your astrolabe for a particular date, you do not use the calendar date. Instead you use the position of the sun in the Zodiac.

To find out the position of the sun in the Zodiac on any day, use the back of the astrolabe. Find the date in question in the calendar on the back. Move the label round until it lines up with this date. Then read along the line of the label to see the position in the Zodiac.

In this example, we see that 20 March is equal to 29 Pisces.

Diagram showing how to use your astrolabe.

Find the Date on the Rete

Remember the place in the Zodiac that you have just found. Turn your astrolabe over, and find this place in the Zodiac marked on the rete.

Rotate the rule on the front of the astrolabe until it rests on this place in the Zodiac.

Diagram showing how to use your astrolabe.

Set the Time

In this example, we are going to find out which stars are above the horizon at 7.30pm on 20 March. The astrolabe uses the 24-hour clock, so we use 19.30 instead of 7.30pm.

Find the location of 19.30 on the rim of the front of the astrolabe. It is half-way between 19 and 20. Hold the rule and rete together, and rotate them as one so that the rule points to 19.30 and also still lines up with 29 Pisces on the Zodiac.

You have now set up your astrolabe.

Diagram showing how to use your astrolabe.

What can I see?

The astrolabe now shows you which stars will be above the horizon at 19.30 on 20 March. The horizon is marked by a line on the plate, the part of the astrolabe beneath the rete. Everything inside this line will be above the horizon: this area is shaded in blue on the diagram.

The plate shows you where in the sky to see the stars. The concentric rings labelled with numbers tell you how high in the sky to look. The central point of these rings is the zenith, the point directly overhead, at an elevation of 90°.

The compass points along the edge of the horizon line tell you in what direction to look.

Diagram showing how to use your astrolabe.

The constellations you see in the sky will be mirror images of the constellations shown on the rete. This is because the rete is drawn as if the stars are all located a fixed distance from the earth on an enormous sphere. The map on the rete is drawn from the perspective of an observer placed outside that sphere. Whilst the stars are not actually arranged this way (they are all at different distances from the Earth), the earth is so small in comparison to the distances to the stars that this method of projection creates an accurate image.

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Supported by a grant from the Friends of the Center for the History of Physics, American Institute of Physics