By Sam Morgan, Updated Mar 24, 2022

Introduction

An astrolabe is a sophisticated instrument that harnesses the positions of the Sun and stars to determine local time, latitude, and even subtle changes in Earth’s axis. Below is a step‑by‑step manual that will help you use this ancient tool with confidence.

Measuring Time with an Astrolabe

1. Align the Alidade with the Sun: Hold the astrolabe so that the alidade (the rotating arm on the outer rim) points directly at the Sun. Adjust until a narrow beam of sunlight passes through the sight holes and lands on the palm of your hand.
2. Read the Sun’s Altitude: While the alidade remains fixed, note the altitude value in degrees where the inner rule intersects the outer scale. If you’re unsure, ask someone to read the measurement for you.
3. Determine Local Time: Set the rete (the inner rotating star map) so that the date and the measured altitude align. The number the rete points to on the outer rim gives the local time in a 24‑hour format (e.g., 15 equals 3 p.m.).

Determining Latitude for Navigation

1. Select a Morning Star: Choose a bright star that appears just after sunrise.
2. Measure Solar Altitude at Noon: Using the same method as above, find the Sun’s highest altitude during the day.
3. Consult the Astrolabe Reference Table: Locate the star you observed on the table’s top row. Cross that with the measured altitude in the table’s column to read your latitude.

Tracking Celestial Events

1. Choose an Event: For example, observe the Sun’s declination to study Earth’s axial tilt.
2. Measure Altitude Consistently: Record the Sun’s altitude at the same time each day, noting the exact hour.
3. Maintain a Daily Log: Compile your measurements into a spreadsheet or notebook. Over months, the variation will reveal the tilt’s effect on solar position.

Safety Warning

Never look directly at the Sun while measuring its altitude. Use a proper solar filter or indirect observation method to protect your eyes.