An astrolabe is a piece of astronomical equipment that dates back to Ancient Greece. You’ll find that it consists of discs and rulers that you can rotate to show the different positions of stars at any given time.
Astrolabes where used for navigation before the sextant was invented.
You can use an astrolabe to tell the time, predict the sunrise or sunset, and measure the altitude of the constellations.
While technology has improved, some astronomers still like to know how this old equipment works. If you’re interested in learning how to use an astrolabe, we have good news, as we can tell you everything you need to know.
Explaining The Astrolabe
To understand how an astrolabe works, you need to know all its features to understand it. The outer ring is called the mater, where degrees are etched into the perimeter. You can also use these to tell the time when you know how to.
The plate is the back disc of the astrolabe. You’ll find grid markings etched onto it. These markings can help you figure out where the stars will be located. For example, it will help you find where the horizon is.
The rete is the rotating disc that overlays the plate. You’ll find that the rete contains many cutouts, which lets you still see the plate. The shapes that remain show where the brightest stars in the sky are located.
You’ll find a ring representing the ecliptic plane in the rete, separated into the other months or zodiac signs.
The rule is a bar that lies straight across the face. It’s attached to the center and rotates. It points towards the mater so it can be used to measure the angle of different objects.
Finally, we also have the alidade, a rotating bar you’ll find on the back, which you would use as the sight. Some astrolabes have wings that let through light to allow you to use them during the day without looking directly into sunlight.
Now that we know the astrolabe’s different parts let’s find out how to use it.
Using The Astrolabe To Find Stars And Constellations
You’ll need to move the alidade onto the calendar date to find stars and constellations using the astrolabe.
You could use today’s date or a future date if you want to know the star’s forecast. Once you align the alidade with the date, you can identify the zodiac date.
Once you’ve done this, turn the astrolabe around and line the rule up so the zodiac date is clear. Doing this, you’ll find the stars and sun’s position for your chosen date. You should ensure it lines up as perfectly as possible to ensure the reading is accurate.
After you’ve done this, adjust the rete and rule simultaneously to align with night-time. You’ll see the time around the perimeter of your astrolabe for when you’re ready to stargaze. Then, you’ll need to position the rete and rule to point at that time.
Look upwards of the horizon line, and you’ll be able to find the location of the stars. Anything over the horizon line will appear visible to you on your chosen date.
By looking at the astrolabe’s concentric rings, you’ll see how high the stars will appear when they appear. If the star’s in the center point of the ring, it will appear directly overhead.
The center of the ring is called the zenith, and if the star is further out, it will appear lower in the sky. Likewise, any stars beneath this line won’t be visible to you.
Use The Astrolabe To Measure Latitude
When measuring latitude with your astrolabe, you must line the alidade up with Polaris or the North Star. To find where this is, you need to locate the Big Dipper and search for the two stars making up the edge of the handle and the furthest from it.
Imagine a line from the lowest star on the ladle to the star in the corner. Continue imagining this line until it is five times the distance between these two stars. The first bright star will be the North Star.
Once it is aligned, check the alidade, see where it points, and note the measurement on the rim. This number is called the zenith angle, then measured in degrees, showing the star’s angle as seen from the ground.
When you know the zenith angle, subtract the angle from the number 90 to get a latitude reading. So long as you know how to locate the North Star, you should have no trouble when you need to measure the latitude.
Use The Astrolabe To Calculate The Time
To calculate the time, you must hold your astrolabe by the top so it hangs downwards. At the top of your astrolabe should be the number “XII” marked on the perimeter. You must adjust your alidade to point at the sun to find the altitude.
You can find the zodiac date by lining the alidade up with the calendar date. For example, if you place the alidade over the current date, you’ll find the zodiac date. Once you know the zodiac date, align the rule with it.
Then, rotate the rule and rete simultaneously so they line up with the sun’s altitude on the base plate. You can then line up the intersection point of both of these with the altitude you measured.
Once you’ve done this, you can find the time by observing where the opposite end of the rule points. As all the numbers are arranged like a clock, you can read the time.
Summary
Using an astrolabe can be intimidating at first. When you know the different parts that keep it together, you’ll find it’s not that challenging to use.
Many astronomers still use astrolabes today, even with all the technology available. These are especially useful if you don’t have all equipment on hand, so we hope this guide has been helpful to you.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.
