Using the Astrolabe
First, a word about Time
Please remember that times are all in True Solar Time – this is different to the time shown on your watch!
Midday in Solar Time is defined as the time at which the Sun is highest in the sky. This worked fine for as long as humans have been around, but it means that midday is at a different time (relatively) in each town or village, since they are spaced at different places on the Earth. The Sun will reach its maximum height over each town, one by one, as the Earth rotates. This actually varies by four minutes of time for every degree of longitude.
When the Railroads came along, they needed meaningful timetables that could be shared over a wide area, and the idea of time zones was born. We are used to the fact that the time in our town is the same as that a few hundred miles away, but it means the Sun will be highest overhead either before or after our midday, unless we are right in the middle of our time zone. Time zones are based on Standard Meridians, which are each 15º wide, and you can correct for 4 minutes of time for each degree of longitude you are from the start of your standard meridian.
Here’s an image that shows both the time zones and Standard Meridians for the continental US. If you are East of your meridian, the correction will be positive. If you’re West of it then it will be negative. However, you always subtract the correction from the solar time. For example, if you’re at 92º longitude, then you’re two degrees west of the Central Time Zone middle of 90º. This means the Sun will be overhead your location eight minutes late, so your correction is -8. For a given solar time of 10:00am (say), then your corrected time will be 10:00-(-8) = 10:08.
The Earth’s orbit around the Sun also makes a difference, since it is elliptical in shape. This gives rise to the Equation of Time. The Sun apparently moves back and forth a few minutes as we go through each year, which accounts for an up to 15 minute difference in the observed versus the actual time. You can correct for this using the Equation of Time end of the Alidade; point it to the current date and you can read the current correction on the scale using the kidney-shaped curve. Subtract this also from the time readings given.
Add these variables to Daylight Savings Time and you may see up to an hour and a half’s difference between Solar Time and the time on your watch. Don’t panic. (There are apps for your phone which will show Solar Time if you want to cross-check this).
Most of the examples in this manual use an Astrolabe built for 30º latitude. If you have one for another latitude, the results of your calculations will be different to those shown here.
How to find Sunrise or Sunset Times
For most calculations with the Astrolabe, we start with the calendar date. This can be today or another date of your choosing.
Start on the back, and move the alidade to the required date. Here, we will use August 26th. This shows an ecliptic longitude of 154°.
Flip over to the front, and rotate the rete so that 154° on the edge of the ecliptic circle sits on the horizon line, on the left (east) side of the dial.
Rotate the ruler so that it lines up on the point where the two scales cross, and it will indicate a time, in this case 05:37 in the morning.
The Astrolabe is also telling you where on the horizon the sun will rise. Notice where the two scales cross again, and read the Azimuth line here. In this case it’s 105° on a Meridian-based plate, which equates to 180-105=75º on a North-based plate; either way, this is just north of East.
The same procedures work for finding the sunset time, only in this case you work from the Western side of the dial, since the Sun sets in the West. On this day, the Sun sets around 6:23pm.
How to find the Current Time
Similar to the procedure above, we find the current time by first using the Alidade pointer on the back of the astrolabe to sight the Sun’s current elevation. DO NOT LOOK DIRECTLY AT THE SUN! Hold the astrolabe by the string so it hangs straight down, then use the shadows of the alidade’s sights on your other hand to line it up. You can enlist the help of a friend to hold the instrument if needs be.
On this morning of May 20th, the Sun is at 40º in the sky. Take the ecliptic longitude for today, which in this case is 60º.
Rotate the Rete so that the ecliptic ring at 60º intersects with the 40º line; morning or evening side as appropriate. (This part is the same as above; for sunrise and sunset you’re using 0º.) Move the pointer so it crosses the intersection and you can read the time on the edge; in this case 08:30 am local solar time (at 30º latitude).
How to find rise/set times of a star
Start with finding the ecliptic longitude from the required date as you did in the previous section. We’ll go with August 26th and 154° again here.
On the front, rotate the rete until the star you want is on the horizon, either East or West depending whether you want rise or set time. Here, we will look to see what time Betelgeuse sets.
Now rotate the ruler to point to 154° on the Ecliptic Longitude scale, and then you can read the time, which in this case shows 14:25, i.e 2:25 in the afternoon.
Using the Stars to tell the time
Use the Alidade on the back to sight on a couple of stars. Suppose you spot Sirius at 20º and Rigel higher up, around 40º. Move the Rete to put the stars in that position. (Stars lower to the horizon will give greater accuracy here).
If it is December 11th, the Ecliptic Longitude that day is 260º. Move the Ruler to that spot on the Rete and you can read the Solar Time, in this case 21:55, at 30º latitude.
To convert to clock time, first add an hour for Daylight Savings if that is in effect. Then subtract your longitude correction. Finally, subtract the Equation of Time correction.
For our observer at 92º longitude, they can subtract the eight minutes of longitude correction, and December 11th has a -7 Equation of Time correction, so 21:55-(-8)-(-7)=22:15.
Introduction
Parts & Plates
Finding Times
Finding Positions
Unequal Hours & Shadow Square