Using the Astrolabe as a Planisphere
You can see which stars are visible to you by setting the date and time on the Astrolabe. First, grab the Ecliptic Longitude for today (in this case 30 September) from the back; this is 189º.
Move the Ruler to 189º on the Rete, then move both together so that the Ruler points to the current solar time.
Everything above the horizon line should now be visible to you in the sky.
Finding Positions of Stars in the Sky
How to find the time at which the Sun will be at a certain Azimuth on a given date
Suppose you want to know when the Sun is going to be at an Azimuth of 30º east of the Meridian, on September 27th. Find the Ecliptic Longitude of that date on the back of the Astrolabe; this is 185º.
Then rotate the Rete until the circle at 185º intersects the 30º azimuth line.
Finally, rotate the Ruler at that intersection and read the Solar time on the Limb, at which the Sun will be at an azimuth of 30º east of the Meridian.
If you’re using a North-based plate with 180º at the top, then the same process applies, but you’re using 150º instead of 30º.
You can also deduce that the Altitude of the Sun at the time is about 44º.
Determining what date and time the Sun will be in a certain position in the sky
Suppose you have a photo in mind that you want to take, and you need the Sun to be in a certain place for the composition. When will that be?
If that place is 40º West of the Meridian and 30º Altitude, we start by rotating the Rete until it meets those two lines.
We read that the Ecliptic Longitude here is 306º.
Rotate the ruler to find the time this occurs; here it is about 14:22 in the afternoon.
Flip to the back, and rotate the Alidade to 306º. Then you can read the date, in this case it will occur on January 25th.
Maximum Altitudes
How to determine when a star will be at its highest Altitude on a given date
Suppose you want to know when Deneb will reach its highest point in the sky on March 20. Flip to the back and get the Ecliptic Longitude for that date; in this case it’s 0º.
Rotate the Rete until Deneb is on the Meridian line, between the North and South Poles. Then rotate the Ruler to 0º and use it to read the time, which will be about 08:40 Solar Time on that day.
Highest Altitude of the Sun in the whole year, for the Astrolabe’s latitude
Rotate the Rete until the 90º mark is lined up on the Meridian, between North and South. This is the maximum altitude for whichever latitude your Astrolabe is set for. At 40º North, this is about 73º.
Highest Altitude of the Sun on a specific date, for the Astrolabe’s latitude
On November 25th, the Sun should be a lot lower than in the summer, and the Ecliptic Longitude is 243º.
Rotate the Rete until 243º lies along the Meridian. At 40º, the Sun will only reach 28º of Altitude on this day.
To compare and contrast, here’s the same calculation at 30º latitude – on the same day, the Sun will reach 39º. Note the difference in positions of the 40º lines; they move quite a bit between the different latitudes.
Finding the Right Ascension and Declination of a Star
Right Ascension (RA) and Declination are the sky equivalents of Latitude and Longitude. RA is the Longitude, and is measured in hours and minutes, from the point of the March Equinox.
Declination is the Latitude equivalent, and is measured in degrees, north or south of the Celestial Equator. This is tilted in respect to the Earth.
We’ll find the RA and Declination of Procyon.
Rotate the Rete until Procyon is on the Meridian line. Use the bottom end of the line, as we have astrolabes with 24-hour markings. (Many use 12-hour markings, repeated twice around the limb)
Line up the Ruler with the Meridian line and use the scale to read the Declination. In this case it is about 5º.
Turn the Ruler to 0º of Ecliptic Longitude and read the RA on the Limb, which is in hours and minutes, in this case 07h39m.
Alternative method:
Many classical astrolabes considered the point of Capricorn 0 (270º, the Winter solstice) as an index of the rete’s position. If Capricorn 0 is aligned on the bottom of the meridian, then Aries 0 (Right Ascension 0) is on the Western horizon. If you move Capricorn 0 to the 1hr mark on the limb, then anything setting on the Western horizon now will have a Right Ascension of 1hr. (We’re using the bottom end of the Meridian because our astrolabes are marked in 24-hour format around the Limb.)
So if you put Procyon on the Western horizon as if it were setting, and then move the ruler to Capricorn 0 (270º of ecliptic longitude), you’ll get the same time (07hr40) as above.
Adding objects to the astrolabe
If you know the RA/Dec of a sky object, you can use a chinagraph pencil to add it onto the acrylic rete. (You could do the same on the wooden rete with a pen, but it won’t wipe off)
Turn the rete until the 270º mark is at the hour for the Right Ascension. Your object will then be along the Western horizon line. Use the ruler to find the declination and mark the rete on the horizon at that point.
For example, we can see where Saturn is in the sky. At this time of writing, it is at 22h 33m RA and -10º 54′ declination. (Get updated numbers here) So move the 270º line to the 22h33′ hour, and see where 10º is along the Western horizon line. Saturn will be there. (If you mark this on the rete, be sure to use a removable marker as the planets tend to move!)
Introduction
Parts & Plates
Finding Times
Finding Positions
Unequal Hours & Shadow Square