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Astrolabe Usage

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Description of Astrolabe Features

  • The transparency overlay has a large outer circle with the Universal Time (see below) marked on it. The set of smaller concentric circles are circles of altitude above the horizon (which is the outermost of these circles). This represents the part of the sky that you can see from your latitude. The heavy line is the limit for altitude = 18 degrees (when the sun reaches this limit it is the beginning or end of twilight). The offcenter circle is the path of the sun through the year the ecliptic. The vertical line passing through the circles of altitude represents the meridian, the imaginary line passing through the zenith (directly over head) and the north celestial pole (an extension of the Earths north pole into the sky).

  • There are two grids with stars. The once with the greatest number of stars is the Northern hemisphere sky down to about magnitude 6. The outermost scale along the outer circle is the date. Next inward is a scale of right ascension (a celestial coordinate equivalent to Earths longitude, expressed in hours). Moving on concentric circles towards the center, the declination is increasing (a celestial coordinate equivalent to Earths latitude). The heavy (bold) circle is at declination = 0 deg, which is called the celestial equator.

  • The second grid with stars (less densely populated) is the side which you may want to customize. I have plotted objects of naked eye and binocular interest from Nortons 2000.0 Star Atlas and Reference Handbook, Ed. I. Ridpath, John Wiley & Sons, Inc. New York. The filled circles represent bright deep sky objects (nebulae, clusters and galaxies) and widely separated binary star pairs. The open circles represent stars which change in brightness (variable stars) which have large ranges in brightness and short enough periods to make school projects.

  • The final sheet has lines of elevation marked off for determining how high a star is above the horizon.

  • The UT (universal time) is the time in Greenwich, England. To make use of the astrolabe we will use UT, so you need to know how many hours from England you are:

    Location No Daylight SavingsDaylight Sa vings
    Honolulu HI -10 hrs N/A
    Tucson, AZ -7 hrs N/A
    Pacific Time Zone -8 hrs -7
    Mountain Time Zone -7 hrs -6
    Central Time Zone -6 hrs -5
    Eastern Time Zone -5 hrs -4

Exercises

View of the Sky for Any Particular Time
  • Select a date and local time, then convert the local time to UT (remember to use a 24 hour clock).

  • Rotate the insert until the date lines up with the UT on the overlay. The stars inside the horizon circle (altitude = 0 degrees) will be what stars are visible.

When an Object Rises or Sets
  • Rising is defined as when an object passes above zero degrees elevation, (although if you have mountains or other obstructions on your horizon you can set the elevation limit for rising to an arbitrary number).

  • Rotate the insert to place the object of interest on the horizon (in the East for rising, and in the West for setting).

  • Look for the date of observation along the outer edge of the insert and read off the UT that it lines up with.

  • Convert from UT to local time this will be the local time that the object rises or sets.

Using the Astrolabe as a Timepiece
  • Select a star that you can identify on the star chart on the insert. Find this star in the sky.

  • Use the back of the astrolabe holder (the side opposite the overlay) and hold the corner where it is marked between the thumb and forefinger.

  • Put your eye for viewing at one of the corners labeled for high or low altitudes depending on whether your object is high or low.

  • Sight along the elevation lines and estimate the altitude above the horizon that your object is (best to just mark it with your finger, then shine flashlight on the grid to read it).

  • Turn the astrolabe over and rotate the insert until the star is on the altitude circle you just measured.

  • Find the date, and read off the UT (and convert to local time).

  • You should notice that there are 2 possible solutions for time, since you can put the object either E or W of the meridian. Therefore you should measure two stars, and write down the 2 possible times you get for each. Only the correct time will match up for both stars. This works best with stars closer to the horizon either due East or due West.

Determining the time of the Year
  • Rotate the insert until the stars inside the horizon circle more or less match up with the sky that you see.

  • Convert your local time to UT, and read off the date from under the UT on the overlay Estimating the Local Sidereal Time

  • Put the date under the current UT, and then sidereal time will be the RA of the object on the meridian

Sunrise, Sunset and Twilights
  • The ecliptic is defined as the path of the Earth around the sun, or as seen from Earth, the path of the sun through the sky projected on the stars. This is drawn as the off center ellipse on both sides of the insert. You can plot the position of the sun along this, to estimate where the sun is at any given time (you can look these up yourself in the magazine Sky & Telescope).

  • Plot the position of the sun on your astrolabe for the date of interest (use water soluble pens on your laminated insert - or a removable sticky). Figure out the rise/set times as in the exercise above.

  • Plot the position of the sun on your astrolabe for the date of interest (use water soluble pens on your laminated insert - or a removable sticky). Figure out the rise/set times as in the exercise above.

  • End of or beginning of astronomical twilight (the first perceptible twilight) is when the sun is 18 degrees below the horizon. You can estimate this time by putting the sun under the 18 degree line on the overlay. Nautical twilight is defined when the sun is 12 degrees below the horizon and Civil twilight when the sun is 6 degrees below.

DateRADecDateRADec
Jan-1 18:41 -23:05 Jul-1 06:40 +23:07
Feb-1 20:58 -17:08 Aug-1 08:45 +18:04
Mar-1 22:48 -07:39 Sep-1 10:41 +08:20
Apr-1 00:42 +04:28 Oct-1 12:28 -03:07
May-1 02:32 +15:01 Nov-1 14:25 -14:22
Jun-1 04:35 +22:02 Dec-1 16:28 -21:46

Finding Moving Objects

  • Magazines readily available, such as Sky & Telescope publish the positions of the planets and sometimes bright comets for subsequent months. You can plot these moving objects non-permanently with pens or stickies to be able to plan when to observe them (rising / setting times etc.). Positions for July 1997:


Last Updated on April 18, 2000

This page has been visited times since October 1999.
Karen Meech, Institute for Astronomy, University of Hawaii
meech@.ifa.hawaii.edu