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Shooting the Stars: There is an App for That

by Laura Collum (10/25/10).

One of the most common sextant one sees is the Navy Mark II. In very good condition with all the accessories it retails for around $600. A rare, double-framed Troughton-made sextant went for $3,200 in an online auction in 2004.

Last May I went on the trip of a lifetime. The opportunity came about when friends asked me a year before if I would like to go to England with them. Without hesitation I said, “Yes!” During the preceding months I made a list of places I wanted to see. I wanted to see the textiles at the Victoria and Albert museum, the Greenwich Observatory holding the first chronometers, and Queen Mary’s dollhouse at Windsor Castle. Anything else would be gravy.

All the nautical and scientific instruments I saw in the National Maritime Museum and Royal Observatory in Greenwich, and in the British Museum in London were this antique dealer’s dream! My friends were very patient with me while I drooled and exclaimed “Ooooh, look at that, that’s what I was telling you about” (Often, when I looked around, they were either sitting down or had wandered off to another part of the museum). They were actually marvelous traveling companions and we had a wonderful time everywhere we went.

Why am I telling you this? Because the nautical instruments I saw were spectacular. And I have been working on articles on navigation instruments for months. And, of course, being at the place (i.e. Greenwich Observatory, the subject of a future article) where the hunt for longitude began was monumental. And, you all know Greenwich is the home of the prime meridian, thus one can actually straddle the line between east and west.

Yours truly (in the light blue coat) is straddling the line between East and West at the Greenwich Observatory.

Travel by sea has proven to be very important in the course of human endeavor. In the past it was much faster than land travel, so commerce and the exploration that increased commerce came to depend on it. The real problem with sea travel was reliable navigation once you got past the storm problem and the foundering on the rocks problem, which, really, was part of the navigation problem. Compasses, discussed in a previous article (Binnacles 101: Get Your Bearings with Nautical Collectibles), helped direct the navigator, but other instruments were used as well. These navigational tools, essential for any journey then, are lovely antiques for us to collect today. Astrolabes, quadrants, cross staves, octants and sextants— all of which I saw in England—are just some of the results of the navigator’s need and the instrument maker’s art. Of course, museums in the United States also have these instruments, but I haven’t been to them and Greenwich is found only in England.

Astronomers observed early on that the star Polaris appeared at a fixed altitude above the horizon at specific locations (in the Northern Hemisphere). Navigators used this fact to devise ways of measuring that altitude so they could know when they were on the latitude of any given port; then sail east or west to the port. Originally, navigators used their fingers held horizontally at arm’s length. Try it; the bottom finger is aligned with the horizon and the top with the star. You must look at both at once, and since you can’t, you must look back and forth quickly. If the mariner needs a “one finger” latitude but the star is at a multiple finger altitude, he must sail south, etc.

The Arabs developed the Kamal, a rectangular piece of wood or horn with a string attached to the center, which was much more accurate than fingers. The observer would line up the horizon on the bottom of the wood and the star on the top. A knot was tied in the string at this point measuring the distance from Kamal to mouth. As with the finger measure, this would indicate the desired latitude, so different knots indicated different ports. The Kamal was inexpensive and easy to use except for the inability to see the horizon and the star at the same time without moving the eye back and forth.

Once the earth was proved to be a globe, the concepts of latitude and longitude were fully realized and the race was on to develop better and more accurate means of measurement.

Author’s Note: Remember from school, latitude lines on the globe go around horizontally, like slicing onion rings. This gives the north/south component. Longitude lines on the globe go north to south like slicing apple wedges and give the east/west component. Because the earth turns on its axis there is a time component to measuring longitude accurately, and because the earth tilts on its axis there is a declination factor to consider in figuring latitude accurately. For a thorough discussion and full understanding of this subject, see books and Internet articles.

A number of instruments came along in a line. The quadrant was a quarter circle (pie shape) of wood or brass with sighting holes along one edge, a scale on the arc, and a plumb bob on string suspended from the apex to the arc; the string crossing the scale gave the altitude. The Portuguese, under King Henry the Navigator, used this instrument during their “expansion on the seas.” Columbus used one on his first voyage. Just think how difficult that plumb bob was to control on a pitching ship. The Greeks developed the astrolabe very early as an astronomical device and the Arabs further developed it to its height of usefulness and beauty.

A 19th century reproduction is not as fine as the early ones seen in museums and fine private collections. Retail for one like this is about $600. Fine early astrolabes go for tens of thousands.

King Henry’s sailors used one modified for marine use with just a simple scale and pivoted alidade (an alidade is just two sighting holes on a pivoted rod. Once Polaris is sighted, altitude is read from the scale). Columbus used one on a later voyage and the mariner’s astrolabe was in use until about 1700.

The mariner’s astrolabe was a simplified version of the astronomical astrolabe with a scale and alidade. The observer held up the astrolabe by the ring and sighted through the holes on the alidade to read the altitude of Polaris.

Next came the cross-staff with a sliding cross arm and scale along the long arm.

When using the cross staff, the cross piece was moved forward or back to line up the horizon on the bottom of the cross piece and the star on the top. The altitude was read on the scale on the shaft.

The observer must slide the cross arm until Polaris is sighted on the top of the cross arm and the horizon on the bottom. The scale is read for altitude. Again, the observer must look at two places at once. By the way, a man using a cross-staff looked similar in position to a man shooting a bow, and so the term “shooting the stars” came into use and is still used today. The measured altitudes were looked up in carefully compiled tables of latitudes so the navigator could calculate the ship’s location.

Longitude was determined by measuring the angle of the sun to the horizon at local noon and consulting tables for calculated locations. The instrument called a back-sight allowed the observer to make measurements with his back to the sun. This was a great improvement from the previous method of looking directly into the sun. The back-sight used shadows and later mirrors. The first half of the 18th century saw much competition among instrument makers to improve the back-sight. In Britain, an American shared the award for a design of a doubly reflecting instrument in the competition instituted by the English Board of Longitude. The mirrors were important because they allowed the observer to see the horizon and star at the same time, instead of looking back and forth, allowing for greater accuracy. This double reflecting instrument was first described by Galileo but was not developed in his time. The early doubly reflecting instrument gave rise to the back staff and eventually to Hadley’s quadrant, the first octant in 1731.

The back staff has two arcs with sliding vanes, the shadow vane A, and the sight vane, B. The two vanes are adjusted so that the shadow from the sun S, cast by vane A falls on the slit in the horizon vane, C, while the horizon is sighted through the sight vane and the slit in C. (hard to do while on a pitching ship while chewing gum!) The sum of the readings of the two scales gives the angle between the sun and the point directly overhead- the zenith distance. This was looked up in books of tables to determine longitude, kinda.

So when you are antiquing, know that any mariner’s astrolabes you see are highly likely to be reproductions since very few survived. According to one source, only 34 originals are known. Original quadrants, cross staves and back-staves are ultra pricey if found.

The large, early octants are known as Hadley’s quadrant. (One sold in an online auction in ’08 for $15,000). By the middle of the 1700s, accurate ways to make scales of arc were devised, so as time went on, the arcs got smaller with increased accuracy, and so did the octant. The early octants were made of walnut or other native wood with the scale made of boxwood. By 1750, they were made of African mahogany, then ebony. In 1760 the scale was made of ivory, the index arm of brass, and a vernier scale was added.

Octant made by Isaac Bradford of Wapping Old Stairs, London. I have a picture taken from a boat on the Thames of Wapping Old Stairs during my recent trip. Retail price of octants with boxes depend on condition and maker can and range from $800 to $2,000 and up.

In 1780 the tangential screw for fine adjustment on the index arm was added. Sextants are mainly just an improvement on the octant in ease of use and accuracy. Modern materials such as brass with black paint to make the frames and index arms and silver for the scales became the norm. Today you can actually purchase a plastic sextant with brass articulating parts and scale if you so choose.

Sextants in keystone-shaped boxes are interesting to collectors. Retail prices vary due to condition and maker and start at around $500.

As of the end of the last century, the United States Naval Academy no longer offers a course on celestial navigation and the use of the sextant. Navigating is now accomplished by electronic devices. More than 2,000 years of navigating by the stars has effectively come to an end1. So if you find yourself on the high seas and want to know where you are, “there is an app for that.”

Author’s Note: While researching details for this article, I came across a nifty little collection of navigation instruments at the Adler Planetarium in Chicago. If I ever get up that way I am going to visit. Of course other museums around the country have navigation instruments this collection seems quite elegant and direct.

1 “Taking the Stars Celestial Navigation from Argonauts to Astronauts”, Ifland, P. Krieger Publishing Company Malabar, Florida. 1998.

Laura Collum is a Worthologist who specializes in decoys, nautical and scientific instruments.

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