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 Subject: Measuring angular size Category: Science > Astronomy Asked by: hose7-ga List Price: \$20.00 Posted: 07 Jul 2005 23:23 PDT Expires: 06 Aug 2005 23:23 PDT Question ID: 541178
 ```What kind of an instrument is used to measure angular size of a distant object ? In my effort to buy such an instrument, I need to know its name.```
 Subject: Re: Measuring angular size Answered By: crabcakes-ga on 08 Jul 2005 01:52 PDT Rated:
 ```Hi hose7, Could you mean a sextant? I found other tools of measuring angles at a distance before I reread your question and noticed it was under science and astronomy. Most of the following tools are used to measure angles of stars, planets and locations. Sextant ======= "A sextant is a measuring instrument used to measure the angle of elevation of a celestial object above the horizon. Making this measurement is known as sighting the object or taking a sight. The angle, and the time when it was measured, are used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at noon to find one's latitude. See celestial navigation for more discussion. The scale of a sextant has a length of 1/6 of a full circle; 60°, hence the sextant's name. An octant is a similar device with a shorter scale, 1/8 of a circle; 45°, which was in use until 1767 when it was quickly replaced by the sextant. In 1767 the first edition of the nautical almanac tabulated lunar distances, enabling navigators to find the current time from the angle between the sun and the moon. This angle is however sometimes larger than 90°, and thus not possible to measure with an octant." http://en.wikipedia.org/wiki/Sextant "Practical celestial navigation usually requires a chronometer to measure time, a sextant to measure the angles, an almanac giving angular schedules of celestial objects and a set of sight reduction tables to help perform the math. With sight reduction tables, the only math required is addition and subtraction. Most people can master the procedure after a day or two of instruction and practice." http://www.algebra.com/algebra/about/history/Celestial-navigation.wikipedia Froogle has many sextants and books for sale: ://www.google.com/froogle?q=sextant&btnG=Search+Froogle Using the sextant http://www.jimthompson.net/boating/CelestialNav/CelestNotes/UsingSextant.htm An angle gauge? =============== "The gauge is used to tally trees in variable area plot sampling. The variable plot method is generally faster than fixed area plot sampling. The gauge usually has basal area per acre factors (BAF) of 5 to 40. The average diameter of the trees to be sampled is what determines which BAF to use. The angle gauge comes in several shapes and serves the same purpose as a cruising prism (see cruising prism below). 2004 price range: \$10-\$33." http://ext.nrs.wsu.edu/handtools/tools/measurements/#clin A clinometer? ============= "Optical Reading Clinometer Model No. PM-5/360 PC Measures:Altitude angle/Percent slope Heavy-duty clinometer for measuring vertical angles. Clinometer has two scales: angle in degrees above or below horizontally level or percent slope. Sighting allows user to simultaneously view object and read slope." http://www.pge.com/003_save_energy/003c_edu_train/pec/toolbox/tll/tool_catalog/misc_instru.shtml "Measuring with a Clinometer A clinometer is a tool that can help you to measure the angular height of the Moon more accurately than with your fists." http://www.learner.org/channel/workshops/lala/moonjactivities.html A protractor? ============= "Use tape to hinge two rulers together end to end at a 90 degree angle. Hold one ruler so that it points straight out to the horizon and the other one points to the straight up to the zenith. Tape a protractor to the rulers so that the base is on the horizon line and the zenith line cuts the 90 degree mark. To determine the altitude of the North Star where you live, hold the rulers horizontally at eye level. Look at the North Star and move the ruler to point at it. The number of degrees or altitude of the North Star can be read on the protractor." http://spikesworld.spike-jamie.com/science/astronomy/c421-03.html A transit? ========== "These units are for the serious builder or contractor who requires greater accuracy. These instruments are designed with a heavy-duty dome shaped head that holds a double ball bearing center, giving the user 5 minute readouts. Model 300B has a 1° vertical arc and two leveling vials. Units use a 3-1/2 x 8 thread." http://historywired.si.edu/object.cfm?ID=186 http://www.contractor-books.com/CB/Optical_Levels/Optical_Levels.htm?source=google "Set up for the class five angle measuring exercises. Get each participant to measure each angle. Rule out any clearly wrong measurments; average the remainder. Now score each participant on how little his/ her readings vary from the class's average. An alternative test for your transit is to go to a large field. Along one edge, mark perhaps 5 locations all in a straight line, perhaps 50 feet apart. (Use a longer distance if you can measure it accurately, and space allows. (You can see they are in line by looking along it!) About 150 feet from the baseline, at a point perpendicular to it's middle, stick a pole in the ground. (The location does not have to be precise.) Now measure angles between the baseline and the pole at each of your locations. Next, prepare a map of what you have measured. Start by drawing a line on a piece of paper. Mark 5 evenly spaced points along it. Draw the angles you recorded. The lines going away from the baseline should all met at one point if your readings are accurate. Remember: Any measuring device must be able to return the same reading every time it measures the same angle, weight, length, etc. It must give the answer in usable units. With the transit, a major element in achieving success will be your skill in devising something that will allow you to point directly at the distant objects which define the direction of the two arms of the angle. How you read off how far around your pointer has swung will be another critical area." http://www.arunet.co.uk/tkboyd/mm1t.htm Your eye and hand? :-) ======================== "One hand span is slightly greater than the separation between the two brightest stars in Orion - Rigel and Betelgeuse - while one and a half hand spans covers the distance between Dubhe and Polaris! For smaller angles you can use even other guidelines. For example, at arm's length, the width of your smallest finger is roughly one degree. A thumb's width is about two degrees, and the distance from the tip of your thumb to its first joint is about three degrees. Although everyone's hands and arms are different, of course, it is still easy enough to take your own measurements of arm length, hand span and so on, and to calibrate your own natural guides to help you estimate angles in the sky." http://www.geocities.com/angolano/Astronomy/PIinSky.html http://cosmos.phy.tufts.edu/~zirbel/ast21/labs/Angles.pdf Kamals, Quadrants, and Astrolabes ================================== http://www.jimthompson.net/boating/CelestialNav/CelestNotes/UsingSextant.htm If none of the above is the tool you had in mind, please do not rate ths answer without letting me know first. Request an Answer Clarification, and I'll respond as soon as possible. Please let me know if one of the above is correct! Regards, Crabcakes Search Terms ============ sextant astrolabe protractor tools for measuring angles + distance measuring angles + astronomy```
 ```The only specific instrument I can think of is a filar micrometer, used to measure the separation of double stars. Most measurements are taken from photographs with a known image scale and are done on a computer. Ian G.```
 ```Or do want an instrument that will allow you to measure the height of something that is a known distance away, or - vice versa - measure the distance by using the known height of a distant object (what a golfers use, based on the height of the flag in the cup)? You can use the instruments Crabcakes describes for this, but then have to apply some geometry. I do not know of the intrument that would do what I suggest, but can imagine such a sighting device with an adjustable scale or calculator to enter the height or distance and read out the other. Is there one? Myoarin```
 ```Thank you for the rating and the nice tip, Hose7. Sincerely, Crabcakes```