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Q: Measuring angular size ( Answered 5 out of 5 stars,   3 Comments )
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:5 out of 5 stars
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.

"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."

"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."

Froogle has many sextants and books for sale:

Using the sextant

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."

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."
"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."

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."

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."

"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

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

How you read off how far around your pointer has swung will be another
critical area."

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."

Kamals, Quadrants, and  Astrolabes

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
tools for measuring angles + distance
measuring angles + astronomy
hose7-ga rated this answer:5 out of 5 stars and gave an additional tip of: $5.00

Subject: Re: Measuring angular size
From: iang-ga on 08 Jul 2005 02:10 PDT
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

Ian G.
Subject: Re: Measuring angular size
From: myoarin-ga on 08 Jul 2005 06:50 PDT
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?
Subject: Re: Measuring angular size
From: crabcakes-ga on 08 Jul 2005 10:57 PDT
Thank you for the rating and the nice tip, Hose7.
Sincerely, Crabcakes

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