Labs from YSI 94 :
Small Angle Calculations.
CARA Yerkes Summer Institute, August 1994
This is a Teacher's Guide and a lab.
- review concepts of circle, radius, diameter, circumference and
the equations used to calculate them
- measure small angles in degrees without using a protractor
- calibrate one hand to use in estimating angles in the sky
- determine the angular size of Jupiter in the night sky
meter stick, blackboard and chalk, calculator
The small angle approximation is a method of doing problems that
would ordinarily require trigonometry, but which may be solved by
simple arithmetic as long as the angle involved is no more than a few
Measuring a degree
Assume the distance from the chalkboard to the back of the
room is 8.3 meters. 8.3 x
2 = 52.15 meters. This is the
circumference of the circle. Dividing by 360 gives 0.144 meters, or
14.4 cm. Place two marks 14.4 cm apart on the chalkboard to
represent one degree as seen from the back of the room.
- Measure the distance from the chalkboard to the back of the
room. Consider this distance to be the radius of a large vertical circle.
- Compute the circumference of the circle by first doubling the
radius, then multiplying by
(pi, = 3.1416).
- One degree is defined as 1/360 of a circle, so divide the
circumference by 360.
- Make a pair of marks on the chalkboard the appropriate
distance apart to represent one degree when seen from the back of
Calibrating a Hand
- Once a degree has been measured, make a series of chalk
marks on the chalkboard one degree apart, covering at least 10
- Have students stand at the back of the room and find the
angular sizes of parts of their hands held at arm's length. In
particular, find a way to estimate accurately 1° and 10° .
Typically, a fist will measure about 10° and some finger
widths will measure 1° .
Determining the angular size of a moon, planet, star, etc.
Jupiter's diameter is 142,800 miles, and its distance from
the Earth at opposition is 628,700,000 miles. For a circle centered on
the Earth reaching to Jupiter, the circumference is 3.95 billion miles.
A degree is 1/360th of that distance or about 11 million miles.
Therefore, Jupiter's apparent size is 0.013 degrees. Since that are 60
minutes in a degree, this is equivalent to .78 minutes of arc. As an
extension to the example, in a 100 power telescope Jupiter would
appear to be 78 minutes of arc in diameter. Since the full Moon
appears to be 30 minutes of arc to the unaided eye, we see that
Jupiter at 100 power would be over twice the size of the
- What is needed are the size and distance of a celestial body.
Think of the distance to the object as the radius of a large circle.
2 to find the circumference
and divide by 360 to find
the size of a degree. Find the ratio of the object size to the size of a
degree of arc to find the degree measurement of the object. Multiply
by 60 to convert to minutes and by 60 again to convert to seconds, if
Find the angular size of Alpha Centauri, assuming a
diameter of 1 million miles (close to that of the sun) and a distance of
4.5 light year (ly) (1 ly = 5.8 x
10 miles). The circle centered
at the Earth
reaching to Alpha Centauri has a circumference of 1.64 x
so the size of a degree of arc is 4.55 x
10 miles. Alpha Centauri is
thus 2.2 x
or 0.008 seconds of arc. Even at 100 power
the apparent size of the disk would be less that a second of arc,
which is not resolvable to the eye.
The small angle technique is applicable to finding the length of a
parsec. A parsec is defined as the distance to a hypothetical star
whose parallax is one second of arc. From such a star the angular
distance from the earth to the sun would appear to be one second of
arc. The arc length is 1 AU (93,000,000 mi) and the are 60 x 60 x
360 seconds in a full circle. Therefore there are 1,296,000 AU in a
full circle around a hypothetical star one parsec away. Dividing by
gives the diameter of the circle which is 412,530 AU. Dividing by 2
gives the radius of the circle which is the distance to a hypothetical
star one parsec away: 206,265 AU. Multiplying by 93,000,000 gives
the equivalent distance in miles.
Important Disclaimers and