Download Measuring Altitude

Measuring Altitude
The Davis Mark 3 Marine Sextant (aka “lifeboat” sextant)
Light Path through a Sextant
The moveable arm adjusts the angle of Mirror B
Sextant Geometry
The arc is about one sixth of 360 degrees, hence a “sextant”
Through geometry, it can be proven that:
altitude (α) = two times angle observed between the mirrors (2 β)
Sextant graphic courtesy of Joaquim Alves Gaspar - own image, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=1314046
View through a Sextant
A sextant produces a split screen, one stationary (from the
horizon mirror) and one adjustable (from the index mirror)
The instrument error called Index Error
must be accounted for prior to an altitude measurement.
Set the sextant to zero
and observe the horizon.
If it is “broken” between
the two images, there is
index error.
Remove the error by
adjusting the index mirrors
until you have a continuous
horizon at zero. Alternatively,
adjust the angle until you
have a continuous horizon.
This angle is your index error.
The index correction is the negative value of the index error. Therefore,
the index correction is always removed from the altitude reading.
Taking a stellar sight
Align horizon image with the horizon
and find a star with your index mirror
Adjust the index mirror until the
star lines up with the horizon
Rock the sextant in an arc motion to confirm
that you were level with the horizon when
making the measurement.
Taking a Sun Sight
Here you will need the index filter.
Never look at the Sun though a
sextant without a filter!
Usually, the lower limb of the Sun is
used to measure solar altitude.
You may also need a horizon filter to
shield the glare of the Sun on the water.
Reading a Sextant
Arc reads whole degrees
“0” on Venier is the benchmark to read Arc
(always read lower value)
VENIER
Venier reads arcminutes
(in 2’ increments)
The first alignment of Vernier
and Arc tickmarks to the right
of zero is the benchmark to
read the Venier.
Remember 60’ = 1o (60 arcminutes = 1 degree)
Sextant Operation Quiz
Name ______________________
1. What is a sextant used to measure?
2. Specifically in celestial navigation, between which two benchmarks does a sextant measure?
3. In what unit(s) does a sextant measure?
4. Before taking sights of the Sun, you align the direct and reflected images of the horizon. Your reading is 1.8’.
a. What is your Index Error?
_________
b. What is your Index Correction? _________
5. You should always read the ( higher or lower ) value on the arc scale of the sextant? (circle answer)
6. What is the purpose of "swinging the arc" when taking a sight with a sextant?
a. To help obtain a clear view of the horizon.
b. To be sure that the sextant is horizontal at the time of the sight.
c. To help focus the body in the sextant telescope.
d. To be sure that the sextant is vertical at the time of the sight.
Answer:____
7. How do you measure the vernier scale on the sextant?
8. What is the reading of this sextant measurement:
9. What is the Index Error of the sextant in our example on the board? Answer: _______
10. What is the sextant reading of the Sun’s lower limb on the board (uncorrected for the Index Error)?
Answer:_______