Download The Marine Sextant

The Marine Sextant
• Learning Objectives
• Know the purpose of a marine sextant.
• Apply proper procedures to determine
the observed altitude (Ho) of a celestial
body.
The Marine Sextant
• A marine sextant is nothing more than a
device designed to measure the angle
between two objects with a great deal of
precision.
• In celestial navigation, these two objects
are
– a celestial body (star, sun, moon, or planet)
– the visible horizon
Use of the Sextant
• A sextant is used to determine the sextant
altitude (hs) of a celestial body.
• First, we have to decide which stars to
observe; this is done using a Rude
Starfinder or other methods.
• When making an observation, the star
should look as shown in the next slide...
Determination of Observed
Altitude (Ho)
• We must make some corrections to hs to
come up with the Ho, which we need to
use the altitude-intercept method.
Determination of Observed
Altitude (Ho)
• These corrections account for
– index error (error in the sextant itself)
– difference between visible and celestial
horizon, due to the observer’s height of eye
– adjustment to equivalent reading at the
center of the earth and the center of the body
– refractive effects of the earth’s atmosphere
Index Correction
• There may be some error present in the
sextant itself; this is known as index
error.
• This is easily determined by setting the
sextant to zero and observing the
horizon; if there is no error, the view
looks like the next slide...
Index Correction
• However, often there is a slight error. In
this case, the view looks a little
different…
Index Correction
• To account for this error, we apply an
index correction (IC).
Dip Correction
• Next, we must account for the difference
between the celestial horizon and the
visible horizon, due to our height of eye.
• This is known as the dip correction.
• The need for this correction is illustrated
on the next slide...
Dip Correction
• The dip correction is dependent upon the
observer’s height of eye.
• Values of the dip correction are tabulated
inside the front cover of the Nautical
Almanac.
Apparent Altitude
• Now, by applying the IC and the dip
correction, we can determine the
apparent altitude (ha).
• Simply put,
ha = hs + IC + dip
Altitude Correction
• The last major correction accounts for
the refractive effects of the earth’s
atmosphere.
• This correction is known as the altitude
correction and is tabulated inside the
front cover of the Nautical Almanac.
• The next slide illustrates the need for this
correction...
Altitude Correction
Determination of Ho
• The corrections needed to convert from
the sextant altitude (hs) to observed
altitude (Ho) are
– IC (sextant error)
– Dip (height of eye)
– Altitude (refractive effects)
Additional Corrections
• These corrections are all that are needed
under normal circumstances to
determine Ho of a star.
• An additional correction is required if the
observation is made under non-standard
conditions of temperature or pressure.
Additional Corrections
• If we are using the sun, moon, or planets,
the problem becomes a bit more
complicated.
• In addition to the corrections we already
mentioned, we must also accout for
– horizontal parallax (sun, moon, Venus, and
Mars)
– semidiameter of the body (sun and moon)
– augmentation (moon)
Additional Corrections
• These additional corrections make
determination of Ho for the sun, moon,
and planets generally more difficult than
those for a star.
• For simplicity’s sake, we’ll stick to
determination of Ho for a star
Use of a Strip Chart
• To walk us through the calculation, we
normally use a form, called a strip chart.
• An example of a strip chart used for
calculating Ho of Dubhe is shown on the
next slide...