The Marine Sextant

1
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.

2
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

3
(No Transcript)
4
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...

5
(No Transcript)
6
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.

7
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 observers height of eye
• adjustment to equivalent reading at the center of
  the earth and the center of the body
• refractive effects of the earths atmosphere

8
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...

9
(No Transcript)
10
Index Correction

• However, often there is a slight error. In this
  case, the view looks a little different

11
(No Transcript)
12
Index Correction

• To account for this error, we apply an index
  correction (IC).

13
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...

14
(No Transcript)
15
Dip Correction

• The dip correction is dependent upon the
  observers height of eye.
• Values of the dip correction are tabulated inside
  the front cover of the Nautical Almanac.

16
Apparent Altitude

• Now, by applying the IC and the dip correction,
  we can determine the apparent altitude (ha).
• Simply put,
• ha hs IC dip

17
Altitude Correction

• The last major correction accounts for the
  refractive effects of the earths 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...

18
Altitude Correction
19
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)

20
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.

21
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)

22
Additional Corrections

• These additional corrections make determination
  of Ho for the sun, moon, and planets generally
  more difficult than those for a star.
• For simplicitys sake, well stick to
  determination of Ho for a star

23
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...

24
(No Transcript)