Celestial Nav Flashcards
In celestial Navigation what is another term for the Geographic Position of Latitude?
Declination
In celestial Navigation what is another term for Longitude?
Greenwich Hour Angle (GHA)
Latitude = Zenith Distance - Declination
Latitude = Declination - Zenith Distance
Latitude = Zenith Distance + Declination
A Noon Sight: Example 1
On the 9th of June a vessel off Newfoundland in an EP of 47* 13’ N 042*23’ W, takes a sight of the sun’s LOWER LIMB when it is on the Meridian.
Name all the parts of the process to find out the Observed Latitude?
Part 1: Converting Merpass on the date to a Local noon at the Ships position - discuss
GO TO THE ALMANAC:
Look up the time of meridian passage at Greenwich for the 9th of June, then adjust for longitude to get the UT time of Local noon at the ships position.
In the Almanac find the date - find Merpass (11:59) GMT - you now need to convert this to local time using Arc to Time corrections.
Using your Longitude find the Arc to Time corrections in the Almanac.
042* = 2h 48m
23’ = 1m 32s
Total = 2h 49m 32s (you must round to whole min) = 2h50min
Merpass + (because psn is West) Arc to time correction = GMT/UT 9th June 14h 49mins.
A Noon Sight: Example 1
On the 9th of June a vessel off Newfoundland in an EP of 47* 13’ N 042*23’ W, takes a sight of the sun’s LOWER LIMB when it is on the Meridian.
Name all the parts of the process to find out the Observed Latitude?
Part 1: Converting Merpass on the date to a Local noon at the Ships position - discuss
What does the “d” value in the Almanac represent?
The difference in Declination for each hour in Minutes. Either + or -
What does Local Hour Angle (LHA) represent?
A distance in degrees from “us” to the sun. Measured no greater than 360
What does GHA (Greenwich Hour Angle) represent?
The distance in Degrees from Greenwich to the sun around the globe. No greater than 360
In the sight reduction tables. What does “same & contrary” refer to?
If refers to the position of the sun to you.
Same = On same hemisphere
Contrary = On opposing hemisphere’s
How do you work out Declination for a Meridian (Noon) Sight?
What two parts must it be split in to?
Declination must be split in two ways - Hours & Minutes
Using the new UT Local Noon time found from the Meridian Passage use the hour and mins to find the declination values.
For example if the hour was 15:04 - 1500 must be read from the day/date page - 04 must be found via the declination values at the back of the Almanac.
“d” value read off from the bottom of the day/date page indicates the value you must use. To know if you are -/+ you must read down the hours and see if the values are increasing or decreasing?
Add or subtract “d” from the declination hour value and this is your Declination at the time of meridian at the ships EP.
Index Error - If the error is on the Arc or off the Arc what should you do?
If the error is on - you must minus it
If the error is off - you must add it
Where do you find the Dip corrections and what correction follows this result?
In the front of the Almanac - measured in meters or feet.
You must then convert the Apparent Altitude correction for the Limb of the sun also called the Altitude Correction - depending on what part of the sun was taken.
Making sure you use the correct column for time of year!
Once the corrections for the sextant have been made, what calculation comes next and what result does this give?
Calculating the Zenith Distance.
Using 89* 60’.0 - you minus the Observed Altitude = Zenith Distance
If you add Apparant Altitude & Altitude Correction together what result does this give and how do you use it next?
Observed Altitude (Ho)
You use this in the Zenith Distance calculation.
What are the 5 headings you must do to complete a Meridian Passage ?
- Look up Mer Pass and convert to Arc to Time
- Look up Declination at the time of Local Noon
- Correct the Sextant
- Calculate Zenith Distance
- Apply Zenith Distance to the Declination to find the Observed Latitude.
In MCA exams - “chosen position” may also be referred to as what?
Assumed Position
In MCA exams the Azimuth (Zn) may also be referred to as what?
True Bearing of the sun
In MCA exams if in the question you have been given UT and NOT Chronometer Time what can you assume?
But what should you do anyway and what result does this give you?
We can assume that no adjustments are necessary & working out will give you an indication of whether or not its an AM or PM sun sight
Sun Sight:
What are the 5 Main Items you must find to plot a sun sight?
- Chosen Latitude
- Chosen Longitude
- Azimut (Zn)
- Intercept (nm)
- Closer or Away?
