Section 1 Flashcards
Problems of Time
UT
Universal Time (new term for Greenwich Mean Time)
GMT
Greenwich Mean Time -> UT
UTC
Universal Time Coordinate, based on atomic clock
LMT
Local Mean Time
- time on navigator’s watch
- is normally set to the time where the vessel is sailing
Deck watch chronometer
accurate time piece set to UTC
used for astro navigation
why a navigator need the accurate time and date? and from where?
to facilitate the taking of sextant sights.
from Greenwich 000* longitude
Is DST applied to UT for astro navigation?
no
Difference UT and UTC?
UTC is based on atomic clocks and does not take account of the worlds rotation slowing down.
UT is based on the yearly average of the time passes over the greenwich meridian.
Time Zones:
- How many?
- One Time Zone has how much Degrees?
- 12 W and 12 E
- 15°
15° =
15* = 1hr
1° =
1* = 4min
What is the celestial sphere?
A imaginary sphere with the sun, stars and planets painted on it
GHA
Greenwich Hour Angle
GHA is measured …
…west of Greenwich all the way round to Greenwich again, 360°.
Whats the GHA of 45°E?
GHA 315°
Whats the “chosen position”?
Its the position for using astro navigation.
It is as close as possible to the estimated position of the vessel but rounded to the nearest whole degree of latitude.
LHA
Local Hour Angle
LHA is the …
… angle in a clockwise direction from the chosen position to the celestial object (Sun, Moon…)
formula
LHA =
LHA =
LHA = GHA - longitude west
LHA = GHA + longitude east
The GHA is the angle…
…west from the Prime Meridian to the Geographic Position (GP) of the celestial body.
The LHA is the angle …
… west from the Observer to the Geographic Position (GP) of the celestial body.
GP
Geographic Position
What are Arc to time tables used?
To convert a longitude to its equivalent in time.
Can you protect yourself against lightning strike?
- Prevent the sinking by installing a lightning protection. The rigging, the mast, the shrouds are all connected to the keel bolt via little spark distance. spark distance is importent because it prevents galvanic corrosion.
Why it is important to have a correct deviation table for your compass?
In case of a lightning the compass will have a deviation.
What is a Sextant?
A angle measuring device.
Two ways to shoot with a Sextant
Way 1:
1. Set Sextant at 0°
2. Set index and horizon shades
3. Look at object
4. hold the index arm and tilt the handle to take the object down to the horizon
Way 2:
1. Estimate object Ho
2. Set index shades
3. Look at horizon
4. Move index arm to and fro until object is on horizon.
A = Horizon shades
B = Horizon mirror
C = Index shades
D = Index mirror
E = Telescope
F = Index bar
G = Arc
H = Magnifying glass
I = Clamp
J = Micrometer
Whats the Global Position?
If a line were taken from the centre of our world through the surface to a celestial object, the point at which it intercepted the surface of the globe, would be the global position of the celestial object.
Whats the Zenith?
If a line were taken from the centre of our world through the position of the observer and out into space this would be the zenith.
Whats the Zenith distance?
The distance from the observer to the GP of the celestial object in degrees, minutes and seconds.
Sextant Check 1: P…
Perpendicularity
-> Index Mirror incorrect
1. Take off Telescope
2. Set arm at ~20°
3. Look past index mirror towards 0° (as sun would)
4. Adjust screw to get it perpendicular
Rechtwinklig / im Lot
perpendicular
Sextant Check 2: S…
Side Error
Horizon mirror not correct
1. Set sextant at 0° 0’
2. Look at star
3. adjust screw furthest away from frame on horizon mirror
4. Get stars on top of each other
Sextant Check 3: I….
Index Error
Mirror not parallel
1. Set sextant at 0° 0’
2. Look at star
3. adjust screw closest to frame on horizon mirror
4. Go back to Check 2: Side Error
Sextant Check: Goals
- Perpendicularity: 0
- Side Error: 0
- Some Index Error is okay
Note it or make a extra line on the drum at the new “0”
Shooting the sun: When do you know its to late ?
The altitude of the Sun will increase with each sight until it is directly overhead.
If the altitude is decreasing when you start taking your sights you are too late.
When to start shooting the sun?
10 minutes before the expected noon.
The altitude of the sun will increase with each sight and then start to decrease as noon passes.
Whats Mer.Pass?
It’s the moment when a celestial body (usually the Sun) crosses your local meridian — meaning it’s at its highest point in the sky for the day, directly due north or south, depending on your hemisphere.
What means “local noon”?
The actual time the sun passes overhead. The local noon of Greenwich can be found in the Almanac.
What do we need for a sun sight?
accurate deck watch chronometer set to UT
and
the time of the meridian passage from the Almanac
When do you know you are to late to shoot the sun?
When the altitude is decreasing when you start taking your sights.
Whats the Zenith?
A line from the centre of the earth through an object (my boat) into the space
? - ? = ZD
90° - Ho = ZD
