Sight reduction and the fix, the almanac behind them, the two latitudes that need no fix, and a teaching track that goes from first principles to the sight reduction tables.
Work a sight
What the almanac gives you
Learn it
Checking offline cache…
A sight
Assumed position
A running fix advances each earlier sight for the run between it and the last.
Sights kept
The fix
For an instant
A name, a star number, or several separated by commas. Aries is always shown, because every star sight is worked from it.
The navigational stars
SHA and declination are computed for the date, with proper motion and precession applied — not copied from a catalogue.
For a date and a DR
Leave Hs empty and this tells you when the sun crosses and how high to expect it. Fill it in afterwards and it gives you the latitude.
For an instant and a longitude
Longitude is what this needs and latitude is what it gives. No limb and no parallax: a star is a point at an infinite distance.
For an instant and a DR
The star horizon is usable between civil and nautical twilight. Before that the horizon is sharp and the stars are not out; after it, the stars are out and the horizon is gone.
A bearing, against the truth
Leave the bearing empty for the body's true azimuth alone. Give the bearing and you get compass error; give the chart variation as well and the error splits into variation and the deviation of your own compass.
A sight to work by the book
This derives the entering arguments, prints the page, and then marks what you read off it. Work the page yourself first — the comparison is only worth something if your Hc came from the table and not from here.
Your working
Hc as degrees and minutes; Z as whole degrees, the azimuth angle the page prints — not Zn.
The navigational triangle
Latitude and declination in degrees, north positive. LHA is the local hour angle of the body, measured west from your meridian.
Pick a drill
The seed is the problem’s name. Give the same kind and seed to
celnav drill at a terminal and you get the same sight, so a drill worked on
deck can be checked anywhere.
Your answer
- 1 Photograph
- 2 Horizon
- 3 Body
- 4 Sight
- 5 Plot
Take a photograph of the sun or the moon with the horizon in the same frame.
Hs—
Ho—
Tilt the line
Move left / right
Size of the circle
Photograph a patch of sky well up from the horizon with four or more stars you can name. Tap each one and say which it is. No time or position is needed — the angle between two stars is a property of the sky.
What this is worth, and what it is not
Not a sextant. A calibrated phone camera is worth about two to three arcminutes — the same order as a plastic lifeboat sextant, and several times worse than a metal one in practised hands. Uncorrected lens distortion can cost tens of arcminutes, so a sight from an uncalibrated camera is an estimate and must be treated as one.