Why a Chart Needs a Place
Two observers looking up at the same instant see different skies, and that is why a location is an input to the calculation rather than a label on the output.
Latitude and Longitude
Two angles measured at the centre of the Earth rather than a grid of distances, and the signed-decimal convention for writing them down, in which a wrong sign moves a place to the other side of the world.
Height Above Sea Level
The one genuine distance in a location, why it leaves both angles untouched, and the short list of things it actually changes.
The Time Zone in a Location
Why a place needs a time zone, why the offset cannot be worked out from the longitude, and why daylight saving is where charts most often break.
When a Local Time Happens Twice
The autumn hour that runs twice, the spring hour that never runs at all, and why the record itself cannot settle which one you meant.
The Sky as a Sphere
The model that treats the sky as a sphere of unlimited radius centred on the observer, keeping direction and deliberately discarding distance.
Great Circles and Small Circles
The distinction that makes every reference circle on this site mean something: whether its plane passes through the centre of the sphere.
The Horizon
The great circle dividing the visible sky from the half hidden below the ground, and the difference between the true horizon and the one you can actually see.
The Zenith and the Nadir
The point directly overhead, its opposite underfoot, and why both depend entirely on where the observer is standing.
North, South, and the Meridian
The four cardinal points on the horizon, and the meridian — the great circle from north through the zenith to south that divides climbing from descending.
The Celestial Poles and Equator
Projecting the Earth's axis and equator outward onto the sky, giving the celestial sphere an axis it turns about and a middle to measure from.
How Latitude Tilts the Sky
The altitude of the celestial pole equals your latitude — the single fact that connects where you stand to the shape of the sky above you.
Sky That Never Sets, Sky That Never Rises
The cap of sky that circles the pole without ever touching the horizon, the matching cap you can never see at all, and how both follow from your latitude.