Caption: Illustrated are 3 types of horizon: true horizon, visible horizon, and celestial horizon (AKA astronomical)---which we usually just call horizon for short.
Features:
- As illustrated, the
true horizon is
circle defined by
the line of sight
from an observation point that is a
tangent line to
a sphere.
- The visible horizon
is the actual local line dividing
Earth features
and sky.
The visible horizon is affected by surface features: e.g.,
trees,
mountains,
water waves,
weather,
atmospheric refraction, etc.
- The celestial horizon
is the great circle
on the celestial sphere
cut by the tangent plane
to the astronomical object
(considered as
an exact sphere) the observer is on at the location
of the observer.
The celestial horizon is the limit of the true horizon when the observing point is actually on the sphere for which it is specified.
- Which meaning of
horizon is meant is usually
known by context.
The celestial horizon is usually what we always mean in astronomy---except when we don't.
- We derive a simple approximate formula
for the true horizon in
Celestial sphere file:
horizon_types_formula_4.html
which may be this file in which case the derivation
is below.
Credit/Permission: User:Acdx,
2006
(uploaded to Wikipedia
by User:Liftan,
2007) /
Public domain.
Image link: Wikipedia:
File:Horizons.svg.
Local file: local link: horizon_types_formula.html.
Extended file: Celestial sphere file:
horizon_types_formula_4.html.
File: Celestial sphere file:
horizon_types_formula.html.