Image 1 Caption: Image 1 The celestial sphere in a cross section diagram (not-to-scale) illustrating the:
Features:
The observer can only see the half of the celestial sphere above the horizon.
A_NCP = L and A_SCP = - L .
All astronomical objects within a circumpolar circle are circumpolar objects.
If they are fixed to the celestial sphere, the circumpolar objects NEVER rise or set.
For an observer in the Northern Hemisphere, circumpolar objects fixed to the celestial sphere in the northern (southern) circumpolar circle are always are always above (below) the horizon.
For an observer in the Southern Hemisphere, the situation is reversed.
Of course, circumpolar objects above the horizon may be invisible due to daylight or weather conditions.
Note, some sources (e.g., Wikipedia: Circumpolar star) do NOT consider astronomical objects that are always below the horizon as circumpolar objects---but who cares what they say.
Note:
We will call just Solar System objects long-term circumpolar objects.
Note, from Kepler's 3rd law we obtain the for circular orbits the fiducial-value orbital period formulae
p_earth_orbit = 2π/(GM) = (84.4902 ... minutes)*(M/M_⊕)*(R/R_eq_⊕)**(3/2) = (1.40817 ... hours)*(M/M_⊕)*(R/R_eq_⊕)**(3/2) = (0.0586737 ... days)*(M/M_⊕)*(R/R_eq_⊕)**(3/2) ,where R is mean orbital radius, gravitational constant G = 6.67430(15)*10**(-11) (MKS units), Earth mass M_⊕ = 5.9722(6)*10**24 kg, and Earth equatorial radius R_eq_⊕ 6378.1370 km.
It is clear that low-Earth-orbit artificial satellites can rise and set pretty often no matter where they are on the celestial sphere. So there is NOT much interest in whether they are technically circumpolar objects or NOT.
Long-term circumpolar objects will rise and set on long enough time scales.
The most notable rising and setting long-term circumpolar object is the Sun which rises and sets once per solar year = 365.2421897 days (J2000) at the Earth's poles: it is always above the horizon in the hemisphere summer and always below the horizon in the hemisphere winter.
In fact, within the Arctic Circle (66.6° N Lat.) and Antacrtic Circle (66.6° S Lat.), there will always be at least a 1 day when the Sun is always a circumpolar object.
Declination and right ascension (RA) constitute equatorial coordinate system which is analogous to geographical coordinate system latitude and longitude.
To do this easily, it is we must define declinations for due north and due south outside of the standard declination range δ∈[-90°,90°]: respectively, these are
δ_dN = L + 90° and δ_dS = δ_dN - 180° = L - 90°where the first formula is obvious from Image 1 and the second formula from the first one.
The altitudes for general declination δ from due north and due south, respectively, are clearly
A_N = -(δ - δ_dN) = -δ + L + 90° and A_S = (δ - δ_dS) = δ - L + 90°.The inverses of these formulae are
δ = -A_N + L + 90° and δ = A_S + L - 90° .For latitude L, we have
L = δ + A_N - 90° and L = δ - A_S + 90° .Recall, latitude L is given as a negative value for southern latitudes.
A concrete example can help to believe them. What is the altitude from due south for the Sun in Las Vegas, Nevada on the solstices and equinoxes? Behold:
A_S = δ - L + 90° In general. = δ - 36.2° + 90° = δ + 53.8° For L_LV ≅ 36.2°. = 23.4° + 53.8° = 77.2° For the summer solstice (∼ Jun21). = -23.4° + 53.8° = 30.4° For the winter solstice (∼ Dec21). = 53.8° For the equinoxes (vernal ∼ Mar21, fall ∼ Sep21).
Note, 23.4° is the famous tilt angle of the ecliptic (i.e., the great circle path of the Sun on the celestial sphere in a solar year = 365.2421897 days (J2000)) from the celestial equator. Note also, 23.4° is the Earth's axial tilt (currently 23.4°) (see also Wikipedia: Ecliptic: Obliquity of the ecliptic). On the summer solstice (∼ Jun21) (winter solstice (∼ Dec21)), the Sun is at its highest (lowest) point on the ecliptic and on the equinoxes (vernal ∼ Mar21, fall ∼ Sep21), the Sun is on the celestial equator.
A_N_NCP = L and A_S_SCP = - LRecall, south latitudes are measured as negative. Note, negative altitude means the astro-body is below the horizon.
Another interesting special case is for the declination of the circumpolar circle at the zenith location. In this case, both A_N and A_S are 90° and we find δ = L for both.