Stars
A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change gradually due to precession of the equinoxes and proper motion, and cyclically due to annual parallax. The declinations of solar system objects change very rapidly compared to those of stars, due to orbital motion and close proximity.
As seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90° − φ (where φ = observer's latitude) appear to circle daily around the celestial pole without dipping below the horizon, and are therefore called circumpolar stars. This similarly occurs in the Southern Hemisphere for objects with declinations less (i.e. more negative) than -90° − φ (where φ is always a negative number for southern latitudes). An extreme example is the pole star which has a declination near to +90°, so is circumpolar as seen from anywhere in the Northern Hemisphere except very close to the equator.
Circumpolar stars never dip below the horizon. Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface (except exactly on the equator). Generally, if a star whose declination is δ is circumpolar for some observer (where δ is either positive or negative), then a star whose declination is −δ never rises above the horizon, as seen by the same observer. (This neglects the effect of atmospheric refraction.) Likewise, if a star is circumpolar for an observer at latitude φ, then it never rises above the horizon as seen by an observer at latitude −φ.
Neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − |φ|, and at the south point, -90° + |φ|. From the poles, declination is uniform around the entire horizon, approximately 0°.
| Observer's latitude, ° | Declination | ||
| of circumpolar stars, ° | of non-circumpolar stars, ° | of stars not visible, ° | |
| + for north latitude, - for south | - for north latitude, + for south | ||
| 90 (Pole) | 90 to 0 |
|
0 to 90 |
| 66.5 (Arctic/Antarctic Circle) | 90 to 23.5 | +23.5 to -23.5 | 23.5 to 90 |
| 45 (midpoint) | 90 to 45 | +45 to -45 | 45 to 90 |
| 23.5 (Tropic of Cancer/Capricorn) | 90 to 66.5 | +66.5 to -66.5 | 66.5 to 90 |
| 0 (Equator) |
|
+90 to -90 |
|
Non-circumpolar stars are visible only during certain days or seasons of the year.
Read more about this topic: Declination
Famous quotes containing the word stars:
“How have I been able to live so long outside Nature without identifying myself with it? Everything lives, moves, everything corresponds; the magnetic rays, emanating either from myself or from others, cross the limitless chain of created things unimpeded; it is a transparent network that covers the world, and its slender threads communicate themselves by degrees to the planets and stars. Captive now upon earth, I commune with the chorus of the stars who share in my joys and sorrows.”
—Gérard De Nerval (1808–1855)
“O love is the crooked thing,
There is nobody wise enough
To find out all that is in it,
For he would be thinking of love
Till the stars had run away”
—William Butler Yeats (1865–1939)
“There seems to be a kind of order in the universe, in the movement of the stars and the turning of the earth and the changing of the seasons, and even in the cycle of human life. But human life itself is almost pure chaos. Everyone takes his stance, asserts his own rights and feelings, mistaking the motives of others, and his own.”
—Katherine Anne Porter (1890–1980)