Celestial pole
Fixed points in the sky around which all stars appear to rotate.
The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. They appear permanently directly overhead to observers at Earth's North Pole and South Pole, respectively. As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per sidereal day.
- definition
- Points where Earth's axis meets the celestial sphere
- north_pole_star
- Polaris (currently within one degree)
- south_pole_star
- Sigma Octantis (more than one degree away)
- coordinate_system
- Declinations of +90° and -90°
- current_epoch
- J2000.0
Lore & Background
The celestial poles are also the poles of the celestial equatorial coordinate system, meaning they have declinations of +90 degrees and −90 degrees (for the north and south celestial poles, respectively). Despite their apparently fixed positions, the celestial poles in the long term do not actually remain permanently fixed against the background of the stars. The Earth's axis is also subject to other complex motions which cause the celestial poles to shift slightly over cycles of varying lengths (see nutation, polar motion and axial tilt). Finally, over very long periods the positions of the stars themselves change, because of the stars' proper motions. To take into account such movement, celestial pole definitions come with an epoch to specify the date of the rotation axis; J2000.0 is the current standard.
Reader's Guide
The north celestial pole currently is within one degree of the bright star Polaris, making it useful for navigation in the Northern Hemisphere: not only is it always above the north point of the horizon, but its altitude angle is always (nearly) equal to the observer's geographic latitude. The south celestial pole lies in the dim constellation Octans. It can be located using the Southern Cross and its pointer stars, or via an equilateral triangle formed with Canopus and Achernar, or using the Magellanic Clouds. An analogous concept applies to other planets: a planet's celestial poles are the points in the sky where the projection of the planet's axis of rotation intersects the celestial sphere.
Did You Know?
- The north celestial pole is currently within one degree of Polaris, the 'North Star.'
- The south celestial pole lies in the dim constellation Octans, with Sigma Octantis more than one degree away.
Frequently Asked Questions
What exactly is the Celestial pole in Lines & Hemispheres?
It is the spot on the imaginary sky-dome where Earth's spin axis, if stretched outward indefinitely, would punch through. Think of it as the one anchor point that stays put while every other star whirls around it.
Which star marks the Celestial pole right now?
In the northern sky, Polaris holds the role, sitting within roughly one degree of the exact point. In the southern sky, Sigma Octantis is the designated marker, though it drifts more than a degree off-center.
Why do all the stars appear to circle the Celestial pole?
Because Earth is the object actually rotating, so the pole acts as a stationary pivot in the observer's frame. Every other point in the sky traces a full loop around it once per sidereal day.
What declination values define the two Celestial poles?
The north pole sits at +90° declination and the south pole at −90°, making them the absolute extremes of that coordinate axis. These reference values are tied to the J2000.0 epoch standard.
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