Wobbling Earth

Very slowly, the Earth is wobbling in space, so that its axis traces out a circle on the celestial sphere. This effect is termed "precession" and one complete wobble takes 25,800 years. Because of precession, the position of both celestial poles among the stars is slowly changing. In the year 2000, Polaris was 3/4° (one and a half Moon diameters) from the exact celestial pole; in 2100 it will be just under V 2° away, the closest it ever comes. The effects of precession are barely noticeable to the human eye.

path of north celestial pole across the sky every 25,800 years

Vega, pole star in AD 13000

angle of tilt remains the same throughout precession angle of tilt remains the same throughout precession

Earth's axis of rotation rotation of Earth around its axis

Alderamin, pole star in AD 8000

path of north celestial pole across the sky every 25,800 years

Alderamin, pole star in AD 8000

Deneb

Vega, pole star in AD 13000

evening star

When setting after the Sun, brilliant Venus Is known as the Evening "Star". Here It lies In evening twilight near the crescent Moon.

Earth's axis of rotation motion of the planets

The Moon and planets are constantly on the move. The Inner planets, Mercury and Venus, stay close to the Sun. Mercury Is particularly difficult to see, as It Is usually low and lost In twilight, but Venus shines brilliantly as the Morning or Evening "Star", depending which side of the Sun It Is on. The outer naked-eye planets, Mars, Jupiter, and Saturn, can be seen In any part of the sky as they orbit the Sun, appearing like bright stars among the familiar constellation shapes.

evening star

When setting after the Sun, brilliant Venus Is known as the Evening "Star". Here It lies In evening twilight near the crescent Moon.

Polaris (current north Pole Star)

star coordinates

Precession gradually changes the coordinates of stars, so for precise work, positions in catalogs are referred to a standard date known as an epoch. Current catalogs and charts are for Epoch 2000. These were exactly correct on 1 January 2000.

Deneb rotation of Earth around its axis

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