Earth rotates once a day from west to east. As a result, the diurnal motion of the Sun and stars, in which they appear to rise in the east and set in the west, can be observed.
Basic Knowledge
Earth rotates counterclockwise (west → east) when viewed from the North Pole. It takes about 24 hours to complete one rotation. Earth's equatorial radius is about 6378km, and its rotational speed at the equator is approximately 1670km/h.
Because Earth rotates from west to east, the Sun and stars appear from Earth's surface to rise in the east and set in the west. This is called diurnal motion. They move about 15 degrees per hour (=360÷24).
In the northern sky, stars move counterclockwise around Polaris. This is because Polaris lies on the extension of Earth's rotation axis toward the North Pole. Polaris is the alpha star of Ursa Minor and points toward the north celestial pole with an accuracy of about 1 degree.
Rotation (the turning of a celestial body around its own axis; Earth completes one rotation in about 24 hours)
Diurnal motion (the apparent movement of celestial bodies caused by Earth's rotation)
Celestial equator and north celestial pole (the Earth's equator and North Pole extended onto the celestial sphere)
Polaris (the alpha star of Ursa Minor; located nearly at the north celestial pole)
Diurnal motion of fixed stars (rising in the east and setting in the west; about 15 degrees per hour)
Transparent hemisphere (a semitransparent hemispherical model used to represent the celestial sphere in observations)
Further Study (Background and Significance)
A famous experiment demonstrating Earth's rotation is Foucault's pendulum (1851). When a long, heavy pendulum is suspended, its plane of oscillation appears to rotate gradually over time. This happens because Earth rotates beneath it and became direct evidence of Earth's rotation.
Polaris has played an important role in the history of navigation. Because the altitude of Polaris (its angle above the horizon) is nearly equal to the north latitude of the observation site, the latitude can be found by measuring Polaris's altitude.
Earth's rotation is actually gradually slowing down. Tidal forces from the Moon slow it by about 1.5ms (milliseconds) per day per century, and several hundred million years ago a day was about 22 hours long. This is also why a leap second is occasionally inserted.
- Rotation = once a day, west → east, counterclockwise (viewed from the north)
- Diurnal motion = east to west (apparent motion from Earth's surface)
- Moves 15 degrees in one hour (360÷24)
- Polaris is the alpha star of Ursa Minor and lies nearly at the north celestial pole
- Polaris's altitude = the north latitude of the observation site
- Foucault's pendulum = evidence of rotation
- Rotational speed at the equator = approximately 1670km/h
Points of Caution (Easily Confused)
① Earth's rotation is west → east, while the diurnal motion of celestial bodies is east → west (apparent). ② In the northern sky, stars move counterclockwise; in the southern sky, they follow an arc from east → overhead → west. ③ Polaris is not the brightest star (it is a second-magnitude star). ④ It moves about 15 degrees per hour (15 degrees = 360÷24).
Practice
- State the direction of Earth's rotation.
- About how many degrees per hour do fixed stars appear to move?
- Which constellation contains Polaris, and which star is it?