rotation

In geometry, a transformation in which a figure is turned about a given point, known as the centre of rotation. A rotation of 180° is known as a half turn.

Three things are needed to describe a rotation: the angle of rotation, the direction of rotation (clockwise or anticlockwise), and the centre of rotation.

For example, in the diagram, triangle A is mapped onto triangle B by a rotation of 90° clockwise with centre (0,0):

The inverse transformation is the rotation of B back onto A. When a shape is rotated but appears not to have moved it is said to have rotational symmetry. Translation, reflection, and enlargement can also transform shapes.

rotation

The Earth rotates in an anticlockwise direction (looking along its axis from north to south) and takes approximately 24 hours to rotate on its own axis. Day and night depend on the Earth's rotation; the half of the Earth facing the Sun is in daylight and the half facing away from the Sun is in darkness.

In astronomy, movement of a planet rotating about its own axis. For the Earth, one complete rotation takes 23 hours and 56 minutes. The Earth rotates in an anticlockwise direction (as viewed looking along its axis from north to south), leading to the appearance from Earth of the Sun moving from east to west in the daily cycle. The rotation of the Earth produces a surface speed at the Equator of about 1,600 kph/1,000 mph. This speed decreases further north and south of the Equator. Artificial satellites use the Earth's natural rotation to orbit the Earth.

As the Earth's axis is tilted with respect to the Sun, the lengths of day and night vary across the globe. At the Equator, day and night are both 12 hours long. During the northern-hemisphere winter, days become shorter with increasing latitude north, until a point where there is continuous night. At the same time, during the southern-hemisphere summer, days become longer with increasing latitude south, until a point where there is continuous daylight. The situation is reversed during the northern-hemisphere summer.