Here, however, we are concerned mainly with two motions- rotation and revolution.
- Rotation is the movement of the Earth on its axis from the west to east.
- It takes 23 hours 56 minutes and 4.09 seconds to complete each rotation .
- A solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian.
- The Earth's rotation is counter-clockwise around its axis.
- The Earth rotates from West to East, which causes the Sun to rise in the East and set in the West.
- The velocity of rotation of an object on the earth’s surface at the equator is about 1700 km per hour and it decrease towards the poles.
- Earth’s rotation results in – causation of day and night; a difference of 1hour between two meridians which are 15 degree apart; deflection of ocean currents and winds; rise and fall of tides everyday.
- The Earth orbits around the Sun once every 365.26 days. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, or a Sun or Moon diameter every 12 hours.
- The Earth's revolution is counter-clockwise around the Sun.
- The orbital speed of the Earth averages about 29.79 km/s (108,000 km/h). 30 km/s is fast enough to cover the planet's diameter (about 12,600 km) in seven minutes, or the distance to the Moon (384,000 km) in four hours.
- Revolution of the earth results in- the change of seasons; variation in the lengths of day and night at different times of the year; difference in altitude of the sun at noon, at different times of the year.
Effect of Earth's Inclined Axis
The inclination of the earth’s axis is an important feature of the earth-sun relationship. In its elliptical movement around the sun, earth’s axis makes a constant angle of 66.5° with the plane of the ecliptic, and is tilted 23.5° from a line perpendicular to this plane. The two facts, i.e., a fixed angle of the earth’s axis to the plane of the ecliptic and the axis always pointing in the same direction, when combined with the earth’s movements, results in varying lengths of day and nights, seasonality and changes in the altitude of sun at different times of the year.
The tilt of the Earth’s axis is the reason we have seasons. Season are period into which the year can be divided as a result of the climatic conditions, largely due to change in the duration and intensity of solar radiation. Seasonal changes are caused by the earth’s revolution round the sun and inclination of earth’s axis to the ecliptic plane and because the axis constantly points towards the pole star.
- During the four seasons – summer, autumn, winter and spring – the position of earth vis-à-vis the sun changes as it revolves.
- The hemisphere that is tilted towards the Sun is warmer because sunlight travels more directly to the Earth’s surface so less gets scattered in the atmosphere. That means that when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere.
- The overhead position of the sun changes, resulting in summer season in the northern hemisphere between Maech 21 and September 23, while it has the winter season between September 23 and March 21, and vice-versa in the southern hemisphere.
- Spring and autumn are shorter seasons, marking the transition between the two main seasons.
- While these four seasons are the pattern in mid-latitudes, the tropical pattern is commonly a wet season and a dry season.
Solstics is one of the two dates in the year on which the sun reaches its greatest altitude north or south of the equator and is directly overhead along one of the lines of the tropics.
- Summer Solistice – It happens on June 21 or 22 of every year. The Longest day and Shortest night is observed in the Northern Hemisphere. This occurs because the Northern Hemisphere is inclined towards the Sun with the North pole pointing towards the Sun. Therefore, the Sunrays fall directly over the Tropic of Cancer. A large portion of Northern Hemisphere receives light from the Sun, hence it is summer in the regions of this hemisphere. But, in the Southern Hemisphere, the region experiences winter season with longer nights than the days.
- Winter Solistice – This happens on December 21 or 22 of each year. The Northern Hemisphere observes shortest day and Longest Night. Whereas, the Southern Hemisphere experiences Longest day and Shortest night. This happens because the South pole is inclined towards the Sun with the Southern Hemisphere receing maximum sunlight, hence, it is summer season in the southern hemisphere but winter season in the northern hemisphere.This position of the Earth is known as Winter Solistice.
When direct sunrays fall over the equator, maximum sunlight is received by the equatorial region. This results in equal length of day and night over all of Earth. At this position, none of the poles are tilted or inclined towards the Sun. This is known as Equinox. In each solar year we have two Equinox’s, one in March (usually around the 20/21st) and one in September (usually around the 22/23rd).
- Types of Equinoxe :
Twilight on Earth is the illumination of the lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere so that Earth's surface is neither completely lit nor completely dark. The word twilight is also used to denote the periods of time when this illumination occurs
The midnight sun :
The midnight sun is a natural phenomenon, observable in latitudes 66.5° north of the Arctic Circle or south of the Antarctic Circle, when the sun remains visible at the local midnight. Around the summer solstice (approximately 21 June in the Northern Hemisphere and 22 December in the Southern Hemisphere) the sun is visible for the full 24 hours, given fair weather.
An Eclipses occurs when the sun, moon and earth are in a straight line
- Lunar Eclipses :
- Solar Eclipses :