ROTATION AND REVOLUTION OF THE EARTH
Rotation of the Earth
The Earth rotates (spins) around its axis. The axis is an imaginary line passing through the center of the Earth. Its two ends on the surface are called NORTH and SOUTH POLES. The Earth completes a rotation in 24 hours (23 hours, 56 minutes and 4 seconds to the exact).
The Earth rotates in an eastward direction opposite to the apparent movement of the sun, moon and stars across the sky. Looking down on a globe from above the North Pole, the direction of rotation is counter-clockwise (anti-clockwise direction). This eastward direction of rotation not only defines the movements of the zone of daylight on the Earth’s surface but also helps define the circulatory movements of the atmosphere and oceans.
The velocity of rotation of the Earth varies depending on the distance of a given place from the EQUATOR (the imaginary circle around the Earth halfway between the two poles). The rotational velocity at the poles is nearly zero. The greatest velocity of rotation is found at the Equator where the distance traveled by a point in 24 hours is largest, the velocity is about 1700 km per hour. At 60 degree parallel, it is half of what it is at the Equator (850 km per hour).
Revolution of the Earth
Earth also revolves around the sun in an elliptical, almost circular, orbit at an average distance from the sun of about 149,000,000 km. This motion is called REVOLUTION. The path in which the Earth describes its motion is called ORBIT. Because of the elliptical shape of the orbit, the distance varies from time to time. About January 3 the Earth is closest to the Sun and is said to be at PERIHELION (from Greek: ‘peri’ = close to; ‘helios’ = sun); its distance then from the sun is approximately 147 million km.
Around July 4 the Earth is about 152 million km from the sun. It is then that the Earth has reached its furthest point from the sun and is said to be at APHELION (Greek: ‘ap’ = away; ‘helios’ = sun). Five million km is insignificant in space and these varying distances from the Earth to the Sun do not materially affect the receipt of energy or Earth
The Period of Revolution
The period of time the Earth takes to make one revolution around the Sun determines the length of one year. Earth takes to complete one revolution of the Sun in 365 days & 6 hours. Because the Earth makes 365 degrees rotations on its axis during the time it takes to complete one revolution of the Sun, a year is said to have 365 days. Because of the difficulty of dealing with a fraction of a day, it has been decided that a year would have 365 days and that in every fourth year, called LEAP YEAR, an extra day would be added in February.
Season Changes Occurred
Another effect of the earth’s revolution is the change in seasons. Season itself is a natural phenomenon that occurs as a result of the annual revolution of the earth around the sun, and is caused by the tilt of the earth’s axis relative to the plane of revolution.
This season can be of various kinds. Areas with moderate climate and poles, will experience seasonal changes that can be observed from changes in the intensity of sunlight to the surface of the earth.
Different seasonal variations in each hemisphere also affect the life of living things on earth. One of them is to cause animals to experience hibernation or migration, and make plants become more active in certain seasons.
Generally, in temperate regions, the season that occurs each year is divided into four groups, as follows:
– Spring (vernal) / Spring
Spring occurs in nontropical areas, and is a winter transition season to summer. Spring in the northern hemisphere, generally takes place from March 21 to June 21, and in the southern hemisphere, generally spring runs from September 23 to December 21.
– Summer (festival) / Summer
This summer generally occurs in countries that have a temperate climate. Summer in each country or in each region also occurs at different times, depending on geographical location.
The region which is located in the northern part of the earth, has a summer which generally takes place on June 21 to September 23, and for the earth’s region in the south, usually takes place on December 21 to March 21.
In many countries that have summer, usually this season is also used as the school holiday season. At that time, most people go on vacation to the beach to sunbathe. Also in this season, many fruits and plants thrive so that they attract more attention to travel.
– Autumn / Autumn
Autumn generally occurs in areas with a temperate climate, which has four seasons. Autumn is a transition season from summer to winter, which occurs in regions with moderate climate zones. Usually, this season is marked by the number of plants that are ready for harvest.
This season can be observed from the length of daytime that lasts shorter than usual. In addition, the precipitation that takes place in some parts of the earth will also increase.
Generally, autumn on the northern part of the earth, takes place on September 23 to December 21, and on the southern part of the earth, takes place on March 21- June 21.
– Winter (winter) / Winter
Winter usually occurs when the temperature of the earth is at the lowest temperature. Generally, only countries that have a subtropical and temperate climate experience winter. In the northern part of the earth, winter generally takes place December 21 to March 21, while in the southern part of the earth, it takes place from June 21 to September 23.
Whereas in tropical climates, usually only two seasons occur, namely the dry season and the rainy season. The following statement:
– Dry season
The dry season is one of the seasons that takes place in the tropics. The season which is also often called the dry season is influenced by the monsoon system. The dry season is characterized by the amount of rainfall per month that is little or no more than 60 mm / month, or 20 mm per ten days, and lasts for one month in a row.
Examples of regions of the earth with a dry season, are Africa, South Asia, Southeast Asia, northeastern Australia, and parts of South America.
– Rainy season
Rainy season is a season that occurs with a marked increase in rainfall in an area, with a certain period of time. The rainy season only occurs in tropical climates.
Characteristic of the rainy season is when the rainfall that occurs in the three dasarian amounted to more than 100 mm / m2 in each dasarian and takes place continuously.
Leap year – Six hours are saved every year from the time period of a year. In four years, these are combined to make a day and under the Georgian calender , it is added to the February 29. such a year has 366 days compared to the 365 days of normal year.
Why Does the Shape of the Moon Appear to Change?
The shape of the moon appears to change because its position changes during its revolution around Earth. The shape of the moon changes according to the reflection of the sun’s light upon its surface.
The moon, which is the second brightest object in the sky, does not actually change its shape as many on earth may observe. The change in appearance is merely a reflection of light from its surface.
When at the new moon phase, the moon is positioned between the sun and Earth. This means its light is not reflected to Earth, but rather the light from the sun bounces from the earth and causes a slight illumination on the moon.
When it moves to the side of the earth during its revolution, a quarter of its side is illuminated by sunlight, making it appear as a crescent shape on Earth. As the moon moves 180 degrees away from the sun, it forms a straight line with Earth, which causes the full moon effect.
All the shapes that are formed by the moon are basically indicators of its position from the sun and earth and how light behaves in relation to each.
OBJECTS IN THE SKY
Stars are glowing balls of gas that undergo nuclear fusion; the Sun is a star.
Planets are moderately large objects orbiting a star. We see planets because they reflect the light of their central star, or in some cases, stars. Planets are generally rocky or gaseous in nature and spherical-shaped.
A new group of objects has been recently defined: the Dwarf Planets or Plutoids. These are objects that orbit the Sun, but have not cleared their orbits. Pluto is an example of a Dwarf Planet.
A satellite orbits a planet; these objects are also called moons. For example, the Earth’s satellite is the Moon – a proper name.
An asteroid is a relatively small, rocky/metallic object usually orbiting a star.
A comet is a relatively small, icy object usually orbiting a star. Asteroids, comets, and miscellaneous small/irregular objects and “dust” are often categorized as Minor Bodies.
A galaxy is a large island of stars, a few hundred million to over a trillion stars.