Explanation : SUNSET

Sunset or sundown is the daily disappearance of the sun below the horizon in the west as a result of Earth's rotation. The time of sunset is defined in astronomy as the moment the trailing edge of the sun's disk disappears below the horizon in the west. The ray path of light from the setting Sun is highly distorted near the horizon because of atmospheric refraction, making sunset appear to occur when the sun’s disk is already about one diameter below the horizon. Sunset is distinct from dusk, which is the moment at which darkness falls, which occurs when the sun is approximately eighteen degrees below the horizon. The period between sunset and dusk is called twilight.

Sunset creates unique atmospheric conditions such as the often intense orange and red colors of the sun and the surrounding sky. The time of sunset varies throughout the year, and is determined by the viewer's position on Earth, specified by longitude and latitude, and elevation. Small daily changes and noticeable semi-annual changes in the timing of sunsets are driven by the axial tilt of Earth, daily rotation of the Earth, the planet's movement in its annual elliptical orbit around the Sun, and the Earth and Moon's paired revolutions around each other. In the summertime, the days get longer and sunsets occur later every day until the day of the latest sunset, which occurs after the summer solstice.

In the Northern Hemisphere, the latest sunset occurs late in June or in early July, but not on the summer solstice of June 21. This date depends on the viewer's latitude (connected with the Earth's slower movement around the aphelion around July 4). Likewise, the earliest sunset does not occur on the winter solstice, but rather about two weeks earlier, again depending on the viewer's latitude. In the Northern Hemisphere, it occurs in early December (influence from the Earth's faster movement near the perihelion, which occurs around January 3).

Sunsets on other planets appear different because of the differences in the distance of the planet from the Sun, as well as different atmospheric compositions.

Sunset pictures :

comment : I choose "sunset" as my explanation task because sunset is one of my favourite natural phenomenon  in this world, and  I would like to explain how it can be happens. So, everyone who open my blog will know it . I also add some pictures in this task and it will be more interesting.

taken from : http://en.wikipedia.org/wiki/Sunset