Day

-

Day, in chronology, period of time required for one rotation of a celestial body, especially the earth, on its axis. This period is shorter or longer depending on whether the sun or another star is used as a reference point; thus, the sidereal day—the time it takes for the earth to rotate once relative to a star not the sun—is 4 min shorter than the mean solar day. The solar day, measured by the interval between meridian passages of the sun, varies in length because of the variation in speed of the earth in its orbit. In consequence, the length of the solar day is averaged over the period of a year, and the mean solar day thus obtained is used for all civil and many astronomical purposes. Each type of day is divided into exactly 24 hr that vary in length proportionately to the respective type of day.

The civil day now begins at midnight, local time. In ancient times, the Babylonian day began with sunrise and with sunset among the Athenians and Jews. The day is still often regarded as starting with sunset in ecclesiastical (particularly Jewish ecclesiastical) usage; until recently, the astronomical day started at noon, and the Julian day still starts at noon.

In common usage day, as distinct from night, is the period of natural light between dawn and dusk. The period of daylight, most nearly constant near the equator, varies with the latitude and the season, reaching a maximum of 24 hr in the polar zones in summer, a phenomenon known as the midnight sun.

Comments

Post a Comment

Popular posts from this blog

Ecliptic

-
Ecliptic, in astronomy, the apparent great-circle annual path of the sun in the celestial sphere , as seen from the earth . It is so named because eclipses occur only when the moon is on or near this path. The plane of this path, called the plane of the ecliptic, intersects the celestial equator (the projection of the earth's equator on the celestial sphere) at an angle of about 23°27’. This angle is known as the obliquity of the ecliptic and is approximately constant over a period of millions of years, although at present it is decreasing at the rate of 48 seconds of arc in each century and will decrease for several millenniums until it reaches 22°54’, after which it will again increase. The two points at which the ecliptic intersects the celestial equator are called nodes or equinoxes. The sun is at the vernal equinox about March 21 and at the autumnal equinox about September 23. Halfway on the ecliptic between the equinoxes are the summer and winter solstices. The sun arrives a
Read more

Solar System

-
Image
Solar System, the Sun and everything that orbits the Sun, including the nine planets and their satellites ; the asteroids and comets ; and interplanetary dust and gas. The term may also refer to a group of celestial bodies orbiting another star (see Extrasolar Planets ). In this article, solar system refers to the system that includes Earth and the Sun. The dimensions of the solar system are specified in terms of the mean distance from Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto , orbits about 39 AU from the Sun. Estimates for the boundary where the Sun’s magnetic field ends and interstellar space begins—called the heliopause—range from 86 to 100 AU. The most distant known planetoid orbiting the Sun is Sedna, whose discovery was reported in March 2004. A planetoid is an object that is too small to be a planet. At the farthest point in its orbit, Sedna is about 900 AU from the Sun. Com
Read more

Characteristics of the Sun

-
Image
The distance between the sun and the earth varies because the earth travels in an elliptical rather than circular orbit. The distance is roughly 100 times the sun’s diameter. Turbulence in the photosphere forms granules of various sizes and sunspots. Temperature is a measure of kinetic energy . The dense plasma in the center of the sun is roughly 2500 times hotter than the surface. Gases in the corona have escaped from the sun’s surface and have a very high velocity. The sun’s spectral type, G2, indicates that it is composed of hydrogen , helium , calcium , iron and other metals.
Read more