Celestial Sphere

Celestial Sphere, imaginary sphere of the heavens, with the earth at its center. The sphere forms the basis for the coordinate systems used in assigning positions to objects observed in the sky. It is also used for designating time intervals and for navigation.

The equatorial system of coordinates establishes a grid in the celestial sphere that is based on the earth's equator and poles, projected outwards to intersect with the sphere. Because the earth is moving around the sun, the appearance of celestial objects such as stars changes on the sphere from day to day. Thus one particular moment of the year is assigned as the time when the celestial grid is established. This moment is the vernal equinox, when the sun's disk passes directly above the equator and marks the beginning of spring in the northern hemisphere (see Ecliptic). Celestial latitude is called declination, and celestial longitude is called right ascension in this equatorial system. Right ascension is measured from the zero-hour circle established by the vernal equinox. The yearly path traced by the sun across the celestial sphere forms a great circle, called the ecliptic, on the sphere. A coordinate system that establishes a grid on the celestial sphere using the ecliptic rather than the equator as its reference plane is also sometimes employed, as are other systems.

The apparent daily movement of the celestial sphere about the earth, caused by the earth's own rotation, is actually about four minutes shorter than the mean solar day.

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.

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 at these points about June 21 and December 22, respectively. The names of the four points correspond to the seasons beginning in the northern hemisphere on these dates. The equinoxes do not occur at the same points of the ecliptic every year, for the plane of the ecliptic and the plane of the equator revolve in opposite directions. The two planes make a complete revolution with respect to each other once every 25,868 years. The movement of the equinoxes along the ecliptic is called the precession of the equinoxes. A correction for precession must be applied to celestial charts to find the true position of the stars at any given time.

The ecliptic is also used in astronomy as the fundamental circle for a system of coordinates called the ecliptic system. Celestial latitude is measured north and south of the ecliptic; celestial longitude is measured east and west of the vernal equinox.

In astrology, the ecliptic is divided into 12 arcs of 30° each, called the signs of the Zodiac. These signs, or “houses of heaven,” are named after the constellations through which the ecliptic passes.

See also Eclipse.

Eclipse

Eclipse, in astronomy, the obscuring of one celestial body by another, particularly that of the sun or a planetary satellite. Two kinds of eclipses involve the earth: those of the moon, or lunar eclipses; and those of the sun, or solar eclipses . A lunar eclipse occurs when the earth is between the sun and the moon and its shadow darkens the moon. A solar eclipse occurs when the moon is between the sun and the earth and its shadow moves across the face of the earth. Transits and occultations are similar astronomical phenomena but are not as spectacular as eclipses because of the small size of these bodies as seen from earth. (see Transit).

Topics:

• Lunar Eclipses
• Solar Eclipses
• Frequency of Eclipses
• Observation of Eclipses

Eclipse

Eclipse->>LUNAR ECLIPSES

The earth, lit by the sun, casts a long, conical shadow in space. At any point within that cone the light of the sun is wholly obscured. Surrounding the shadow cone, also called the umbra, is an area of partial shadow called the penumbra. The approximate mean length of the umbra is 1,379,200 km (857,000 mi); at a distance of 384,600 km (239,000 mi), the mean distance of the moon from the earth, it has a diameter of about 9170 km (about 5700 mi).

A total lunar eclipse occurs when the moon passes completely into the umbra. If it moves directly through the center, it is obscured for about 2 hours. If it does not pass through the center, the period of totality is less and may last for only an instant if the moon travels through the very edge of the umbra.

A partial lunar eclipse occurs when only a part of the moon enters the umbra and is obscured. The extent of a partial eclipse can range from near totality, when most of the moon is obscured, to a slight or minor eclipse, when only a small portion of the earth’s shadow is seen on the passing moon. Historically, the view of the earth’s circular shadow advancing across the face of the moon was the first indication of the shape of the earth.

Before the moon enters the umbra in either total or partial eclipse, it is within the penumbra and the surface becomes visibly darker. The portion that enters the umbra seems almost black, but during a total eclipse, the lunar disk is not completely dark; it is faintly illuminated with a red light refracted by the earth’s atmosphere, which filters out the blue rays. Occasionally a lunar eclipse occurs when the earth is covered with a heavy layer of clouds that prevent light refraction; the surface of the moon is invisible during totality.