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Celestial Sphere

Celestial Sphree The positions of stars and other heavenly bodies are described by coordinate systems imposed onto an imaginary celestial sphere with the Earth (or the Sun) located at the center as shown in Figure 08-01e. The red arrow indicates the sphere's apparent daily movement westward (corresponding to the Earth's eastward rotation - counter-clockwise). There are 4 commonly used coordinate systems on the celestial sphere:
  • Celestial (Equatorial) Coordinates - It is defined by the Earth's rotational axis with the North pole pointing to the polaris. The right ascension starts at the vernal equinox from 0 h (hour) to 24 h counter-clockwise. The declination is 0o at the celestial equator. It runs north with a positive value and ends with +90o at the North Pole; while it runs

Figure 08-01e Celestial Sphere [view large image]

south with a negative value and ends with -90o at the South Pole. This is the system most commonly used in astronomy.

  • Ecliptic Coordinates - It is defined by the orbital plane of the Earth revolving around the Sun, i.e., the ecliptic plane. The path follows roughly the 12 constellations of the zodiac. The longitude also starts at the vernal equinox from 0o to 360o counter-clockwise. The latitude is perpendicular to the longitude. It is 0o at the ecliptic plane. It runs north with a positive value and ends with +90o at the North Ecliptic Pole; while it runs south with a negative value and ends with -90o at the South Ecliptic Pole. This system is used to describe solar system objects with the Sun at the center.
  • Ecliptic The Zodiac is a band of sky 18o wide across the ecliptic (Figure 08-01f): the ancients divided it from the gamma point (at the vernal equinoxe near Aries about 2000 year ago) into 12 signs 30o wide each, and to each sign gave the name of its most representative constellation. As the positions of the Earth and the other celestial bodies change, the Sun, the planets and the Moon are projected onto the Zodiac. During the year the Sun passes through all the signs as it moves along the ecliptic.

    Figure 08-01f Ecliptic and Zodiac [view large image]

    Each night we view a slightly different part of the Zodiac because of this revolution. The precession of the equinoxes has since moved the gamma point about 30o toward the constellation Pisces.

    In Figure 08-01,a the Earth moves around the Sun at the center; while the Earth is at the center in Figure 08-01,b. Thus for example at the Vernal (Spring) Equinox (~ March 21), an observer in the northern hemisphere would see the Libra and Virgo constellations at night; while the Sun is said to be at the sign of the Aries in day time. The configuration is the same; the different perception depends only on the points of view.

  • Horizon Coordinates - It is defined by the observer's horizon. The azimuth starts at the north (the meridian) from 0o to 360o
    Horizon Coordinates Star Tracks clockwise. The altitude is 0o at the horizon. It runs up with a positive value and ends with 90o at a point vertically overhead - the zenith. Many sky charts are drawn in this system corresponding to certain time and place on Earth with the observer at the center (Figure 08-01g).

    Figure 08-01g Horizon Coordinates

    Figure 08-01h Star Tracks
    [view large image]

    Figure 08-01h,a shows the star tracks at mid northern latitude. The north circumpolar stars are present all the time at that latitude, but the constellations would be observed in different
    orientation at a fixed time (same as for the other star tracks) depending on the 4 seasons. For example, on March 31 each year a number of constellations come into view at mid night with Virgo at the center (Figures 08-01f and 08-01l). On the other hand, the south circumpolar stars never appear at that latitude regardless of seasons. The insert in Figure 08-01h is a solargraph, which is a photo of the Sun's paths taken each day from June 21 to December 21. Dark gaps are caused by cloud covers. These are the paths defined by the red and green circles in Figure 08-01h,b. The Ecliptic is traced out by the Sun moving within the band. It is at an angle of 23.5o to the equatorial plane as shown in Figure 08-01e.

  • Galactic Coordinates - It is based on the plane of the Milky Way, which is inclined about 63o to the celestial equator, and centered on the Sun, with the zero point of longitude and latitude pointing directly at the galactic center. Before 1958, the zero point of galactic latitude and longitude was taken to lie at R.A. 17h 45.6m, Dec. -28o 56.2' (in Sagittarius). Galactic latitude (b) is measured from the galactic equator north (+) or south (-); galactic longitude (l) is measured eastward along the galactic plane
    Galactic Coordinates from the galactic center (Figure 08-01i). In 1958, because of increased precision in determining the location of the galactic center, based on observations of the 21-centimeter line, a new system of galactic coordinates was adopted with the origin at the galactic center in Sagittarius at R.A. 17h 42.4m, Dec. -28o 55' (epoch 1950). The new system is designated by a superior Roman numeral II (i.e., bII, lII) and the old system by a superior Roman numeral I. Galactic coordinates are used to specify the position of objects in the Milky Way as observed from the Earth.

    Figure 08-01i Galactic Co- ordinates [view large image]

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