What Is Galactic Coordinate

00 00 (by definition)

a Diffuse objects ranging in size from ~5' (Crab) to (Andromeda).

a Diffuse objects ranging in size from ~5' (Crab) to (Andromeda).

as a spinning top precesses about the vertical. Because the equatorial coordinate system is, by definition, locked to the earth's equator and the vernal equinox, the coordinate system slowly slides around the sky as the earth precesses. The coordinates of any given star will therefore change slowly (<1 per year), and one must assign an epoch or date to any such coordinates. (See Section 4.3 for more on precession.)

Epoch B1950.0 was the common system for catalogs and charts in the latter part of the past century. It is based on the Besselian year and refers to the earth's orientation at (approximately) midnight of New Year's eve in Greenwich, or more precisely 22 h 09 m UT (Universal Time, essentially the time in Greenwich England) of 1949 Dec. 31. Epoch J2000 (based on the Julian year) is now the standard for current astronomy. It refers to the orientation of the earth at about noon in Greenwich on 2000 Jan. 1. (See Chapter 4 for more on the keeping of time and the definition of "epochs".)

The celestial positions of several important objects at the two standard epochs are shown in Table 1.

Why equatorial coordinates?

Why do astronomers use the continually changing equatorial coordinates? In early telescopes, the mechanical rulings that indicate the two angular scales, right ascension and declination, are fixed to the mount which is fixed to the earth; thus the mechanical pointers (attached to the movable telescope) show the pointing direction relative to the mount and hence relative to the earth.

Traditionally the telescope has been mounted so one of its rotation axes is aligned parallel to the earth's rotation axis and the other perpendicular to it. This permits a star to be tracked throughout the night with rotations about only one axis, the one parallel to the earth's spin axis. The indicator for telescope rotation angle about this axis gives the angle of the pointing east or west relative to the meridian that passes through the zenith. This angle is called the hour angle and is typically indicated in units of "hours". With knowledge of the time of day, this angle yields the right ascension of the pointing direction.

The indicator for the other axis reads the pointing angle relative to the earth's spin axis, or equivalently relative to the earth's equator. This is the declination of the pointing direction for the epoch of the observation, e.g., epoch 2002.50 for the ~2002 July 1. Conversely, if one wishes to use the mechanical indicators to point a telescope toward a given (faint) star on this date, one must know or calculate its epoch 2002.5 celestial coordinates and take into account the time of day.

This rationale for the use of equatorial coordinates is becoming irrelevant for the many computer-driven telescopes. Nevertheless the system shows no signs of being superseded. Conversions of coordinates from epoch to epoch are becoming trivial; they are routinely carried out on the computers that control telescope pointing at most observatories. Thus, if you know the position of the desired star in any epoch, e.g., B1900, the computer will point the telescope there on your observation date. Computers remove the pain associated with this cumbersome system, so why change it?

Galactic coordinates Another coordinate system in common use is the galactic coordinate system. In this system, the equator on the celestial sphere is defined to be a great circle that runs along the Milky Way.

A schematic of the Galaxy is shown in Fig. 3. It is a disk-shaped cluster of some 1011 stars. The visible stars tend to cluster in spiral arms, and the sun is one unimportant star well removed (~25 000 LY) from the center. The central plane of the Galaxy contains a high concentration of stars. It thus appears to us as a band in the sky that (more or less) follows a great circle on the celestial sphere. It is this great circle that is chosen to be the equator for the galactic coordinate system as illustrated in Fig. 4.

The two coordinates used to define a celestial position are galactic longitude (l) and galactic latitude (b). These are analogous to latitude and longitude on the earth. The angles are measured in degrees with latitude increasing toward the north and longitude running from 0° to 360° in the direction shown (counter-clockwise when viewing from the north galactic pole), with zero in the approximate direction of the center of the Galaxy. This is also a right-handed system. One often sees the

Side view Top view

Side view Top view

Figure 3.3. Simplified sketches of our Milky Way system of stars showing spiral arms of increased matter density, the bulge, the corona, a dark matter halo, globular clusters, and the position of the sun. The stars, gas, and dust rotate in the counterclockwise direction about the center. As the dust and gas pass through the slowly rotating spiral arms, shocks cause new star formation to take place. [Adapted from Abell, Exploration of the Universe, 3rd Ed., Holt Rinehart Winston, 1975, p. 484]

Figure 3.3. Simplified sketches of our Milky Way system of stars showing spiral arms of increased matter density, the bulge, the corona, a dark matter halo, globular clusters, and the position of the sun. The stars, gas, and dust rotate in the counterclockwise direction about the center. As the dust and gas pass through the slowly rotating spiral arms, shocks cause new star formation to take place. [Adapted from Abell, Exploration of the Universe, 3rd Ed., Holt Rinehart Winston, 1975, p. 484]

Figure 3.4. Galactic coordinates. The plane of the Galaxy defines the galactic equator on the celestial sphere. The angles that specify the location of a celestial body are measured from the sun. Galactic longitude, l, is measured approximately eastward from the direction of the galactic center in units of degrees (0° to 359.9°) as shown. Galactic latitude, b, is measured in degrees (0° to ±90°) from the galactic equator, similar to latitude on the earth's surface. The north galactic pole (dark circle) is shown. The celestial sphere is quite small in this figure; in fact, its radius is infinite. The earth observer is located close to the sun.

Figure 3.4. Galactic coordinates. The plane of the Galaxy defines the galactic equator on the celestial sphere. The angles that specify the location of a celestial body are measured from the sun. Galactic longitude, l, is measured approximately eastward from the direction of the galactic center in units of degrees (0° to 359.9°) as shown. Galactic latitude, b, is measured in degrees (0° to ±90°) from the galactic equator, similar to latitude on the earth's surface. The north galactic pole (dark circle) is shown. The celestial sphere is quite small in this figure; in fact, its radius is infinite. The earth observer is located close to the sun.

North celestial pole

North galactic pole

North celestial pole

North galactic pole

Double Stars Coordinate Transform
Figure 3.5. Celestial sphere showing the celestial (B1950) and galactic equators, their north poles, and the line of nodes.

superscript "II" applied to the l and b to designate the modern second definition of the galactic coordinates. We omit the "II" in this text as there is no longer any ambiguity; the "II" system has been in use for several decades.

The galactic system was defined by the International Astronomical Union (IAU) in terms of B1950.0 equatorial coordinates as follows (Fig. 5). The north galactic pole (NGP) is defined to be precisely at

«ngp(B1950) = 12 h49 m = 192.25° (North galactic pole) (3.1) <5ngp(B1950) = +27°24'= +27.4° (3.2)

The values given are not rounded off; they are exact values which implies that the celestial and galactic equators are tilted relative to each other by 90-27.4 = 62.6°.

The usual convention in such a coordinate transformation is to assign zero of longitude to an intersection of the two planes (the ascending node), but this is not at the galactic center. Accordingly, the zero is set such that the galactic longitude of the north celestial pole (NCP) is precisely at incp = 123° (3.3)

which places the zero of galactic longitude near, but not exactly at, the galactic center. The two great circles intersect at two nodes. The line of nodes (Fig. 5) is t

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  • Amanuel
    What is galactic coordinate?
    1 year ago

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