The Stars

Suppose that a star is at x degrees north celestial latitude and y degrees west celestial longitude. If you stand at the point on the surface corresponding to x°N and y°W, then a straight, infinitely long geometric ray originating at the center of Earth and passing right between your eyes will shoot up into space in the direction of the star (Fig. 1-3).

As you might guess, any star that happens to be at the zenith will stay there for only a little while unless you happen to be standing at either of the


Star at

Celestial latitude = x Celestial longitude = y

Straight ray / of sight y

Observer at Latitude = x Longitude = y

Earth's center Figure 1-3. Celestial latitude and longitude.

PART 1 The Sky geographic poles (not likely). Earth rotates with respect to the stars, completing a full circle approximately every 23 hours and 56 minutes. In a few minutes, a star that is straight overhead will move noticeably down toward the western horizon. This effect is exaggerated when you look through a telescope. The greater the magnification, the more vividly apparent is the rotation of Earth.

The next time you get a chance, set up a telescope and point it at some star that is overhead. Use the shortest focal-length eyepiece that the telescope has so that the magnification is high. Center the star in the field of view. If that star is exactly overhead, then its celestial latitude and longitude correspond to yours. For example, if you're on the shore of Lake Tahoe, your approximate latitude is 39°N and your approximate longitude is 120°W. If you have a telescope pointing straight up and a star is centered in the field of view, then that star's celestial coordinates are close to 39°N, 120°W. However, this won't be the case for long. You will be able to watch the star drift out of the field of view. Theoretically, a star stays exactly at a given celestial longitude coordinate (x, y) for an infinitely short length of time—in essence, for no time at all. However, the celestial latitude of each and every star remains constant, moment after moment, hour after hour, day after day. (With the passage of centuries, the celestial latitudes of the stars change gradually because Earth's axis wobbles slowly. However, this effect doesn't change things noticeably to the average observer over the span of a lifetime.)

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