Daily motion

The universe is a mass of swirling motions, but most of the time, you can ignore all but one of them. That one is daily motion (diurnal motion), caused by the rotation of the Earth. You will see it immediately if you aim a 100 x telescope at a star with the drive motor turned off.

As you know, celestial objects rise in the east, move across the sky, and set in the west. But as Figures 2.1 and 2.2 show, that is not the whole story. The motion is not directly from east to west; instead the whole sky rotates like a globe with Polaris at its north pole.

In the southern sky, each object rises somewhere on the eastern horizon (not necessarily due east!), passes across the sky, and sets somewhere in the west. Its path may be long or short. In the far southern sky, objects rise just east of south, climb only a short distance above the horizon, and set again a short time later, just west of south.

Hint: Maps of the sky have north at the top, east at the left (not right as on a terrestrial map), in order to match the view that you see when facing south and looking up. Get used to facing south to get your bearings when looking at the sky.

The most northerly celestial objects are circumpolar; that is, they do not rise or set at all. Instead, they twirl around the north celestial pole, which is conveniently marked by the star Polaris. Each revolution takes one day (24 hours).

Opposite the circumpolar region, there is a region in the far south containing stars that never rise. That is why Alpha Centauri, for instance, is not visible from the continental United States.

The diagrams show the sky as seen from New York. Farther north, everything in the south is lower, everything in the north is higher, and the circumpolar region is larger. Within the Arctic Circle, the Sun can become circumpolar; that's how

ZENITH

(directly overhead)

ZENITH

(directly overhead)

ZENITH

(directly overhead)

ZENITH

(directly overhead)

NORTH

NORTH

Figure 2.2. The same, but looking north. Celestial objects twirl around Polaris once every 24 hours.

they get the Midnight Sun. Even in England, the Sun is so nearly circumpolar in mid-June that the sky does not get completely dark.

At more southerly latitudes, the opposite is the case. From Florida, you can see the star Can opus, which is due south of Sirius and below the New York horizon.

Seen from the equator, Polaris lies on the northern horizon, and nothing is circumpolar, but nothing is hidden from view in the south; you can see the entire sky. From the southern hemisphere (Australia, for instance), Polaris is below the horizon but the south celestial pole is high in the sky; from there, you can see all of the southern sky but not all of the north.

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