Rotation of the Sky

The celestial sphere itself is fixed. But, because the Earth rotates, from our point of view the celestial sphere appears to rotate. The Earth rotates, roughly once per day, towards the east. This means the celestial sphere appears to rotate, roughly once per day towards the west. The stars, of course, rotate about an axis through the north and south celestial poles. It follows then that the stars appear to rotate around the celestial poles, and move from east to west.

From our previous discussion of how the celestial sphere appears in horizon co-ordinates, we know that an northern hemisphere observer sees the north celestial pole at an altitude given by their latitude, Φ. The diagram below then illustrates the paths of some stars over the course of a night, for a northern hemisphere observer.

Gemini star trails

Star trails over Gemini North, Hawaii. Which end of the star trails marks the start of the exposure? Credit: Gemini Observatory

The stars appear to revolve from east to west about the celestial poles on circular paths parallel to the celestial equator once per day. Some stars (like the red star above) never set and remain visible at night all year. These are called circumpolar stars. A circumpolar star at its maximum altitude above the horizon is said to be at its upper culmination. Similarly, a circumpolar star at its minimum altitude above the horizon is said to be at its lower culmination. Stars further from the pole rise, attain a maximum altitude above the horizon (when they are said to transit) and then set below the horizon (e.g. the yellow star above). These stars, unsurprisingly, are called rise/set stars. Which stars are circumpolar depends on the latitude of the observer; stars within an angle Φ of the north celestial pole are circumpolar for an observer at northern-hemisphere latitude Φ. From the diagram, and what you know about celestial co-ordinates, we can see that stars with a declination greater than 90-Φ are circumpolar. Similarly, stars within an angle Φ of the south celestial pole never rise for our northern hemisphere observer, like the blue star in the diagram above.

diurnal motion

Because the Earth rotates, the stars appear to rotate in the night sky.

The animation below lets you explore the path of stars for observers at various latitudes. Some things to think about; how do stars move for an observer at the poles? At the equator? How is the situation described above differ for observers in the southern hemisphere? These are some of the questions we'll deal with in the lab part of this course.

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