Celestial Sphere And Star Charts

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Astronomy 101
Celestial Sphere and Star Charts
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Until relatively modem times people believed the stars were firmly attached to some sort of invisible sphere
which turned once per day around the Earth. While we now know this is not true, it was, and remains, an
effective model for predicting the positions of many objects in the sky..
The celestial sphere is a geocentric (earth-centered) model of the universe. You will need to sight down
through the sphere to the earth globe inside to determine what objects will be visible from different
locations on the earth.
The Sun, Moon, and planets are not shown on the sphere since their positions are not constant with respect
to the stars. We will locate these later. Study Figure 1, and find the corresponding features on your sphere.
Figure 1
The North and South Celestial Poles are projections of the Earth's north and south poles onto the celestial
sphere. The sky appears to rotate daily about these points. The Celestial Equator is the projection of the
Earth's equator onto the celestial sphere. It defines the zero point for declination (which is analogous to
latitude on the Earth running from +90 degrees at the north celestial pole to -90 degrees at the south
celestial pole). The Ecliptic is the path the Sun appears to follow through the sky over the course of a year.
The Vernal Equinox is the intersection of the ecliptic with the celestial equator. It is where the Sun crosses
the celestial equator on its way north. A line through the vernal equinox to the north and south celestial
poles establishes the zero for Right Ascension (which is analogous to longitude on the Earth).
Since the celestial sphere appears to rotate once every 24 hours, right ascension is measured in hours,
minutes and seconds; increasing eastward from the Vernal Equinox. 24 hours thus corresponds to 360
degrees.
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