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Geology/Physics 360
Chapter 1: The Celestial Sphere
The celestial sphere and the celestial
• The celestial sphere is what is above you when
you walk outside
• It acts as a model of the heavens
• Where the sky meets the ground is the
• Two points in the sky do not move as the
earth rotates (the north and south celestial
• Polaris the north star is at the celestial north pole
• The earth rotates counterclockwise (as viewed from
above) therefore stars rise in the east and set in the
west (so does the sun and the moon)
• The line that the sun traces across the celestial
sphere is the ecliptic
• The sun reaches 23.5 degrees N of the celestial
equator on June 21 (the solstice)
• When does it reach 23.5 degrees S of the celestial
• When does it reach the celestial equator? (the
How would stars rotate if you were at
the North Pole?
As the earth moves around the sun the constellations in the sky tend to shift also
Revolving = motion
around the sun
Rotation = the
spinning motion
what causes night
and day
Stonehenge in England probably was built to help people understand the
The simplest way to determine a star’s position on the celestial sphere using
the topo-centric or the altitude-azimuth coordinate system
The equatorial coordinate system
The declination is measured in degrees above the celestial equator
The right ascension is measured in hours, minutes and seconds in the
easterly direction from the vernal equinox position on the celestial equator
Screen capture of HNSKY with cursor positioned on Arcturus
We will briefly discuss the orbit of the moon in class. We will return to the
moon later in the semester (it has its own chapter)
The moon goes through its cycle of phases in 29.5 days however the sidereal
month is the time the moon takes to complete an orbit relative to the distant
stars of one complete cycle. This we bring into play a new time system and a
new concept of measuring time. Sidereal time or time according to the stars.
The orbit plane of the moon around the earth is slightly tilted with respect to
the ecliptic. This is what causes solar and lunar eclipses.