Download Daily "Motions" of the Celestial Sphere

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The Celestial Sphere
 An imagery construction, useful for charting the positions and motions of
objects visible in the sky.
 Stars, planets, etc can be mapped onto the celestial sphere just as cities
can be mapped on the Earth. Celestial objects appear to be “attached” to
the celestial sphere, since the distances to these objects cannot be
discerned simply by looking at them.
 At any given time, an observer on Earth actually sees a hemisphere, since
the body of the Earth blocks ½ of the sky. The horizon defines the
boundary of that hemisphere, separating the visible sky from the ground.
The Daily "Motions" of the Celestial Sphere
We live on “Spaceship Earth”, which is moving through the cosmos. Earth’s
motions affect our view of the sky, on daily, yearly, and much longer
Earth rotates once every 23 hrs 56 mins with respect to the stars – an
interval known as a sidereal day.
A solar day is 24 hrs -- one rotation of the Earth with respect to the Sun.
Due to the Earth’s physical rotation, the celestial sphere appears to rotate
around the north celestial pole - the projection of Earth's rotation axis onto
the sky. The celestial sphere “completes” one rotation in a sidereal day.
Polaris, also known as the Pole star, marks this axis in the northern sky.
Celestial objects rise in the east and set in the west.
There is no equivalently bright southern pole star.
The celestial equator marks the projection of the Earth's equator onto the
An Observer's View of the Sky
One half of the celestial sphere is always in view to an observer on the Earth’s
surface. The portion of the sky visible to any particular observer depends on the
observer’s location on Earth.
 Your latitude (how far north or south of the equator you are located) defines
the amount of the celestial sphere visible from a specific location on the
 At the north and south poles only 1/2 of the celestial sphere is visible. The
other half of the sky can never be seen. It remains permanently below the
 At the equator the entire celestial sphere is visible during the course of a
For all other observers:
 Some stars are always visible, meaning these stars never set below the
horizon. These stars are known as circumpolar stars; they appear to
“circle the pole” during the course of a night.
 Some stars are never visible. A portion of the sky never rises above the
observer’s horizon.
For northern hemisphere observers, the altitude of Polaris is equal to the
observer’s latitude.
The Sky From Denton, Texas
Latitude for Denton, TX is 33o 13' North
Polaris is located 33o 13' above the northern horizon
All stars with declinations greater than + 57o "circle the pole". These stars are
always visible from Denton.
All stars with declinations less than  57o never rise above the horizon. We
can never see them from here. An example is the Southern Cross, part of the
constellation Crux.
The Annual "Motions" of the Celestial Sphere
Earth completes 1 revolution (an orbit) around the Sun every 365.24 days defined as 1 year.
From our perspective, the Sun appears to move against the background of
stars at a rate of ~1o per day.
The Sun's motion describes a smooth, circuit around the sky. This path is
known as the ecliptic.
The zodiac is the band of constellations through which the Sun passes.
These constellations lie along the ecliptic. The Sun’s movement through
these and ONLY these constellations is the reason that early civilizations
created mythologies about the constellations of the zodiac.
Twelve zodiacal constellations exist, from Aries to Pisces.
The visibility of each constellation changes throughout the year, as the Sun
appears to "move" along the ecliptic. Those constellations near the Sun’s
position on the sky are not visible, since they rise and set with the Sun.
Each month the Sun moves through about 15 degrees, and one zodiacal
constellation. Constellations opposite the Sun on the sky are visible all night
The Reason for the Seasons
The ecliptic is tilted 23.5 degrees in relation to the celestial equator.
Physically, this tilt implies that the Earth’s rotation axis is not aligned with its
orbit around the Sun.
During the northern summer, the Sun is located north of the celestial equator
on the sky, during northern winter, the Sun lies south of the celestial equator.
This ecliptic tilt causes the amount of daylight (and the amount of solar
heating) to vary during the year – more in the summer, less in winter, for
those of us in the northern hemisphere.
The seasons exist because of this ecliptic tilt, NOT due to the Earth's
changing distance from the Sun. The variable amount of sunlight striking the
Earth’s surface, causes the basic change in daily temperatures.
Solstices occur when the Sun is located at the maximum southern (first day
of winter) or northern (first day of summer) distance from the celestial
Equinoxes occur when the Sun crosses the celestial equator. This happens
on the first day of fall and first day of spring.
The seasons are reversed in the southern hemisphere.