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LocatingCelestial Objects
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In this lesson, you will learn...
what the celestial sphere and celestial equator
are, and terminology related to them
how to describe the location of objects in the
night sky using both the Declination - Right
Ascension system and the Altitude - Azimuth
system
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Looking Up...
Early astronomers believed that all of the stars we
see in the night sky were fixed on a celestial sphere
surrounding the Earth.
Although we now know that this is not true, it iseasier to use this idea to locate stars in the sky.
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The Celestial Sphere
The celestial sphere is a dome that surrounds
the Earth.
It is divided in much the same way as we
divide Earth with the North and South
celestial poles as an extension of the Earths
axis.
The celestial equator is a projection of Earths
equator onto the sphere.
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The
Celestial Sphere
showing the
celestial North
and South poles
and the
celestial equator.
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As the Earth rotates inside the celestial sphere
(or, as early astronomers believed, the sphere
rotates around us) the stars appear to rise in
the east and then set in the west.
This of course includes our own star, the Sun.
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Locating Stars
Astronomers use a coordinate system similar
to the latitude-longitudesystem used on Earth
to locate stars in the sky.
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There are two different
coordinate systems:
But first, a little terminology
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The eclipticis the apparent path of the Sun in one year. It is also
along the plane in which the Earth revolves around the Sun.
The summer solstice is the day of the year that the northern
hemisphere gets the most sun.
The winter solstice is when we get the least sunlight (and the
southern hemisphere gets the most).
The vernal equinox isthe day in the spring
when we get exactly 12
hours of day and 12
hours of night.
The autumnal equinox
is the same thing, but
occurs in the fall
(autumn).
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System 1:
Declination - Right Ascension
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Declination
Declination is analogous to latitude and is
measured in degrees from the celestial
equator.
Measurements of declination are either
positive (north) or negative (south).
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Right Ascension
Right ascension is analogous to longitude and
is measured in hours, minutes, and seconds,
from the point of vernal equinox (where the
ecliptic and celestial equator meet).
Since a circle is 360o(and it has been broken
into 24 parts) each hour is 15o.
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Breaking that hour (15o) into 60 pieces gives
us minutes, and breaking minutes into 60
pieces given us seconds.
For example:
If the star was
located at an angle
of 50o, this would
equate to 3 hours
or 3h 20 m.
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Example:
The star in this
picture has...
Declination: 45o
Right Ascention: 3h
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System 2: Altitude-Azimuth
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Altitude
Altitudeis the angular distance (measured
from 0oto 90o) of a celestial object above the
observers horizon.
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Azimuth
Azimuthis the compass angle from due N to
the location of the celestial object.
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Example:
The star in this picture appears to have an altitude of
approximately 40o(which means it's zenith angle is
50o) and an azimuth of about 200o(just over halfway
around the circle, starting from North and goingclockwise).
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Angular Sizes We can measure angles in the sky
The angular sizeof an object like the Sun or
the Moon is the angle it appears to span in
your field of view.
The angular distance between a pair of
objects is the angle that appears to separate
them..
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Angular size of
moon ~ o
Angular distance between the
pointer stars ~ 5o
To
Polaris