AY 101 Lecture Notes - Lecture 8: Equatorial Coordinate System, Celestial Sphere, Celestial Equator
Naked-Eye Astronomy
Human awareness of the universe began when people realized that they could observe
objects in the sky, and that what they saw changed over days, months, and years.
The sky
To any observer on the ground looking at the sky with the naked eye, the sky appears
to be a vast spherical bowl, a celestial sphere that extends from all points along the
horizon to the zenith, the point directly overhead.
Astronomical objects seen in the sky are so far away that observers viewing them
without the aid of a telescope have no intuitive sense of which objects are closer than
others. This lack of depth perception causes everything to appear equidistant on the
sky. Any object's position on this sphere can be determined by two coordinates, which
designate the object's horizontal and vertical location. The vertical coordinate is
determined by measuring an altitude angle upwards from the nearest point on the
horizon, and the horizontal coordinate is established by measuring an azimuth
angle from due north eastwards along the horizon to that nearest point. This system is
called either the horizon coordinate system or the alt‐azimuth system(see Figure 1).
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Figure 1
The horizon coordinate system.
Stars and constellations
About 6,000 stars are visible to the naked eye when you can achieve the darkest
conditions. The positions of the stars relative to each other remain fixed from night to
night and year to year. In general, stars in the same area of the sky have no physical
relationship to each other, but the very human tendency to impose order upon otherwise
random distributions yields patterns of brighter stars, or constellations. Many
constellations of the Northern Hemisphere have been inherited from antiquity, including
Ursa Major, the Big Bear, of which the more familiar Big Dipper is part, and Orion, the
hunter, which can be seen in the winter sky. Many Southern Hemisphere constellations
were defined in the last century to fill in unlabeled regions of the sky.
The sky is now officially divided into 88 constellations, which are used in modern
astronomy for naming purposes. For example, the star alpha Ursa Majoris (α UMa) is
located in the Big Bear constellation. Therefore, Ursa Majoris comes from the name of
the constellation, Ursa Major, and the Greek letter α indicates that it's the brightest star
within that constellation. In addition to modern names, some 90 or so stars also have
names from antiquity. For instance, α UMa is also known as Dubhe.
Over the course of the night, stars move across the sky from east to west as a
consequence of Earth's rotation on its axis. The stars appear to move in circular paths
around a celestial pole, or either of two points on the celestial sphere where the
extensions of Earth's axis would intersect. In the Northern Hemisphere, the celestial
pole is coincidentally marked by the relatively bright star alpha Ursa Minoris, also known
as Polaris. Simple geometry shows that the altitude angle of the pole star above the
northern horizon is equivalent to the latitude of the observer on Earth.
To an observer in the Northern Hemisphere, stars that are always above the northern
horizon are known as circumpolar stars; an observer in the Southern Hemisphere
would see circumpolar stars around the south celestial pole. Stars that are further to the
south and that rise and set sometime during the night are called equatorial
stars.Equatorial stars rise in the east, move diagonally into the southern sky, achieving
their highest position above the horizon on the meridian (the great circle that extends
from due north on the horizon, through the zenith, to due south on the horizon). From
the meridian, these stars move westward until they set below the western horizon.
The celestial equator is that great circle formed on the celestial sphere by extending
the plane of the Earth's equator. The equator intersects the horizon due west and due
east. An immediate consequence of Earth's rotation is identifying the preferred
directions of north, south, east, and west around the horizon, called cardinal
directions, by which humans naturally orient themselves.
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Minute by minute, over the course of the night, both the altitude and azimuth of a star
continually change. To more easily observe and track celestial objects, scientists have
defined celestial coordinate systems that are fixed upon the sky and thus move with the
stars. The equatorial coordinate system is a projection onto the sky of Earth's latitude
and longitude coordinate system. Celestial latitude,known as declination, is the
angular position north or south of celestial equation. Celestial longitude,measured
around the celestial equation, is known as the right ascension (see Figure 2).
Declination is measured in degrees, minutes of arc, and seconds of arc. Recognizing
that the stars appear to move once around the sky in one day (24 hours), right
ascension is measured not in degrees, but in hours, with 24 hours of right ascension (to
distinguish from 24 hours of time) equal to 360 degrees.
Figure 2
Celestial sphere features.
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Document Summary
Human awareness of the universe began when people realized that they could observe objects in the sky, and that what they saw changed over days, months, and years. Astronomical objects seen in the sky are so far away that observers viewing them without the aid of a telescope have no intuitive sense of which objects are closer than others. This lack of depth perception causes everything to appear equidistant on the sky. Any object"s position on this sphere can be determined by two coordinates, which designate the object"s horizontal and vertical location. This system is called either the horizon coordinate system or the alt azimuth system(see figure 1). About 6,000 stars are visible to the naked eye when you can achieve the darkest conditions. The positions of the stars relative to each other remain fixed from night to night and year to year.
