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Astronomy 263/264
Celestial Planisphere
Summary
The purpose of this lab is to teach you about the depth and the
curvature of the sky, and how to navigate through the night sky by
utilizing a celestial planisphere and a star locator. After this
lab, you should be able to use a planisphere or star locator to
locate constellations in the night sky on a given date.
Concepts
A celestial planisphere can be
thought of as a "globe" that represents the sky, similar to how a
normal globe represents the Earth. The stars that we see at night can
be thought of as points of light that are on the inner surface of a
glass sphere surrounding the Earth. This glass sphere is the "globe"
of our celestial planisphere. In this lab, we use a planisphere to
study how the sky changes over the course of a night, and over the
course of a year.
A star locator is a flat version of
the celestial planisphere, just like a mercator map is a flat version
of part of the globe. The main advantage of a star locator is that
it's much more portable than a celestial planisphere. Unfortunatly, it
does have the some disadvantages. Remember how the countries near the
edge of the map (like Greenland) are distorted? In the same way, the
constellations near the edge of the star locator are distorted.
Since the earth's axis "points" out of the north pole at the star
Polaris, it appears that all the constellations in the sky rotate
around Polaris during the night.
Circumpolar constellations are
those constellations which never set during the course of an evening.
You can use the celestial planisphere to determine what constellations
are circumpolar for Lafayette's latitude, or for the North Pole. You
can also use the star locator to determind the circumpolar
constellations for Lafayette's latitude.
At our own latitude (about 39 degrees North), close to half of the sky
dips below the horizon during the evening. Therefore just a small number
of constellations like Ursa Minor and Cassiopeia are circumpolar. If
we travel all the way to the equator, Polaris will be on the northern
horizon -- the entire sky will rise and set during the course of the
evening. So if you were standing on the equator, you would not see any
circumpolar constellations.
The zenith is the point directly
above our heads when we stand outside. On the celestial planisphere,
this point is the topmost point of the globe. On the star locator, the
zenith is the very center of the oval.
The celestial equator is the projection
of the Earth's equator on to our imaginary glass sphere. The
ecliptic is the path that the Sun
and the planets travel in our night sky. On the celestial planisphere,
the ecliptic is the ring of dates printed on the sphere. In the star
locator, the ecliptic is shown as a thin, labeled, white line.
Last updated on January 22, 2000.
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