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Astronomy 263/264
Voyager Lab--Spring 1, The Moving Stars
Summary
This lab uses Voyager II to demonstrate how the stars move across the
sky during the course of a night, the motion of circumpolar constellations,
the motion of non-circumpolar constellations, and how the astrology system
of star signs is 3000 years out of date.
Concepts
We know that the Earth rotates about its axis once every 24 hours in
such a way that, at any location on Earth, the Sun appears to rise in
the East and set in the West. Similarly, the constellations seem to
move across the night sky from East to West.
There are a special group of constellations that do not appear to move
from East to West during the night. Instead, they circle around
Polaris, the North Star. These are the
circumpolar constellations.
The reason why these constellations behave differently from the others
has to do with their position in the sky with respect to the Earth's
axis. Those constellations near Polaris are high enough in the sky
that they never go below the horizon. So instead of behaving like the
"ordinary" constellations that "circle" the Earth, and dip below the
horizon and thus disappear from sight, the circumpolar constellations
circle Polaris and are visible the entire night. By using Voyager to
observe some circumpolar constellations, you can see that the stars
rotate counterclockwise around
Polaris.
Astrology attempts to explain your personality type by determining what
constellation "sign" the Sun was in on the day that you were born.
Astronomy is a science, while astrology has no scientific basis.
However, many people are interested in it, so we will explore one
aspect of astrology in this lab.
Although today the Earth's axis points approximately towards Polaris,
this was not always so. The Earth's axis precesses like a gyroscope (or spinning top)
with a period of 26,000 years.
Astrology's association between constellations and birthdates was
established a long time ago. In the interim the Earth's axis has
precessed sufficiently for the Sun to appear to have moved into the
neighboring constellation. Therefore, to get the correct
correspondence between the date you were born and the constellation
that the Sun was in, we have to go back in time 3000 years. That's
when the zodiacal constellations were invented. Voyager allows us to
observe how the Earth's access has precessed in the past 3000 years.
3000 years ago, the Earth's North pole pointed somewhere between the
dippers. Right now we are lucky because it is pointing almost
precisely to a star -- Polaris. 2000 years from now, it will point
toward the constellation Cepheus. With every such change, the order of
zodiacal constellations changes as well! The more we move away in time
from when the Zodiac was defined, the less correct it becomes. A
person who is born 'under Sagittarius' in the year 4000 will actually
be born 'under Libra'!
Last updated on March 29, 2000.
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