Stars
Stars have been important to every culture They have been
used in religious practices and for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently afixed to a
heavenly sphere, and they were all but immutable. The stars were grouped
together into constellations and were used to track the positions of the
planets, and the inferred position of the Sun.The motion of the Sun
against the background stars (and the horizon) was used to create
calendars, which could be used regulate agricultural practices.The
Gregorian calendar, used nearly everywhere in the world, is a solar
calendar based on the angle of the Earth's rotational axis relative to the
nearest star, the Sun.
In spite of the apparent immutability of the heavens, Chinese astronomers
were aware that new stars could appear and then fade away.
Early European astronomers such as Tycho Brahe identified new stars
in the night sky (later termed novae) suggesting that the heavens were not
immutable. In 1584 Giordano Bruno suggested that the stars were actually
other suns, and may have other planets, possibly even Earth-like, in orbit
around them, an idea that had been suggested
earlier by such ancient Greek philosophers as Democritus and Epicurus.
By the following century the idea of the stars as distant suns was
reaching a consensus among astronomers. To explain why these stars exerted
no net gravitational pull on the solar system, Isaac Newton suggested that
the stars were equally distributed in every direction, an idea prompted by
the theologian Richard Bentley.
The Italian astronomer Geminiano Montanari recorded observing variations
in luminosity of the star Algol in 1667. Edmond Halley published the first
measurements of the proper motion of a pair of nearby "fixed" stars,
demonstrating that they had changed positions from the time of the ancient
Greek astronomers Ptolemy and Hipparchus. The first direct measurement of
the distance to a star (61 Cygni at 11.4 light years) was made in 1838 by
Friedrich Bessel using the parallax technique. Parallax measurements
demonstrated the vast separation of the stars in the heavens.
|
