Shifting Stars

Timelines of Events

• 280 BCE

  • Timocharis noted that Spica is located 8° west of the fall equinox.

• 4th century

  • Yu Xi, a Chinese astronomer notices and precession.

• 1543

  • Nicolaus Copernicus explains precession as a motion of Earth’s axis.

• 1687

  • Isaac Newton demonstrated precession to be a consequence of gravity.

• 1718

  • Edmond Halley discovered that, except for the relative motion between stars and reference points on the celestial sphere, stars have a gradual motion relative to each other.

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The Equinoxes Move Over Time

• Hipparchus of Nicaea observed that a star by the name of Spica had moved 2° east of the fall equinox point on the celestial sphere.
• Equinox: The transit of a planet's subsolar point through its Equator.
• Celestial sphere: A imaginary sphere encircling Earth, with stars located at specific points.
  • Defined points and curves are used as references on the surface of this sphere to describe the positions of stars and other celestial objects.
  • The sphere has north and south poles.
  • Celestial Equator: A circle lying above Earth’s equator.
  • Ecliptic: A significant circle on the sphere that traces the apparent path of the sun against a background of stars throughout the year.
  • It intersects at two points: Spring and Fall Equinox
  • Spring Equinox: Two moments of the year when the Sun is directly above the equator and day and night are of equal length; the March Equinox.
  • Fall Equinox: The times of the year when the number of daylight hours and the number of nighttime hours is almost equal; the September Equinox
• Precession of the equinoxes: The gradual drift of these two points relative to star positions.
  • Precession: A shift in the direction of a rotating body's rotational axis.
• Hipparchus put this precession down to a “wobble” in the movement of the celestial sphere, which he believed to be real and to rotate around Earth.
• Wobble: The direction that the Earth's spin axis is rotating due to the gravitational pull of the sun and moon.

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Facts about Hipparchus

• Hipparchus was an astronomer, geographer, and mathematician from Greece. He lived during the Hellenistic era. He was born in Nicaea and most likely died on Rhodes. 
• From 147 BC to 127 BC, he worked as an astronomer; the greatest prehistoric astronomical observer is regarded. 
• He was the first Greek to leave behind quantitative and precise models of the Sun and Moon's motion. 
• He accomplished this by using observations and perhaps mathematical methods that the Chaldeans from Babylonia had amassed over many years. He had a trigonometric chart with him. 
• The table appears to have been used by him to solve some spherical trigonometry issues. He may have been the first to create an accurate method to forecast solar eclipses using his solar and lunar theories, trigonometry, and other mathematical techniques. 
• Other notable accomplishments of his include the discovery of precession and the creation of the Hipparchus Catalog, the first thorough star catalog produced in the western world.
• The armillary sphere, which first appeared in his century and was used by him to create the star catalog, and the astrolabe are additional inventions that he may have made. The work of Hipparchus is heavily dependent on it in many areas, and it would take three centuries for Claudius Ptolemaeus' synthesis of astronomy to render it obsolete.
• A satellite called Hipparcos, an asteroid, and craters on the Moon and Mars are all named after Hipparchus.

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