Shifting Stars

Timocharis

He notes that Spica is located 8° west of the fall equinox.

Yu Xi

In the 4th century, A Chinese astronomer notices and precession.

8° west

Timocharis notes that Spica is located ______ of the fall equinox.

1543

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

280 BCE

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

1687

Isaac Newton demonstrates precession to be a consequence of gravity.

1718

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

Edmond Halley

He discovers that, except for the relative motion between stars and reference points on the celestial sphere, stars have a gradual motion relative to each other.

Isaac Newton

In 1687, he demonstrated precession to be a consequence of gravity.

Nicolaus Copernicus

In 1543, he explains precession as a motion of Earth’s axis.

Hipparchus of Nicaea

He first 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.

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.

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.

Wobble

The direction that the Earth's spin axis is rotating due to the gravitational pull of the sun and moon.

**Hipparchus Catalog**

the first thorough star catalog produced in the western world.

armillary sphere

Used by Hipparchus to create the star catalog.