Ancient Astronomy part 2.pptx

1. Astronomy
Ancient Astronomy
Part 2

3. Earth’s Motion
•Diurnal motion
•Annual motion
•One sidereal day
•One mean solar day
•Precession of equinoxes

4. Earth’s Motion
causes diurnal
motion, annual
motion and
precession of
equinoxes in the
celestial sphere.

5. Diurnal motion
•Refers to the apparent
daily motion of the
celestial sphere from
east to west about a
fixed axis passing
through Earth’s north
and south poles.

6. Annual motion
•Refers to the apparent
shift in the location of
the stars to the west,
relative to the sun.
•Greeks astronomers
observed that the
stars return to their
original position every
year.

7. Annual motion
•This motion causes
an apparent
displacement of the
sun among the
constellations by
almost one degree
each year.

8. Annual motion
•This motion
creates a
difference of
almost four
minutes
between a
sidereal day and
a mean solar
day.

9. One sidereal day
•(Is equal to 23 hours,
56 minutes, and 4
seconds) is the time
it takes Earth to
rotate on its axis so
that the distant stars
appears in the same
position in the sky.

10. One mean solar day
•(equal to 24 hours) is the time
it takes Earth to rotate on its
axis so that the sun appears in
the same position in the sky.

11. In the ancient times, Earth’s
precession, the annual, gradual
shift in the orientation of Earth’s
axis, is known to be caused by
Precession of Equinoxes.

12. Precession of Equinoxes
•Refers to the westward
motion of the
equinoxes along the
ecliptic relative to the
fixed stars and opposite
to the motion of the
sun along the ecliptic.

13. Precession of Equinoxes
• This was discovered by
Hipparchus, a Greek
Astronomer. He studied the
Babylonian and Chaldean
observations of the celestial
bodies and found out that
the precession of the
equinoxes is not due to the
shifting in the position of the
stars but due to the motion
of the Earth itself.