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Motions
Astronomy 1-1
Lecture 02-1
Motions in the Sky
When viewed from the north pole of the solar system
Earth moves counterclockwise about the Sun and rotates
counterclockwise about its own axis
Moon moves counterclockwise about the Earth and
rotates counterclockwise about its own axis
Astronomy 1-1
Lecture 02-2
Diurnal Motion
The Daily Motion of the Sun and the
rotation of the stars
http://lasp.colorado.edu/~bagenal/1010/graphics/apparent_motion.html
Astronomy 1-1
Lecture 02-3
Night View
At the same time every night, stars appear
in different positions due to the motion of
the Earth about the Sun.
Astronomy 1-1
Lecture 02-4
Definitions
Zenith
Point directly overhead
Meridian
A line going from North Pole through Zenith
to the South Pole
Astronomy 1-1
Lecture 02-5
Daily Time Keeping
Our day is based on the Solar Day
Apparent Solar Day
Time for Sun to go from overhead to overhead
Depends upon the season
Mean Solar Day
Defined to be 24 hours
Our clocks are based on this
Sidereal Day
Time for Earth to make one full rotation about its own
axis
It has a value of 23h 56m 4.1s
Astronomy 1-1
Lecture 02-6
Daily Time Keeping
Both the Apparent and
Mean Solar day often do
not correspond to an
Earth rotation, about its
own axis, of 360 degrees.
The Sidereal day
corresponds to an Earth
rotation of just 360
degrees. This is the
correct rotation period
of the Earth.
Astronomy 1-1
Lecture 02-7
Daily Time Keeping
Apparent Solar Day
Sidereal Day
Astronomy 1-1
Lecture 02-8
North Celestial Pole
A point in the northern hemisphere that
stars apparently circle
Currently the north celestial pole points
towards Polaris
This was not always so, and will not be so in
the future
The Earth's rotation axis is affected by
external forces
The axis orientation changes with
respect to the stars, it precesses
A complete precession takes ~26,000 yrs
Noticed by ancient astronomers
Similar to rotating gyroscope
Astronomy 1-1
Lecture 02-9
Precession of Earth’s Rotation Axis
Also known as the precession of
the equinoxes
One such complete precessional
cycle in a period of
approximately 26,000 years
Being a nonspherical shape, the
gravitational tidal forces of
the Moon and Sun apply
torques as they attempt to
pull the equatorial bulge into
the plane of the ecliptic
Astronomy 1-1
Lecture 02-10
Seasons
Earth's axis of rotation is tilted at 23.5 degree with respect to orbital plane of Earth
around Sun
This tilt causes the hemispheres to alternate in the amount of our Sun’s light and heat
they receive through the year
This is responsible for the seasons that are experienced
Holds for all planets
Astronomy 1-1
Lecture 02-11
Northern Hemisphere Summer
More daylight hours, more direct sunlight
In Northern Hemisphere summer, Earth is actually slighter further from the
Sun than in the winter
Earth is CLOSEST to the Sun (91 million miles) in Winter ~January 3
Earth is FARTHEST from the Sun (94 million miles) in Summer ~July 4
Astronomy 1-1
Lecture 02-12
Celestial Sphere
Coordinate system based on the Earth’s
perspective
A sphere whose equatorial plane coincides with
the equatorial plane of the Earth
The north celestial pole is coincident with the
north pole of the Earth
Plane of sun's movement not the same as
equatorial plane
The two planes are tilted with respect to each
other at an angle of 23.5 degrees
The path of the sun is called the ecliptic
Astronomy 1-1
Lecture 02-13
Celestial Sphere
Astronomy 1-1
Lecture 02-14
Coordinates on the Celestial Sphere
They are:
Declination (latitude)
It is measured in units of degrees, minutes, and
seconds. This is the angular distance above or
below the celestial equator.
Right Ascension (longitude)
It is measured in units of hours, minutes, and
seconds. Its origin is the vernal equinox point and
is measured to the east.
Astronomy 1-1
Lecture 02-15
Major Points on the Ecliptic
Four major points along the ecliptic:
VERNAL EQUINOX
Point where Sun crosses the equatorial plane from below to
above. Occurs around March 21.
SUMMER SOLSTICE
Highest point above equatorial plane. Occurs around June 21.
AUTUMNAL EQUINOX
Point where Sun crosses the equatorial plane from above to
below. Occurs around Sept 23.
WINTER SOLSTICE
Lowest point below equatorial plane. Occurs around Dec 21.
Astronomy 1-1
Lecture 02-16
Time Keeping II
As with the day, there are several types of years.
SIDEREAL YEAR
True orbital period of the Earth around the Sun.
365.2564 Mean Solar Days
TROPICAL YEAR
Time for passage from one vernal equinox to the next.
365.2422 Mean Solar Days
GREGORIAN YEAR
Calendar based on tropical year. This is the one that is used.
Error of only 1 day in 3300 years.
365.2425 Mean Solar Days
Astronomy 1-1
Lecture 02-17
Calendar
If the calendar that is used were based on the
sidereal year, the seasons would slowly shift
with respect to the calendar because the
Earth's rotation axis precesses.
This precession causes the relative facing of the
Earth towards the Sun at a given point on the
Earth's orbit to change from year to year.
Astronomy 1-1
Lecture 02-18
Planetary Motions
Inferior Planet
Planet closer to the Sun than the Earth.
Mercury and Venus
Superior Planet
Planet further from the Sun than the Earth.
Mars, Jupiter, Saturn, Uranus, Neptune,
and Pluto
Astronomy 1-1
Lecture 02-19
Important Points Along a Planet's Orbit
Inferior Planets
Inferior Conjunction
Planet is directly between
Earth and Sun.
Superior Conjunction
Sun is directly between
Earth and planet.
Maximum Elongation
Point on the planet's orbit
where it has the largest
angular distance between
it and the Sun as viewed
from Earth.
Astronomy 1-1
Lecture 02-20
Important Points Along a Planet's Orbit
Superior Planets
Opposition
Earth is directly between
the planet and the Sun.
Conjunction
Sun is directly between the
planet and Earth.
Quadrature
Planet is see at 90 degrees
with respect to the Sun
as viewed from Earth.
Astronomy 1-1
Lecture 02-21
Mars’ Interesting Motion
Mars in its motion seems to go backwards
What is the cause of this apparent motion?
Astronomy 1-1
Lecture 02-22
Retrograde Motion
This apparent temporary
backward motion of a
superior planet along its
orbit is caused by the
Earth's faster motion
along its own orbit.
The Earth overtakes the
planet.
Astronomy 1-1
Lecture 02-23