EAS 105-THE PLANETS
Prof. Robert L.
Nowack
Lecture 2
The
Universe - The whole of space and what it contains
Earth is a small planet in a Solar System of
planets revolving around the Sun. Four
of the eight planets of the system are enormous compared to Earth. (Jupiter is 400 times as massive as Earth.) The Sun is the truly massive object of the
system: (1000 times as massive as Jupiter; 1/3 of a million times as massive as
Earth). The Sun's gravitational
attraction is what keeps the planets in revolution about it. The Sun is an enormous ball of hot vaporized
gas. The surface temperature is ~ 10,000
degrees Fahrenheit and many millions of degrees at the center. Yet, the Sun is an ordinary Star.
The
Moon’s orbit around the Earth can fit entirely within the disk of the Sun!!! Most of the small twinkling objects in the
night sky are other stars more or less like the Sun, but in distances measured
from Earth in Light Years. A Light Year
is the distance light travels in one year, where c = 3.0 x 105 km/sec
( =186,000
miles/sec ).
r = c x t = 3.0
x 105 km/sec x t sec is the distance light travels in seconds.
Ex.) Light
travels 186,000 miles (300,000 km) in 1 sec.
Ex.) It
takes light 8 minutes to go from the Sun to the Earth.
Ex.) It
takes light ~ 5.5 hours to go from the Sun to Pluto.
The nearest star is in the Alpha Centauri
System. It takes ~ 4.27 years for its
light to reach us. Because the speed of
light is a constant in a vacuum, we are looking back in time when we look out
into space.
Many visible stars make up our galaxy, The
Milky Way. The solar system is part of
this expansive star system. The Sun is
2/3 out to the edge of the galaxy. The
Milky Way is seen as an irregular luminous band across the night sky.
But this Galaxy is not the end. Other galaxies can be seen in the night sky;
even clusters of galaxies.
Large Spiral Galaxy Andromeda
Constellations are groups of stars that appear
from Earth’s perspective to maintain a "fixed" relative orientation
in the night sky.
The Big Dipper
From Earth’s standpoint, it appears Earth is
at the center of a giant hollow sphere (with small lights) which is called The
Celestial Sphere. As with the Sun, this
background Celestial Sphere tends to move in the sky, rising in the East and
setting in the West.
(1) Fixed
Stars form a convenient reference system for studying the motion of Earth and the
planets.
With
respect to the "fixed" stars, the Earth rotates about an axis through
the north and nouth poles in a counterclockwise fashion once every 23 hours, 56
minutes and 4 seconds.
At any given time, the Sun illuminates one-half
of the Earth.
Sun "washes out" or out-shines stars during local day.
Below are time lapsed photographs of the
night sky. Streaks are paths of stars.
Time exposure showing star trails as a result
of the apparent rotation of the Celestial Sphere.
Time exposure showing star trails in the
region of the North Celestial Pole. The
bright trail below the center was made by Polaris (the North Star), which is
about 1 degree away from the True Pole.
Celestial Poles are apparent fixed points in
the sky. The north celestial pole is now
very near the star Polaris. The apparent
rotation of the celestial sphere could be explained by either:
(1) Daily
rotation of entire sky (ancient viewpoint)
(2) Rotation
of the Earth itself (modern view)
(1) Since
the time of Copernicus, it is generally accepted that it is the Earth that
turns. But, can it be proved directly on
Earth? Jean Foucault a French scientist,
proved it in 1851. He used a 60 meter
pendulum with a 25 kg (55 lb) mass on the end. Foucault started the pendulum swinging. After several minutes, the pendulum swing was
seen to twist or rotate with respect to the sand and, hence the Earth
below. If Earth was stationary, the
pendulum would just swing in one plane making one line in the sand.
(Ex.) At
the North Pole of the Earth, the pendulum would return to the same orientation
in 24 hr. Essentially the Earth spins
underneath the swinging pendulum.
(Ex.) Sitting
at the North Pole, the fixed stars would "appear" to rotate about the
overhead Zenith.
For a given North Latitude between the
Equator and the North Pole (such as here in West Lafayette), the North Star
will appear fixed in the sky with other stars circling around it throughout the
night.
Note: The angle of the North Star
above the LOCAL horizon equals the latitude of where you are. The latitude of
(2) The
Earth “revolves” around the Sun in about 365 days in a counter-clockwise orbit looking from “above
the plane of the Solar System”.
The Earth moves approximately 1o
per day in its orbit about the Sun. The
Sun, as seen from the Earth at different times of the year, blocks out
different background stars. In one year,
the Sun will block out a belt of stars and constellations called the Zodiac on
a path called the Ecliptic.
The
Zodiac
The Earth and other planets all revolve
around the Sun in nearly the same plane (as if rolling on a table ). Thus, the Moon, the planets and the Sun are
all found in the sky with respect to background stars in a narrow belt ~ 18o.
This zone in the sky is called the Zodiac. The Sun passes through Twelve constellations
in this zone over the course of 1 year.
These constellations called signs, each span about 30o. This apparent motion actually results from
the revolution of the Earth around the Sun.
Your “sign” is the position of the Sun in the
Zodiac at your birth. This is Sun sign Astrology. Astrology uses the position of the Sun and
planets at ones birth to predict the future of the individual.
Tradition places 12 constellations along the
Zodiac, but official borders now include 13!
Attempts to verify astrological predictions
by experiment have proved negative in general.
As such, astrology lies outside this course on the scientific study of
the Solar System.
(3) The
Earth sustains its axis of rotation like a top as it orbits around the Sun.
The earth's spin axis is inclined with
respect to its orbit plane about the Sun.
Positions in Earth's
Orbit
About
June 22 – Summer Solstice
About
Sept. 23 – Autumn Equinox (or Equal nights)
About
December 22 – Winter Solstice
About
March 21 – Vernal Equinox (or Spring)
The inclination of the Earth's spin axis with
its orbit plane gives rise to seasons.
On June 22, there is more direct sunlight in
northern hemisphere. This occurs in the Summer
in the northern hemisphere.
On December 22, there is more direct sunlight
in southern hemisphere. This occurs in
the Summer in the southern hemisphere.
Even the Greeks distinguished between the
“fixed” stars and the wandering “stars” which moved with respect to the background
Celestial Sphere. The term Planet means wanderer
in Greek. The days of the week are named
after the “7” known wandering “stars” in ancient times:
“Star” English French
Sun Sunday Dimanche
Moon Monday Lundi
Mars Tuesday Mardi
Mercury Wednesday Mercredi
Jupiter Thursday Jeudi
Venus Friday Vendredi
Saturn Saturday Samedi