The Sky
http://www.phy.ohiou.edu/~mboett/PSC100/spring12/PSC100_spring12.html
If you look up into the sky,
towards the south, then …
1. east is to your right, north is behind you, west
is to your left.
2. east is to your left, north is behind you, west is
to your right.
3. east is to your left, west is behind you, north is
to your right.
4. east is to your right, west is behind you, and
north is to your right.
5. east is behind you, west is to your left, north is
to your right.
North
East
West
South
The Celestial Sphere
• Zenith = Point on
the celestial sphere
directly overhead
• Nadir = Point on the
c. s. directly
underneath (not
visible!)
• Celestial equator =
projection of the
Earth’s equator onto
the c. s.
• North celestial pole
= projection of the
Earth’s north pole
onto the c.s.
The Celestial Sphere (II)
90o - l
l
• From geographic
latitude l (northern
hemisphere), you
see the celestial
north pole l
degrees above the
northern horizon;
• From geographic
latitude –l (southern
hemisphere), you
see the celestial
south pole l
degrees above the
southern horizon.
• Celestial equator culminates 90o – l above the horizon.
Example:
New York City: l ≈ 40.70
Celestial
North Pole
40.70
Horizon
North
Celestial
Equator
49.30
Horizon
South
The Celestial South Pole is not visible
from the northern hemisphere.
Athens, OH, is located at l ≈ +39o.
Where in the sky would you see the
highest point of the celestial equator?
1.
2.
3.
4.
5.
North, 39o above the horizon.
South, 39o above the horizon.
North, 51o above the horizon.
South, 51o above the horizon.
South, 45o above the horizon.
Athens, OH: l ≈ 390
Celestial
North Pole
390
Horizon
North
Celestial
Equator
510
Horizon
South
The Celestial Sphere (III)
Apparent Motion of the
Celestial Sphere
Apparent Motion of the Celestial Sphere II
Sun’s position in the morning
A View of the
Sugarloaf
Mountain (Rio de
Janeiro, Brazil)
Where will the sun be
in the evening before
sunset?
1.
2.
3.
4.
5.
Far off to the right
Far off to the left
Near the Zenith
Near the Nadir
Close to where it is now.
The Magnitude Scale
First introduced by Hipparchus (160 - 127 B.C.):
• Brightest stars: ~1st magnitude (mv = 1)
• Faintest stars (unaided eye): 6th magnitude
(mv = 6)
More quantitative:
• 1st mag. stars apear 100 times brighter than
6th mag. stars
• 1 mag. difference gives a factor of 2.512 in
apparent brightness (larger magnitude =>
fainter object!)
The magnitude scale system can be extended
towards negative numbers (very bright) and
numbers > 6 (faint objects):
Sirius (brightest star in the sky): mv = -1.42
Full moon: mv = -12.5
Sun: mv = -26.5
The Sun and its Motions (I)
Earth’s rotation is causing the day/night cycle.
What is the name for the plane of
the Earth’s orbit around the sun?
1.
2.
3.
4.
5.
Earth plane
Solar plane
Epileptic
Ecliptic
Epiphany
The Sun and its Motions (II)
Due to Earth’s revolution around the sun, the sun appears to
move through the zodiacal constellations.
The Sun’s apparent path on the sky is called the Ecliptic.
Equivalent: The Ecliptic is the projection of Earth’s orbit onto the
celestial sphere.
What is causing the seasons?
1. Brightness variations of the sun.
2. The Earth being closer to the sun in the
summer and further away in the winter.
3. A steeper angle of incidence of the sun’s rays
in the summer than in the winter.
4. A denser cloud cover in the winter than in the
summer.
5. Stronger heat output from within the Earth in
the summer than in the winter.
The Seasons (I)
The Earth’s axis of rotation is inclined vs. the normal to
its orbital plane by 23.50, which is causing the seasons.
The Seasons (II)
• The Seasons are only caused by a varying angle of
incidence of the sun’s rays.
• They are not related to the Earth’s distance from the sun.
• In fact, the Earth is slightly closer to the sun in (northernhemisphere) winter than in summer.
Steep incidence
→ Summer
Shallow incidence
→ Winter
Light from
the sun
The Seasons (III)
Earth’s orbit (eccentricity
greatly exaggerated)
Earth in
January
Earth in July
Sun
The Earth’s distance from the sun has only a very
minor influence on seasonal temperature variations.
When it’s summer in the U.S., it
is … in Argentina
1.
2.
3.
4.
5.
spring
summer
fall
winter
midnight
The Seasons (IV)
Northern summer =
southern winter
Northern winter =
southern summer
What happens to a spinning top
that you set on a table top
slanted?
1. It will immediately fall over
2. It will quickly adjust itself to a perfectly upright
position.
3. It will wobble around in a circular motion
around the vertical.
4. It will wobble around back and forth through
the vertical position.
5. It will take off and become airborne.
Precession (I)
Gravity is pulling on a slanted top. => Wobbling around the vertical.
The Sun’s gravity is doing the same to the Earth.
The resulting “wobbling” of the Earth’s axis of rotation around the vertical to
the Ecliptic takes about 26,000 years and is called precession.
Precession (II)
As a result of precession,
the celestial north pole
follows a circular pattern
on the sky, once every
26,000 years.
It will be closest to
Polaris ~ A.D. 2100.
~ 12,000 years from now,
it will be close to Vega in
the constellation Lyra.
There is nothing peculiar about Polaris at all
(neither particularly bright nor nearby etc.)
What is the radius of the circle that the
celestial north pole traces out on the
celestial sphere in the course of a
precession cycle?
1.
2.
3.
4.
5.
90o
45o
39o
23.5o
66.5o
The Motion of the Planets (I)
The planets are orbiting the sun almost
exactly in the plane of the Ecliptic.
Venus
Mercury
The Moon is orbiting Earth in
almost the same plane (Ecliptic).
The Motion of the Planets (II)
• All outer planets (Mars,
Jupiter, Saturn, Uranus,
Neptune and Pluto)
generally appear to move
eastward along the
Ecliptic.
• The inner planets Mercury
and Venus can never be
seen at large angular
distance from the sun and
appear only as morning or
evening stars.
Which planet is the most difficult
one to observe?
1.
2.
3.
4.
5.
Mercury
Venus
Mars
Jupiter
Saturn
Mercury appears at
most ~280 from the sun.
It can occasionally be
seen shortly after
sunset in the west or
before sunrise in the
east.
Venus appears at most
~ 460 from the sun.
It can occasionally be
seen for at most a few
hours after sunset in
the west or before
sunrise in the east.