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)• 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.

l

90o - l

Example:New York City: l ≈ 40.70

Horizon

North

Celestial North Pole

40.70

South

49.30

Celestial Equator

The Celestial South Pole is not visible from the northern hemisphere.

Horizon

Athens, OH, is located at l ≈ +39o. Where in the sky would you see the highest point of the celestial equator?

1. North, 39o above the horizon.2. South, 39o above the horizon.3. North, 51o above the horizon.4. South, 51o above the horizon.5. South, 45o above the horizon.

Athens, OH: l ≈ 390

Horizon

North

Celestial North Pole

390

South

510

Celestial Equator

Horizon

The Celestial Sphere (III)

Apparent Motion of the Celestial Sphere

Apparent Motion of the Celestial Sphere II

1. Far off to the right

2. Far off to the left

3. Near the Zenith

4. Near the Nadir

5. Close to where it is now.

Where will the sun be in the evening before

sunset?

A View of the Sugarloaf

Mountain (Rio de Janeiro, Brazil)

Sun’s position in the morning

The Magnitude ScaleFirst 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. Earth plane2. Solar plane3. Epileptic4. Ecliptic5. 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 (northern-

hemisphere) winter than in summer.

Light from the sun

Steep incidence → Summer

Shallow incidence → Winter

The Seasons (III)

Sun

Earth in JulyEarth in January

The Earth’s distance from the sun has only a very minor influence on seasonal temperature variations.

Earth’s orbit (eccentricity greatly exaggerated)

When it’s summer in the U.S., it is … in Argentina

1. spring

2. summer

3. fall

4. winter

5. 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 over2. 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. 90o

2. 45o

3. 39o

4. 23.5o

5. 66.5o

Radius

The Motion of the Planets (I)

The planets are orbiting the sun almost exactly in the plane of the Ecliptic.

The Moon is orbiting Earth in almost the same plane (Ecliptic).

Jupiter

MarsEarth

Venus

Mercury

Saturn

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. Mercury

2. Venus

3. Mars

4. Jupiter

5. 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.