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Lecture 2

Motions in the Sky

September 10, 2018

What is your year in school?

A. New freshman

B. Returning freshman

C. Sophomore

D. Junior

E. Senior

F. I’ve been here, like,

forever

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What is your major?3

A. Arts or Communication

B. Natural Resources

C. Professional Studies

D. One of the Sciences

E. One of the Humanities (languages,

philosophy, political science, etc.)

F. Undeclared

Why are you taking this class?

A. Astronomy seems interesting to

me.

B. Only because it satisfies my

GEP requirements.

C. It was the only class that would

fit in my schedule.

D. A friend recommended it to me.

E. Some other reason.

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Units of Measure

• We’ll use mainly the metric system, based on the

meter, kilogram, and second

• We’ll often use scientific notation:

• You should know the metric prefixes and

scientific notation

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1 light-year 9,461,000,000,000,000 m

9.461 10 m

=

=

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Angular Size

• Astronomers measure angles in degrees and

arc minutes (60 per degree) and arc seconds

(60 per arc minute)

• The size of an object is related to its

distance from Earth and its observed

angular diameter

physical diameter distance angular diameter 180

57.3

rD

=

=

D

r

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Estimating Angles

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Jupiter has a physical diameter of 143,000 km

and is 635,000,000 km from Earth. What is

its angular size?

A. 46.5 arcsec

B. 129 arcsec

C. 635 arcsec

D. 1910 arcsec

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Jupiter has a physical diameter of 143,000 km

and is 635,000,000 km from Earth. What is

its angular size?

A. 46.5 arcsec

B. 129 arcsec

C. 635 arcsec

D. 1910 arcsec

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( )5

8

57.357.3

57.3

57.3 1.43 10 km0.0129

6.35 10 km

3600 arcsec0.0129 46.5 arcsec

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r DD D r

r

= = =

= =

= =

Yearly Motions

• The Earth travels around the Sun once every

year.

• The Earth is tilted by 23.5 degrees with

respect to the orbital plane.

E

E

E

E

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• The Sun lines up with different constellations

during the year.

These stars are overhead during the

day. Sun is too bright to see them.

These stars are visible at night.

Presently

These stars are visible at

night.

These stars are overhead during the

day. Sun is too bright to see them

6 months later

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What is a Day?

• Solar Day = amount of time for Earth

to rotate once on its axis with respect

to the Sun.

• Sidereal day = amount of time for

Earth to rotate once with respect to

the stars.

– Shorter than solar day by 4 minutes.

• Since solar and sidereal days are

NOT the same, a star will typically

rise 4 minutes earlier each day.

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Ecliptic

• Path of the sun

through the

celestial sphere.

• Not aligned

with Celestial

Equator due to

tilt of the Earth

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Zodiac• The 13 constellations through which the sun

passes during the course of a year.

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Interactive Figure (might not work in every browser)

Constellation Traditional Actual (2000) Days

Capricornus Dec. 22-Jan. 21 Jan. 21-Feb. 16 26

Aquarius Jan. 22-Feb. 21 Feb. 16-Mar. 11 24

Pisces Feb. 22-Mar. 21 Mar. 11-Apr. 18 38

Aries Mar. 22-Apr. 21 Apr. 18-May 13 25

Taurus Apr. 22-May 21 May 13-June 22 40

Gemini May 22-June 21 June 22-July 21 29

Cancer June 22-July 21 July 21-Aug. 10 20

Leo July 22-Aug. 21 Aug. 10-Sept. 16 37

Virgo Aug. 22-Sept. 21 Sept. 16-Oct. 31 45

Libra Sept. 22-Oct. 21 Oct. 31-Nov. 23 23

Scorpius Oct. 22-Nov. 21 Nov. 23-Nov. 29 6

Ophiuchus Nov. 29-Dec. 18 19

Sagittarius Nov. 22-Dec. 21 Dec. 18-Jan. 21 34

The Real Signs of the Zodiac15

If we could observe background stars in daylight,

how would the Sun appear to move against this

background because of the Earth’s orbit around the

Sun?

A. 1° per day, from west to east

B. 1° per day, from east to west

C. 15° per hour, from west to east

D. 15° per hour, from east to west

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Planetary Motion

The planets all move in

the same plane about the

Sun so they all appear to

move along the zodiac

(near the ecliptic).

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Summer Solstice North Celestial Pole

South Celestial PoleCelestial Equator

Ecliptic

Summer Solstice:Sun reaches highest point

in sky for N. Hemisphere.

(June 21)Longest period of

daylight in Northern

Hemisphere

E

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Autumnal EquinoxEqual periods of day and night for entire Earth

E

North Celestial Pole

South Celestial PoleCelestial Equator

Ecliptic

Autumnal Equinox:Sun crosses Celestial

Equator. (September 21)

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Winter Solstice

Longest period of

darkness in Northern

Hemisphere

North Celestial Pole

South Celestial PoleCelestial Equator

Ecliptic

Winter Solstice:Sun at lowest point in

sky for N. Hemisphere.

(December 21)

E

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Vernal EquinoxEqual periods of day and night for entire Earth

North Celestial Pole

South Celestial PoleCelestial Equator

Ecliptic

Vernal Equinox:Sun crosses Celestial

Equator. (March 21)

E

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The ecliptic crosses the celestial equator at ___

A. Two points, known as solstices

B. Two points, known as equinoxes

C. One point, known as the vernal

equinox

D. One point, known as the meridian

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The Seasons• The Seasons are due to the tilt of the Earth

with respect to the orbital plane.

– Not distance from sun

• Flash Applet

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Why are the seasons warm and cold?• During the Summer the sun is higher in the sky

so the light is more direct.

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• Sun is above the horizon longer in the

summer.

Why are the seasons warm and cold?

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During summer in the northern hemisphere, why is

it still cold at the North Pole, even though they get

24 hours of daylight?

A. The Earth is further from the

Sun in the summer.

B. The North Pole is further from

the Sun than Stevens Point

C. The light is still very indirect

even in the summer.

D. None of the above.

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