For Wednesday, Jan. 28

Reading: Section 2.3

Assignments: Math Review (due today)

Mini-Project #1 (due Mon. Feb. 2)

Homework #1 (due Mon. Feb 2)

Thought QuestionIf you look at this star in the south part of the sky, what direction would have a

larger declination?A. higher in the skyB. lower in the skyC. to the eastD. to the west

HORIZONS WE

Winter Constellations

TO SIRIUS (BRIGHTEST STAR IN SKY)

visible in evening in winter

ORION:

(home to Betelgeuse and the Great Nebula)

RA: about 5h

DEC: about 0°

Being a Good Astronomer

Be able to predict:• How stars will appear to move for different people on Earth• How high a star can get in the sky (knowing RA and DEC)• What time of year it is best to observe a star (knowing RA

and DEC)

ATMOSPHERE

• A long photo of the northern sky:

A Good Telescope

• a star’s RA and DEC don’t change during night

• to keep pointing at one RA and DEC, most telescopes rotate with the sky

TO NORTH STAR

Telescopes in Space

• In space, a telescope can continuously point at a star without needing to rotate

The Sky from San DiegoMost stars rise in east half of sky,

and set in west half:

NORTH CELESTIAL POLE

LOOKING SOUTH

N

W

S

E

HORIZON

circumpolar: stars that don’t rise or set for a person

LOOKING NORTH

stars circle in 23h56m

The North Star

• altitude of North Celestial Pole (North Star) = your latitude!

The Sky from San Diego

if lower part of circle doesn’t hit the horizon, the star is circumpolar…

NORTH CELESTIAL POLE

N

W

S

E

HORIZON

the further the star is from a celestial pole, the bigger the circle it makes around the sky:

LOOKING NORTH

stars circle in 23h56m

33°

Thought QuestionA star rises in the NE in San Diego. How long will it be “up” (above the horizon)?A. 24 hoursB. Between 12 and 24 hoursC. 12 hoursD. Between 0 and 12 hours

HORIZON

NORTH CELESTIAL POLE

N

E

W

S

Star MotionsA star on celestial equator (DEC = 0º) will:• rise due E• set due W• spend exactly 12 hr above horizon

NORTH CELESTIAL POLE

N

W

S

E

HORIZON

Exactly ½ the star’s path is above the horizon

The Sky from San Diego

• meridian: arc from north through zenith to south

stars always get to be highest in sky there…

SN

E

W

N

E

W

S

HORIZON

NORTH CELESTIAL POLE

NORTH CELESTIAL POLE MERIDIAN

CELESTIAL EQUATOR

ZENITH

• Imagine cutting celestial sphere from N to S:

The North Star

• altitude of North Celestial Pole (North Star) = your latitude!

MERIDIANNORTH CELESTIAL POLE CELESTIAL

EQUATOR

• A star’s maximum altitude depends on its declination and your latitude:

Star Altitude

MERIDIAN

Thought Question:What is the maximum altitude that a star with

declination +15º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.)

A. 18º

B. 33º

C. 42º

D. 48º

E. 57º

F. 72º

G. The star is not visible from San Diego

Thought Question:What is the maximum altitude that a star with

declination -25º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.)

OR: 0 (The star is not visible from San Diego)

Enter value in degrees using the arrow keys ( to change number; to change between digits), then hit SEND

Thought Question:What is the maximum altitude that a star with

declination +40º can reach as seen from San Diego? (San Diego is at a latitude of about 33º.)

Enter value in degrees using the arrow keys ( to change number; to change between digits), then hit SEND

Thought Question:What is the minimum altitude that a star with declination +40º

can reach as seen from San Diego? (San Diego is at a latitude of about 33º.)

A. 17º above the horizon

B. 7º above the horizon

C. 7º below the horizon

D. 17º below the horizon

For Friday, Jan. 30

Reading: review Section 2.3

Assignments: Mini-Project #1 (due Mon. Feb. 2)

Homework #1 (due Mon. Feb 2)

Telescope observing session on campus early next week?

Summer Constellations

CYGNUS:

lies in Milky Way

RA: about 20h

DEC: about +40°

Vega

What about the Sun?Earth’s motion around Sun slowly changes which

constellations are visible at night:

Thought Question:You go out tonight (Jan. 28) and see a bright star in the

constellation Cancer to the south at midnight. One week later at midnight this same star …

A. will be somewhat southwest.B. will again be due south.C. will be somewhat southeast.D. won’t be visible (below the horizon).

The Sun’s Path

• ecliptic: Sun’s apparent path around celestial sphere during a YEAR

Sun’s RA and DEC change planets, Moon stay near

ecliptic

Sun’s DEC

Summer solstice (June 21) +23.5º

Fall, spring equinoxes

(Mar. 21, Sept. 21)

0º

Winter solstice (Dec. 21) -23.5º

Solar and Sidereal (Star) Days

SUN

• solar day: time between when Sun appears in same place in sky (24 hrs on average)

• sidereal day: time between when a star appears in same place in sky (23 hr 56 min)

SUN

Which type of day corresponds to exactly one rotation of Earth?

Thought Question:If the Earth orbited the Sun but did not

rotate, which of the following would be true?

A. The Sun would not rise or set for someone on Earth.

B. A solar day would equal a year.

C. A sidereal day would equal a year.

VIEW FROM ABOVE EARTH N. POLE

Solar Time is NOT Star Time

1 sidereal (star) day = 23 hr 56 min

= 0.997 solar day

(4 minutes shorter than 1 solar day)

Star time at sunset gets later by:

about ½ hr per week

about 2 hr per month

(Earth moves 1/12th of way around Sun)

The star clock runs fast…

Right Ascension and Star Time• use hours, minutes, and

seconds of time for RA

1h = 60m

1m = 60s

• 0h ≤ RA < 24h

• RA increases going east

• Local Sidereal Time (LST): current RA on your meridian (“star time”)

The Sky from San DiegoLooking S, you see stars up for 12

hrs or less

NORTH CELESTIAL POLE

N

W

S

E

HORIZON

Looking N, stars are up for 12 hrs or more

LOOKING NORTH increasing RA

LST+6h LST-6h

RA=LST

The Sky from San DiegoStars are rising in east half of sky, and setting in west half:

LOOKING SOUTHLOOKING NORTHLST LST-2hLST+2h

LST+4 h LS

T-4

h

LSTLST-2h LST+2h

LST+12h

LST-6h LST+6h

NENW SE SW

…like a clock where the face moves instead of the hands

Star time = RA on your meridian right now

STEPS:

1. Start on the day that star time and solar time match up at midnight (Sept. 21)

2. Determine star time at midnight on the day you want

3. Determine star time for the clock time you want

4. Stars having RA within about 6 h of that star time will be visible

Telling Star Time

Date: RA of Sun: LST at midnight:

Spring equinox (March 21)

0h

(BY DEFINITION)

12h

Summer solstice (June 21)

6h 18h

Fall equinox (Sept. 21)

12h 0h

REMEMBER THIS

ONE!!

Winter solstice (Dec. 21)

18h 6h

What’s Up?1,2. Determine star time at midnight on the day you want

What Part of the Sky is Up?What is local sidereal time (LST) at midnight?Example: January 28

• LST = 0h at midnight on fall equinox (Sept. 21)

• LST at midnight advances 2 h every month (and about 0.5 h every week)

• Jan. 28 is 4 months and 1 week after Sept. 21

• LST 8h 30m

Thought Question:What will the star time be at midnight on

May 14?A. 3h 30m

B. 4h

C. 4h 30m

D. 15h 30m

E. 16h

F. 16h 30m

What Part of the Sky is Up?What is local sidereal time (LST) at 9pm?

Example: January 28

• LST at midnight 8h 30m

• 9 pm is 3 h earlier, so LST 5h 30m

How much of the sky can you see?

0h(MERIDIAN)

-2h 2h

SE SWS

How much of the sky can you see?

About 6h on either side of meridian

+4h

+6h

+2h

SEENE

Thought Question:

If the local sidereal time is 17 h and you are looking for a star having an RA of 14 h, which of the following is true?

A. The star will reach its highest point in 3 hours.

B. The star reached its highest point 3 hours ago.

C. It isn’t possible to tell using the information given.

increasing RA

LST+6h LST-6h

RA=LST

For Monday, Feb. 2Reading: Section 2.4

Assignments: Mini-Project #1 (due Mon. Feb. 2)

Homework #1 (due Mon. Feb 2)

Mini-Project #2 (due Wed. Feb. 11)

QUIZ #1 SCHEDULED FOR FRIDAY, FEB. 6

PLANETARIUM SHOWS:Tues. Feb. 3: NOON, 1 PMWed. Feb. 4: NOON, 1 PM, 3 PMThur. Feb. 5: NOON

Mon. Feb. 9: 3 PMTues. Feb. 10: 1 PM, 3 PMWed. Feb. 11: 1 PM

Seasons on Other Planets?

Earth’s Orbit and Tilt

• Earth’s axis is tilted compared to the axis of Earth’s orbit

Why Does the Axis Matter?In June:

• More than 50% of Sun’s light falls on N hemisphere

• You spend more than half a day on sunlit half of Earth (Sun is N of celestial equator)

The Sun’s Path

• ecliptic: Sun’s apparent path around celestial sphere during a YEAR

crosses celestial equator in two places

ONLY goes through zodiac constellations

(and Ophiuchus)Sun’s DEC

Summer solstice (June 21) +23.5º

Fall, spring equinoxes

(Mar. 21, Sept. 21)

0º

Winter solstice (Dec. 21) -23.5º

Thought Question:

What is the minimum altitude the Sun can reach at noon in San Diego? (San Diego’s latitude is 33º.)

(Enter your answer in degrees, rounded to the nearest whole number.)

Sun in WinterWhen Sun is S of celestial equator:• daytime is less than 12 hr long• Sun is low in sky at noon (33.5° on Dec. 21)• rises in SE, sets in SW

NORTH STAR

LOOKING SOUTH:

N

W

S

E

HORIZON

SummerSSE SW

WinterSSE SW

Sun in Summer

NORTH STAR

LOOKING SOUTH:

N

W

S

E

HORIZON

When Sun is N of celestial equator:• daytime is more than 12 hr long• Sun is high in sky at noon (80.5° on June 21)• rises in NE, sets in NW

The Sun’s Altitude

Sunlight coming in at a shallower angle gets spread over a greater area on Earth’s surface, and warms the surface less

NORTH STAR

N

W

S

E

HORIZON

ALTITUDE

The Sun’s Altitude

Sunlight coming in at a shallower angle gets spread over a greater area on Earth’s surface… how much greater?

ALTITUDE

ALTITUDEANGLE

90°ANGLE

increase in area:

Would the Sun appear to change in size during the year?

What season is it in the southern hemisphere during summer here?

HYPOTHESIS 1 HYPOTHESIS 2

(NOT TILTED) (TILTED)

What if Earth’s orbit was like this:

(TOP VIEWS)

AND

and Earth’s axis was like this:

(SIDE VIEWS)

Thought Questions:

A. YES YESB. YES NOC. NO YESD. NO NO

A. SUMMER SUMMERB. SUMMER WINTERC. WINTER SUMMERD. WINTER WINTER

Sun Appearance• take a picture of Sun

from the same place every few days at noon

• any noticeable difference in size of Sun?

Antarctic Ice map

Thought Question:What would seasons be like on this planet as it

orbits the Sun?

A. There would be seasons, but the temperature changes would be more extreme than on Earth.

B. The seasons would be just like those on Earth.C. There would be seasons, but the temperature

changes would be less extreme than on Earth.D. There would not be any seasons on this planet.

Seasons on Uranus