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 Question
If you look at this star in the south part of the sky,
what direction would have a larger declination?
A. higher in the sky
B. lower in the sky
C. to the east
D. to the west
HORIZON
E
S
W
Winter
Constellations
visible in evening in winter
ORION:
(home to Betelgeuse and the
Great Nebula)
RA: about 5h
DEC: about 0°
TO SIRIUS
(BRIGHTEST
STAR IN SKY)
Being a Good
Astronomer
ATMOSPHERE
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)
• A long photo of
the northern sky:
A Good
Telescope
TO NORTH
STAR
• 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
Telescopes in Space
• In space, a
telescope can
continuously
point at a star
without
needing to
rotate
The Sky from San Diego
circumpolar: stars that don’t rise
or set for a person
Most stars rise in east half of sky,
and set in west half:
LOOKING
NORTH
LOOKING
SOUTH
NORTH
CELESTIAL
POLE
E
S
N
W
HORIZON
The North Star
• altitude of North Celestial Pole (North Star) = your latitude!
The Sky from San Diego
the further the star is from a celestial pole, the
bigger the circle it makes around the sky:
33°
LOOKING NORTH
radius = ( 90 - DEC )
if lower part of circle doesn’t hit the horizon, the
star is circumpolar…
NORTH
CELESTIAL
POLE
E
S
N
W
HORIZON
Thought Question
A star rises in the NE in San Diego. How long will it be “up” (above the
horizon)?
A. 24 hours
B. Between 12 and 24 hours
C. 12 hours
D. Between 0 and 12 hours
NORTH
CELESTIAL
POLE
E
N
S
W
HORIZON
Star Motions
A star on celestial equator (DEC = 0º) will:
• rise due E
• set due W
• spend exactly 12 hr above horizon
NORTH
CELESTIAL
POLE
Exactly ½ the star’s path is
above the horizon
E
S
N
W
HORIZON
The Sky from San Diego
ZENITH
NORTH
CELESTIAL
POLE
NORTH
CELESTIAL
POLE
CELESTIAL
EQUATOR
E
E
S
N
W
MERIDIAN
HORIZON
S
N
W
• meridian: arc from north through zenith to south
stars always get to be highest in sky there…
The North Star
• Imagine cutting celestial sphere from N to S:
NORTH
CELESTIAL
POLE
MERIDIAN
CELESTIAL
EQUATOR
• altitude of North Celestial Pole (North Star) =
your latitude!
Star Altitude
• A star’s maximum altitude depends on its
declination and your latitude:
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.)
Enter value in degrees
using the arrow keys
( to change number;
 to change
between digits),
then hit SEND
OR: 0 (The star is not visible from San Diego)
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
• solar day: time between
when Sun appears in
same place in sky
(24 hrs on average)
SUN
• 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
(7 days ´ 4
minutes
day
)
 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 Diego
Looking N, stars are up for 12
hrs or more
Looking S, you see stars up for 12
hrs or less
RA=LST
LOOKING
NORTH
LST-6h
LST+6h
NORTH
CELESTIAL
POLE
E
S
N
W
HORIZON
The Sky from San Diego
Stars are rising in east half of sky, and setting in west half:
LOOKING NORTH
LST
LST+2h
LST-2h
LOOKING SOUTH
LST-2h
LST
LST+2h
LST+12h
NW
NE
…like a clock
where the
face moves
instead of the
hands
SE
SW
Telling Star Time
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
What’s Up?
1,2. Determine star time at midnight on the day you want
Date:
RA of Sun:
LST at
midnight:
Spring equinox
(March 21)
0h
12h
Summer solstice
(June 21)
6h
18h
Fall equinox
(Sept. 21)
12h
0h
Winter solstice
(Dec. 21)
18h
(BY DEFINITION)
REMEMBER
THIS ONE!!
6h
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.
B.
C.
D.
E.
F.
3h 30m
4h
4h 30m
15h 30m
16h
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
0h
(MERIDIAN)
SE
S
How much of the sky can you see?
SW
NE
E
How much of the sky can you see?
About 6h on either side of meridian
SE
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.
RA=LST
LST+6h
LST-6h
For Monday, Feb. 2
Reading: 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 PM
Wed. Feb. 4: NOON, 1 PM, 3 PM
Thur. Feb. 5: NOON
Mon. Feb. 9: 3 PM
Tues. Feb. 10: 1 PM, 3 PM
Wed. 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
Sun’s DEC
Summer solstice (June 21)
+23.5º
Fall, spring equinoxes
(Mar. 21, Sept. 21)
0º
Winter solstice (Dec. 21)
-23.5º
 crosses celestial equator in
two places
 ONLY goes through zodiac
constellations
(and Ophiuchus)
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 Winter
When 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
LOOKING SOUTH:
NORTH STAR
E
S
N
W
HORIZON
SE
S
SW
Summer
SE
Winter
S
SW
Sun in Summer
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
LOOKING SOUTH:
NORTH STAR
E
S
N
W
HORIZON
The Sun’s Altitude
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
E
S
N
W
HORIZON
The Sun’s Altitude
ALTITUDE
Sunlight coming in at a shallower angle gets spread over a greater
area on Earth’s surface… how much greater?
90°
ANGLE
ALTITUDE
ANGLE
width of beam
sin (ALT) =
width on ground
increase in area:
1
sin ( ALT)
Thought Questions:
HYPOTHESIS 1
HYPOTHESIS 2
What if Earth’s orbit was like
this:
(TOP VIEWS)
AND
and Earth’s axis was like this:
(SIDE VIEWS)
Would the Sun appear to
change in size during
the year?
What season is it in the
southern hemisphere
during summer here?
(NOT TILTED)
A.
B.
C.
D.
A.
B.
C.
D.
YES
YES
NO
NO
SUMMER
SUMMER
WINTER
WINTER
(TILTED)
YES
NO
YES
NO
SUMMER
WINTER
SUMMER
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