Transcript Lecture 2 ppt - University of Connecticut

PHYS 1025 – Introductory Astronomy
Lecture 2, Either Semester
- syllabus, information about the course
http://astronomy.uconn.edu
- observing sessions:
Monday – Thursday 9:00 pm, weather permitting
http://www.phys.uconn.edu/observatory
Richard Crudo
Celestial Sphere
Our lack of depth
perception when we
look into space creates
the illusion that Earth is
surrounded by a
celestial sphere. In
reality, stars that appear
very close together in
our sky may actually lie
at very different
distances from Earth.
Remember that we are on the INSIDE of the
sphere (on Earth) looking out!
Points on the Celestial Sphere
• North and south celestial poles
• Celestial equator
• REMEMBER: These are
points /lines on the celestial
sphere and NOT on the Earth
• From now on:
equator = celestial equator
The Dome of the Local Sky
•Zenith
•Nadir
•Horizon
•Meridian
•Transit
Horizon coordinate system
- coordinates are measured with respect to horizon
- change with time and depend on observer
• Azimuth:
– 0 to 360 degrees around horizon from north towards east
– 0° = North, 90 ° = East, 180 ° = South, 270 °= West
• Altitude:
– 0 to 90 degrees up from horizon
– 0 ° = Horizon, 90 ° = Zenith
Ecliptic Plane
Plane containing the Sun and planets
Ecliptic is tilted 23.5° with respect to the Equator
Eclipses can only occur when the moon crosses this plane
• Ecliptic:
– The Sun's apparent annual path among the constellations
• Zodiac Constellations
– The constellations on the celestial sphere through which the ecliptic passes
– Origin of Astrology (Zodiac Sign)
Cardinal Points on the Ecliptic
• Vernal Equinox
• Sun rises due East and sets
due West
• Length of day = length of
night = 12 hours
• Summer Solstice
• Sun is highest in the sky (this
is why it’s so hot during
summer)
• Autumnal Equinox
• Winter Solstice
• Sun is lowest in the sky (this
is why it’s so cold during
winter)
Equatorial coordinate system
- coordinates fixed on the celestial sphere
- time and observer independent
• declination (dec)
– Analogous to latitude, but on the celestial sphere; it is the angular
north-south distance between the celestial equator and a location on
the celestial sphere.
– Measured in degrees:
» 0 ° to 90 ° – north from celestial equator
» 0 ° to -90 ° – south from celestial equator
• right ascension (RA)
– Analogous to longitude, but on the celestial sphere; it is the angular
east-west distance between the vernal equinox and a location on the
celestial sphere.
– Measured in units of time: hours, minutes, seconds
» 0 h – 24 h from Vernal Equinox towards east
» Ex. Sirius has RA =
6 h 45 m
OR
6:45
Don’t confuse RA with time on your watch!
Equatorial coordinate system
Comparing latitude and longitude to
declination and right ascension
RA and Dec of the Cardinal Points on the Ecliptic
Vernal Equinox
– Sun appears on March 21
– RA = 0h Dec = 0˚
Summer Solstice
– Sun appears on June 21
– RA = 6h Dec = 23.5˚
Autumnal Equinox
– Sun appears on Sept. 21
– RA = 12h Dec = 0˚
Winter Solstice
– Sun appears on Dec. 21
– RA = 18h Dec = -23.5˚
RA and Dec of the Cardinal Points on the Ecliptic
23.5°
Vernal Equinox
Declination
– Sun appears on March 21
– RA = 0h Dec = 0˚
Equator
0h
6h
12 h
18 h
Summer Solstice
– Sun appears on June 21
– RA = 6h Dec = 23.5˚
24 h Autumnal Equinox
– Sun appears on Sept. 21
– RA = 12h Dec = 0˚
Winter Solstice
Ecliptic
-23.5°
– Sun appears on Dec. 21
– RA = 18h Dec = -23.5˚
Example: where is Vega?
Its declination tells us
that it is 38°44′ north of
the celestial equator. We
can interpret its right
ascension in two ways:
As an angle, it means
Vega is about 279° east
of the vernal equinox
As a time, it means Vega
crosses the meridian
about 18 hours 35
minutes after the spring
equinox.
Understanding Local Skies
3 classes of stars:
circumpolar north - always visible
circumpolar south - never visible
rising and setting
Understanding Local Skies
The sky at the North Pole.
Understanding Local Skies
The sky at the equator
Understanding Local Skies
The sky at 40˚N latitude.
Understanding Local Skies
The sky at 30˚S latitude.
The altitude of the celestial pole in your sky is equal
to your latitude.
Everything in the sky rotates around the north celestial pole
Sidereal Time
Sidereal time
1) Time measured according to the position of
stars in the sky rather than the position of the Sun
in the sky.
2) How long ago the vernal equinox has
transited
3) It’s the Right Ascension of ANY transiting
star.
Problem 1
• What is the Sidereal Time at noon, December 21?
1. Sidereal Time = RA of transiting Star
2. What star is transiting at noon?
Answer: Sun
3. What is significant about Dec. 21?
Answer: Winter Solstice: Sun has an RA of 18 hours
4. Therefore, the Sidereal Time at noon on Dec. 21
is 18:00
Equator
Problem 2
• Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°)
from an observatory at latitude 50° N?
1. Set up a picture:
2. Do the math:
Zenith
Dec = 50°
NCP
Equator
NCP
The horizon is 90° from your zenith
Zenith has a dec = your lat = 50°
The lowest point in the sky that you can
see has a dec of:
50 ° - 90 ° = - 40 °
3. The star is 5 ° below the
Horizon…so we can’t see it
Celestial Sphere
50°
40°
Lat = 50°
Earth
Horizon
-5° Horizon Equator
Dec = -40°
Kapteyn’s Star
Dec = -45°
Problem 4
• A star lies 15 degrees due east of zenith at 10
PM; when will it transit?
Zenith
1. Recall that 1 hour is equal to 15 degrees. Why is this?
Answer: Earth rotates 360 degrees in 24 hours  360 / 24 = 15 degrees per hour
2. So we know the star will transit 1 hour before or after 10
PM. Since the star is east of the meridian, it hasn’t yet
transited. (all stars rise in the east and set in the west as
time passes)
West
East
Therefore, the star will transit in one hour:
11 pm
Problem 5
• What is the maximum altitude and the azimuth of the sun at noon,
September 21 in Storrs, CT.?
Zenith
Dec = 42°
1. Set up a picture:
NCP
Celestial Sphere
2. Do the math:
Storrs has a lat of 42 ° N
NCP
The horizon is 90° from your zenith
Zenith has a dec = your lat = 42°
The lowest point in the sky that you can
see has a dec of:
42° Earth
42 ° - 90 ° = - 48 °
Equator Sun
Dec = 0°
Lat = 42°
48°
Equator
Horizon
Horizon
Dec = -48°
The sun has a dec of 0 ° on the Autumn Equinox
It’s altitude is then: 48 °
Since it is transiting at noon in the south, it’s azimuth must be 180 °
48 Degrees