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Richard Crudo. 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. Celestial Sphere.

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richard crudo
Richard Crudo

PHYS 1025 – Introductory AstronomyLecture 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

celestial sphere
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
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
The Dome of the Local Sky
  • Zenith
  • Nadir
  • Horizon
  • Meridian
  • Transit
slide5
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
slide6

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

slide7
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
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)
slide9
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
Equatorial coordinate system

Comparing latitude and longitude to

declination and right ascension

slide11

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˚
slide12

RA and Dec of the Cardinal Points on the Ecliptic

23.5°

  • 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˚

Equator

Declination

0 h

6 h

12 h

18 h

24 h

Ecliptic

-23.5°

example where is vega
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
Understanding Local Skies

3 classes of stars:

circumpolar north - always visible

circumpolar south - never visible

rising and setting

understanding local skies1
Understanding Local Skies

The sky at the North Pole.

understanding local skies2
Understanding Local Skies

The sky at the equator

understanding local skies3
Understanding Local Skies

The sky at 40˚N latitude.

understanding local skies4
Understanding Local Skies

The sky at 30˚S latitude.

the altitude of the celestial pole in your sky is equal to your 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

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
Problem 1
  • What is the Sidereal Time at noon, December 21?
  • Sidereal Time = RA of transiting Star
  • What star is transiting at noon?

Answer: Sun

  • What is significant about Dec. 21?

Answer: Winter Solstice: Sun has an RA of 18 hours

  • Therefore, the Sidereal Time at noon on Dec. 21

is 18:00

problem 2

NCP

Equator

Problem 2

50°

40°

Horizon

-5°

  • Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°)

from an observatory at latitude 50° N?

  • Set up a picture:
  • Do the math:

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

Zenith

Dec = 50°

NCP

Equator

Celestial Sphere

Lat = 50°

Equator

Horizon

Dec = -40°

Earth

Kapteyn’s Star

Dec = -45°

problem 4
Problem 4
  • A star lies 15 degrees due east of zenith at 10

PM; when will it transit?

  • 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

  • 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)

Therefore, the star will transit in one hour:

11 pm

Zenith

West

East

problem 5

NCP

Problem 5

48°

42°

Horizon

Zenith

Dec = 42°

  • What is the maximum altitude and the azimuth of the sun at noon,

September 21 in Storrs, CT.?

  • Set up a picture:
  • Do the math:

Storrs has a lat of 42 ° N

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 ° - 90 ° = - 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 °

NCP

Equator

Celestial Sphere

Sun

Dec = 0°

Lat = 42°

Equator

Earth

48 Degrees

Horizon

Dec = -48°