1. PHYS 1025 – Introductory Astronomy Lecture 2, Either Semester
- syllabus, information about the course
- observing sessions:
Monday – Thursday 9:00 pm, weather permitting
Richard Crudo

2. Celestial Sphere Remember that we are on the INSIDE of the
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!

3. 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

4. The Dome of the Local Sky
Zenith
Nadir
Horizon
Meridian
Transit

5. Horizon coordinate system
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, ° = East, ° = South, °= West
Altitude:
0 to 90 degrees up from horizon
0 ° = Horizon, ° = Zenith

6. 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

7. Zodiac Constellations
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)

8. 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)

9. Don’t confuse RA with time on your watch!
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!

10. Equatorial coordinate system
Comparing latitude and longitude to
declination and right ascension

11. 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˚

12. 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°

13. 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.

14. Understanding Local Skies
3 classes of stars:
circumpolar north - always visible
circumpolar south - never visible
rising and setting

15. Understanding Local Skies
The sky at the North Pole.

16. Understanding Local Skies
The sky at the equator

17. Understanding Local Skies
The sky at 40˚N latitude.

18. Understanding Local Skies
The sky at 30˚S latitude.

19. The altitude of the celestial pole in your sky is equal to your latitude.
Everything in the sky rotates around the north celestial pole

20. 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.

21. 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

22. Problem 2 Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°)
Equator
Problem 2
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°
Equator
NCP
Lat = 50°
Horizon
Dec = -40°
Celestial Sphere
Earth
Kapteyn’s Star
Dec = -45°
NCP
Horizon
50°
40°
-5°
Equator

23. 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
East
West

24. Problem 5
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 °
Zenith
Dec = 42°
Equator
NCP
Lat = 42°
Horizon
Dec = -48°
Celestial Sphere
Earth
Sun
Dec = 0°
NCP
Horizon
42°
48°
Equator
48 Degrees