1. Phys. 102: Introduction to Astronomy
Circles and Time

2. Clicker Question
Which observer(s) would see the star travel on the diurnal circle shown?
a) b)
North

3. Diurnal Circles Each celestial object circles the observer each day
Observer sees part of each circle
Observer sees full diurnal circle
Observer sees half of the diurnal circle
Celestial Equator
Observer sees none of the diurnal circle

4. View of Observer In the observer’s frame of reference
Observer sees full diurnal circle
Stars ALWAYS visible
View of Observer
Celestial Equator
Stars visible for 12 hours/day
Observer sees half of the diurnal circle
In the observer’s frame of reference
Observer sees none of the diurnal circle
Stars NEVER visible

5. Antares is at azimuth 170º and altitude 30º
Altitude & Azimuth
Position of an object in the sky
Azimuth = Angle from north through east
Altitude = Angle from horizon to object
North
South
Meridian
Antares is at azimuth 170º and altitude 30º
Antares
Altitude
Azimuth

6. Sun's Summer Diurnal Circle
Time of Day
Position of Sun with respect to Observer
Noon = Sun on observer’s meridian
Sun's Summer Diurnal Circle
North
South
Meridian
West
East
Wow! It’s solar noon!

7. Time of Day Position of Sun with respect to Observer
West
East
Wow! It’s solar Midnight!
Meridian
Summer Sun's Diurnal Circle
North
South
Position of Sun with respect to Observer
Midnight = Sun opposite observer’s meridian

8. Time of day To Sol Earth Rotates Once Each Day
360° with respect to Earth-Sun line
All Earthlings ride along
To Sol

9. Sunrise, Sunset … Everything in the sky (sun, moon, stars, etc.)
Rises in the east
Sets in the west
each day 0°
0hr
15°
1hr
30°
2hr
Measuring Circles:
3hr
45°
60°
4hr
360° = 24 hr
75°
5hr
15° = 1 hr
18hr
270°
6hr
90°
Each hour, the sun moves 15 degrees in the sky
1° = 4 min or
15’ = 1 min
Every 4 minutes, the sun moves 1 degree = 60’ in the sky
180°
12hr

10. Observer’s View of the Day
Sun rises in east,
moves 15°/hour from East to West
transits at noon
sets in west

11. Standard Clock Time Every Longitude at slightly different time
It’s 6 pm (sunset).
Every Longitude at slightly different time
It’s 9 pm.
It’s 3 pm.
It’s midnight.
It’s noon.
It’s 3 am.
It’s 9 am.
It’s 6 am (sunrise).

12. Clock Time = Position of Sol
It’s 6 pm (sunset).
Observers move through times
It’s 9 pm.
It’s 3 pm.
It’s midnight.
It’s noon.
It’s 3 am.
It’s 9 am.
It’s 6 am (sunrise).

13. Daylight Saving Time Shifts times one hour later (USNO Explanation)
It’s 7 pm (sunset).
Shifts times one hour later (USNO Explanation)
It’s 10 pm.
It’s 4 pm.
It’s 1 pm.
It’s 1 am.
It’s 4 am.
It’s 10 am.
It’s 7 am (sunrise).

14. Time Zones: 24, roughly 15° apart
Clock Time
6 pm
5 pm
Time Zones: 24, roughly 15° apart
4 pm
9 pm
3 pm
2 pm
1 pm
12 am
12 pm
9 am
3 am
6 am

15. Time Zones Politics complicates things … Prime Meridian
Time Zone Center at 0° E
Time Zone ~15° wide

16. North America Time Zones
120° W
Pacific Time Zone
90° W
Central Time Zone
105° W
75° W
Mountain Time Zone
Eastern Time Zone

17. Coordinated Universal Time
UTC (UT or Zulu)
Time at Greenwich
no Daylight saving
Conversion
EST (Eastern Standard Time) = UTC – 5hr
eg. 2pm (14:00) EST = 19:00 UT
EDT (Eastern Daylight Time) = UTC – 4hr
eg. 2pm (14:00) EDT = 18:00 UT
UTC = Standard Time on Prime Meridian

18. Solar Time vs. Clock Time
Solar time varies across time zones
Time Zone’s Solar Noon
Clock Noon FOR ALL
Sun’s path seen from time zone center
Eastern Observer’s Solar Noon
Western Observer’s Solar Noon
Time Zone
East side
West side
Rising
Setting

19. Solar Time vs. Clock Time
Solar time varies across time zones
Time Zone’s Solar Noon
Clock Noon FOR ALL
Solar noon is (Degrees)(4 minutes/degree) earlier than clock noon
Degrees East of TZ center
Eastern Observer’s Solar Noon
Time Zone
East side
West side
Rising

20. Clicker Question
Sept-Îles, Quebec, 66.25° W is in the Eastern Time Zone (center: 75° W). Solar noon occurs at
a) 11:25 am, b) 11:51:45 am, c) 12:08:45 pm, d) 12:35 pm
Degrees East of TZ center
Eastern Observer’s Solar Noon
Time Zone
East side
West side

21. Solar Time vs. Clock Time
Solar time varies across time zones
Time Zone’s Solar Noon
Clock Noon FOR ALL
Clock Noon FOR ALL
Solar noon is (Degrees)(4 minutes/degree) earlier than clock noon
Solar noon is (Degrees)(4 minutes/degree) Later than clock noon
Degrees East of TZ center
Degrees West of TZ center
Eastern Observer’s Solar Noon
Western Observer’s Solar Noon
Time Zone
East side
West side
West side
Rising
Setting

22. Clicker Question
Yuma, AZ, 114.5° W is in the Mountain Time Zone (center: 105° W). Solar noon occurs at
a) 11:22 am, b) 11:50:30 am, c) 12:09:30 pm, d) 12:38 pm
Degrees East of TZ center
Degrees West of TZ center
Eastern Observer’s Solar Noon
Western Observer’s Solar Noon
Time Zone
East side
West side
West side

23. to north celestial pole
Celestial Navigation
Finding Latitude & Longitude from
Altitude of Polaris (NCP)
Transit time of star
to observer’s zenith
to celestial equator
to north celestial pole N S
Observer at 20° N

24. Star Transit Time Gives position of star with respect to the sun
eg. Find transit time of Aldebaran on December 15
Standard Time
12 pm
9 pm
6 pm
3 pm
9 am
12 am
6 am
3 am
2 pm
1 pm
4 pm
5 pm
11 pm
Aldebaran’s Position on 12/15 nearly opposite Sol!

25. Star Transit Time Gives position of star with respect to the sun
eg. Find transit time of Vega on July 15 (Daylight time)
Standard Time
Daylight Time
1 pm
10 pm
7 pm
4 pm
10 am
1 am
7 am
4 am
3 pm
2 pm
5 pm
6 pm
12 pm
9 pm
6 pm
3 pm
9 am
12 am
6 am
3 am
2 pm
1 pm
4 pm
5 pm
12 pm
Vega’s Position on 7/15 nearly opposite Sol!

26. Celestial Navigation
Difference between observed and expected transit times gives longitude
Observer watches star transit.
Star’s Transit
Clock’s Time Zone Longitude
Clock is set to some time zone.
Observed transit time disagrees with ephemeris.
Degrees East of TZ center
Longitude difference from clock’s time zone center = (Time difference)(15°/hour)

27. Celestial Navigation Colorado Day!
Example: Transit of Deneb on August 1
Colorado Day!
Observer sees Deneb transit at 11 pm EDT
At 1 am Deneb will transit TZ center at 75° W
Looks up transit time in FG
On 8/1 Deneb transits at 1 am
Early  East of TZ center
Longitude difference from clock’s time zone center = (2 hours)(15°/hour) = 30° East
Observer’s Longitude = TZ center – Latitude difference = 75° W - 30° = 45 ° W

28. Today’s FUN!
Time Zones
For western time zones, determine center longitude (do the math … it’s easy!)
Convert times of events to UTC
UTC – 5 hours = EST
UTC – 4 hours = EDT
For cities listed, determine clock time of solar noon
(date may change!)
4 min = 1° = 60’
1 min = 15’
10’ = 2/3 of 15’
10’ = 40 sec
10’ W
12:00:40

29. Today’s FUN! Time Zones Celestial Navigation
For western time zones, determine center longitude (do the math … it’s easy!)
Convert times of events to UTC
UTC – 5 hours = EST
UTC – 4 hours = EDT
For cities listed, determine clock time of solar noon
Celestial Navigation
Find Latitude from altitude of NCP
Find Longitude from ephemeris & observation
USE an ATLAS to find out where you are!!!
(date may change!)