# Phys. 102: Introduction to Astronomy

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Phys. 102: Introduction to Astronomy
Circles and Time

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

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

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

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

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!

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

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

Sunrise, Sunset … Everything in the sky (sun, moon, stars, etc.)
Rises in the east Sets in the west each day 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

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

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).

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).

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).

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

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

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

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

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

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

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

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

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

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

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!

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!

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)

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

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

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!)