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Today’s APOD
READ Essay 2 – Keeping Time in the text, pp. 177-186
Review on Oct. 152nd Exam Friday, Oct. 17
The Sun Today
A100 Oct. 13 Keeping Time
Celestial Timekeeping
Astronomical Time PeriodsHow do we
define the day, month, year, and planetary time periods?
How do we tell the time of day?
When and why do we have leap years?
Solar day: The Sun makes one circuit around the sky in 24 hours
The “day” has two lengths
Sidereal day: Earth rotates once on its axis in 23 hrs, 56 min, and 4.07 sec.
The difference between the solar day and the sidereal day
The “year” is also complicated to define
Sidereal year: Time for Earth to complete one orbit of Sun
Tropical year: Time for Earth to complete one cycle of seasons
Tropical year is about 20 minutes (1/26,000) shorter than a sidereal year because of Earth’s precession.
When and why do we have leap years?
The length of a tropical year is about 365.25 days.
In order to keep the calendar year synchronized with the seasons, we must add one day every four years (February 29).
For precise synchronization, years divisible by 100 (e.g., 1900) are not leap years unless they are divisible by 400 (e.g., 2000).
How do we tell the time of day?
Apparent solar time depends on the position of the Sun in the local sky
A sundial gives apparent solar time
Mean Solar Time
Length of an apparent solar day changes during the year because Earth’s orbit is slightly elliptical.
Mean solar time is based on the average length of a day.
Noon is average time at which Sun crosses meridian
It is a local definition of time
An analemma gives the position of the Sun in the sky at noon through the year.
Mean Solar Time
Universal TimeUniversal time (UT) is defined to be the
mean solar time at 0° longitude.It is also known as Greenwich Mean Time
(GMT) because 0° longitude is defined to pass through Greenwich, England
It is the standard time used for astronomy and navigation around the world
Indiana is 5 time zones west of the Greenwich meridian (actually… it’s 6!)
Origin of Time ZonesOriginally each town kept its own timeArrival of railroads required standardizationTime zones suggested by Canadian Sanford
Fleming in 1878Adopted by U.S. railroads on Nov. 18, 1883International Prime Meridian Conference in
1884 formalized Greenwich as the Prime Meridian and set up the time zones
Adopted officially in the US by the Standard Time Act in 1918
Standard Time & Time Zones
US Time Zone
s
The official U.S. time - clock
Time Zones in Indiana
Indiana is on the western side of the eastern time zone
Geographically, Indiana’s longitude should put it in the central time zone.
On eastern time, the sun rises and sets late (effectively, we are on double daylight savings time in the summer
Daylight Savings Time for Indiana
Sunset is late in the summer
Kids go to school in the dark most of the year.
Eastern Daylight Time
5:00
6:00
7:00
8:00
9:00
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In the U.S., daylight savings time is observed from the second Sunday in March until the first Sunday in November (NOTE ERROR IN TEXT!)
Sunset Time, 2008
Eastern Daylight Time
3:00 PM
4:00 PM5:00 PM
6:00 PM
7:00 PM
8:00 PM9:00 PM
10:00 PM
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Date
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set
Sun’s Path in
the Local Sky
Special Latitudes:•the poles•the equator•23.5 degrees N•23.5 degrees S•66.5 degrees N•66.5 degrees S
The official U.S. time - clock
Special LatitudesArctic Circle
(66.5°N): Sun never sets on summer solstice
Tropic of Cancer (23.5°N): Sun directly overhead at noon on summer solstice
Special LatitudesAntarctic Circle
(66.5°S): Sun never sets on winter solstice
Tropic of Capricorn (23.5°S): Sun directly overhead at noon on winter solstice
Sun’s Path at North Pole
Sun remains above horizon from spring equinox to fall equinox
Altitude barely changes during a day
Sun’s Path at EquatorSun rises straight
up and sets straight down
North of celestial equator during spring and summer
South of celestial equator during winter and fall
Sun’s Path at Tropic of Cancer
Sun passes through zenith at noon on summer solstice
Sun’s Path at Arctic CircleSun grazes horizon
at midnight on summer solstice
Planetary Periods
Planetary periods can be measured with respect to stars (sidereal) or to apparent position of Sun (synodic).
Planetary Periods
Difference between a planet’s orbital (sidereal) and synodic period depends on how far planet moves in one Earth year
Read Essay 2 in the Text Review on Wednesday 2nd exam on Friday, Oct. 17
Dates to Remember