A100
Oct. 13
Keeping Time
READ Essay 2 – Keeping Time in
the text, pp. 177-186
Review on Oct. 15
2nd Exam Friday, Oct. 17
Today’s APOD
The Sun Today
Celestial Timekeeping
Astronomical Time Periods
How 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?
The “day” has two lengths
Solar day: The
Sun makes one
circuit around
the sky in 24
hours
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
An analemma gives the position
of the Sun in the sky at noon
through the year.
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
Mean Solar Time
Universal Time
Universal 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 Zones
Originally each town kept its own time
Arrival of railroads required standardization
Time zones suggested by Canadian Sanford
Fleming in 1878
Adopted by U.S. railroads on Nov. 18, 1883
International 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
Zones
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
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!)
Eastern Daylight Time
8:00
7:00
6:00
Date
12/1
11/1
10/1
9/1
8/1
7/1
6/1
5/1
4/1
3/1
2/1
5:00
1/1
Time of Sunrise
9:00
Sunset is
late in the
summer
Kids go to
school in
the dark
most of
the year.
Date
Sunset Time, 2008
12/1
11/1
10/1
9/1
8/1
7/1
6/1
5/1
4/1
3/1
2/1
10:00 PM
9:00 PM
8:00 PM
7:00 PM
6:00 PM
5:00 PM
4:00 PM
3:00 PM
1/1
Time of Sunset
Eastern Daylight Time
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 Latitudes
Arctic Circle
(66.5°N): Sun
never sets on
summer solstice
Tropic of
Cancer
(23.5°N): Sun
directly
overhead at
noon on summer
Special Latitudes
Antarctic 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 Equator
Sun 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 Circle
Sun 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
Dates to
Remember
 Read Essay 2 in the Text
 Review on Wednesday
 2nd exam on Friday, Oct. 17