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

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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

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8:00 PM9:00 PM

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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