Seasons and Calendar

Lecture 4

2-5 What causes the seasons

2-6 The effect of changes in the direction of Earth’s axis of rotation

2-7 The role of astronomy in measuring time

2-8 How the modern calendar developed

You do not need to study about the celestial coordinates (Box 2-1).

Misconception about seasons Common misconception is that the season is due to the changing distance between the Sun and Earth

Sum

mer

Winter

Wrong Idea!

The truth is opposite!Earth is about 3% closer to the Sun during the Northern hemisphere winter.

• Why are there seasons?• Northern and southern hemisphere

seasons are opposite, why?

all because of the tilt of the Earth spin axis

Seasons(a) The Sun in winter

The Sun is high in the midday summer sky…

… so a shaft of sunlight is concentrated onto a small area, which heats the ground effectively and makes the days warm.

• Why are there seasons?• Northern and southern seasons are

opposite, why?

all because of the tilt of the Earth spin axis

Seasons(b) The Sun in winter

The Sun is low in the midday winter sky…

… so the same shaft of sunlight is spread out over a larger area and less heating of the ground takes place.

Seasons

summer days are longer AND Sun stays higher in the skythan winter days.

longer daysstronger sunlight

shorter daysweaker sunlight

seen from North Pole, rotation and orbital direction of

Earth are bothcounter-clockwise

Orbital motion of Earth

Figure 2-12

Northpole

South pole incontinuous daylight

North pole incontinuous darkness

South pole incontinuous darkness

North pole incontinuous daylight

23 1/2°

Southpole

Earth’sorbit

Northpole

Southpole

Summer in the northern hemisphere; winter in the southern hemisphere

Autumn in the northern hemisphere; spring in the southern hemisphere

Winter in the northern hemisphere; summer in the southern hemisphere

Spring in the northern hemisphere; autumn in the southern hemisphere

Sun’s yearly path in the celestial sphere

Ecliptic something to do with eclipses (detailed in the next chapter).

equinox : in Latin, “equal night”Spring (vernal) equinoxAutumnal equinox

day and night have the same length (12hr)

solstice : “stand still” stops moving northward or southward…

Summer solsticeWinter solstice

However, these seasonal prefixes are for the northern hemisphere only!

Equinoxes and solstices

Celestialequator

Northcelestialpole

N

W

S

EDec. 21 Mar. 21

Sept. 22June 21

Horizon

Zenith

During summer in the northern hemisphere, the Sun rises in the northeast and sets in the northwest.

On the first day of spring and the first day of fall, the Sun rises precisely in the east.

In winter in the northern hemisphere, the Sun rises in the southeast and sets in the southwest.

Figure 2-16

ArcticCircle

Tropicof Cancer

Equator

Tropic ofCapricorn

AntarcticCircle

N

S

Sun’srays

Earth at winder solsticeThe Sun does not rise north of the Arctic Circle…

…the Sun is directly overhead at noon on the Tropic of Capricorn…

…and the Sun does not set south of the Antarctic Circle.

Earth at summer solstice

ArcticCircle

Tropicof Cancer

Equator

Tropic ofCapricorn Antarctic

Circle

N N

SS

ArcticCircle

Tropic of Cancer

Equator

Tropic ofCapricorn

AntarcticCircle

Sun’srays

The Sun does not set north of the Arctic Circle…

…the Sun is directly overhead at noon on the Tropic of Cancer…

…and the Sun does not rise south of the Antarctic Circle.

Moon Zodiac : band around

Ecliptic where all planets, Sun, Moon are located.

There are 12 constellations in Zodiac

PrecessionChange in the orientation of the rotation axis of a rotating object

http://www.youtube.com/watch?v=TbJ5JNn77UM

Precession Earth spin axis precesses with a period of 26,000 years.

Changing zodiac Zodiac signs need to be changed every ~2000 years!

Sundial : tracking time using the location of the Sun

apparent solar time

Meridian : a circle paths through NP, SP, and Zenith.

A celestial object crosses Meridian twice a day, once above the horizon and the other below the horizon.

Transit : Crossing of the upper meridian by an object.

Apparent solar day : time between two successive transits by the Sun

Time, Meridian, and TransitKorean sundial 600 years ago

Sun as a Timekeeper

Not a good one!

To avoid the irregular move of the apparent Sun, astronomers invented an imaginary Sun called “Mean Sun”.

Mean Sun moves along the equator at a uniform rate.

The difference b/w mean and apparent solar times can be as large as ~30 minutes.

Time Zones : rough interval of 15° longitude for the convenience.

At noon at the center of a time zone, mean Sun transits.

Mean Solar Time

Sidereal Time

Earth onMarch 22

To vernal equinox

Sun

1°

Earth onMarch 21

1°Local solar noon on March 21 is at this location on Earth.

Earth moves about 1° around its orbit in one day…

…so Earth must make a complete rotation plus 1° to bring this location to local solar noon on March 22.

star as a timekeeper

used by astronomers

Calendar gets complicated because of fractional days (0.2422 days).

Roughly, in every four years, there will be one extra day. add one extra day in every 4th year. Leap Year (if the year number is a multiple of 4, then the year is a leap year).

Then, in every four year, we are adding little too much (4×0.2422 days = 0.9688 days, but we added 1.0 day). The difference is 0.0312 days over 4 years or 3.12 days in 400 years.

Length of a year ~ 365 ¼ days.

Sidereal year : time required for the Sun to return to the same position w.r.t. stars365.2564 days

Tropical year : time required for the Sun to return to the vernal equinox. Because of the precession, it is shorter than the sidereal year.365.2422 days

In leap years, we add one extra day in February (Feb 29).

Calendar : Leap Year system.

3.12 days extra over 400 years If the leap year is a multiple of 100, then the year is no longer a leap year. However, if a year is a multiple of 400, then it is a leap year.

In 1582, Pope Gregory XIII introduced a new calendar system.

- 1 year = 365.2425 days - year dividable by 4 leap year - year dividable by 100 not a leap year - year dividable by 400 leap year

Over 400 years, we will have 0.03 days too short. to adjust, we add leap seconds occasionally.

1. If year is dividable by 400 a leap year2. If year is dividable by 100 not a leap year3. If year is dividable by 4 leap year

Ex)

Year 2000 = leap year2001 = no2004 = yes2100 = no2400 = yes…

Current Leap Year system = Gregorian Calendar

In summary…

Important ConceptsApparent solar timeMean solar timesidereal timeprecessionCalendar

Important TermsEclipticprecessionequinoxsolsticemeridiantransittropical yearleap year, leap second

Chapter/sections covered in this lecture : sections 2-5 through 2-8