1. Seasons and Calendar
Lecture 4

2. 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).

3. Misconception about seasons
Common misconception is that the season is due to the changing distance between the Sun and Earth
Summer
Winter
Wrong Idea!
The truth is opposite!
Earth is about 3% closer to the Sun during the Northern hemisphere winter.

4. Seasons The Sun is high in the midday summer sky…
(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 hemisphere seasons are opposite, why?
 all because of the tilt of the Earth spin axis

5. Seasons The Sun is low in the midday winter sky…
(b) The Sun in winter
Why are there seasons?
Northern and southern seasons are opposite, why?
 all because of the tilt of the Earth spin axis
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.

6. Seasons
summer days are longer AND Sun stays higher in the sky than winter days.
shorter days
weaker sunlight
longer days
stronger sunlight

7. Orbital motion of Earth
seen from North Pole,
rotation and orbital direction of Earth are both
counter-clockwise

8. Figure 2-12
Spring in the northern hemisphere; autumn in the southern hemisphere
Winter in the northern hemisphere; summer in the southern hemisphere
North
pole
North pole in
continuous daylight
North pole in
continuous darkness
North
pole
South
pole
23 1/2°
South pole in
continuous darkness
South pole in
continuous daylight
Earth’s
orbit
South
pole
Autumn in the northern hemisphere; spring in the southern hemisphere
Summer in the northern hemisphere; winter in the southern hemisphere

9. Sun’s yearly path in the celestial sphere
Ecliptic  something to do with eclipses (detailed in the next chapter).

10. Equinoxes and solstices
equinox : in Latin, “equal night”
Spring (vernal) equinox
Autumnal equinox
day and night have the same length (12hr)
solstice : “stand still”  stops moving northward or southward…
Summer solstice
Winter solstice
However, these seasonal prefixes are for the northern hemisphere only!

11. Figure 2-16 Zenith Celestial equator North celestial pole W S N
Horizon E
For an observer at 35 deg north.
Dec. 21
Mar. 21
June 21
During summer in the northern hemisphere, the Sun rises in the northeast and sets in the northwest.
Sept. 22
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.

12. Earth at winder solstice
Arctic
Circle
Tropic
of Cancer
Equator
Tropic of
Capricorn
Antarctic N S
Sun’s
rays
…the Sun is directly overhead at noon on the Tropic of Capricorn…
The Sun does not rise north of the Arctic Circle…
…and the Sun does not set south of the Antarctic Circle.

13. Earth at summer solstice
The Sun does not set north of the Arctic Circle…
Earth at summer solstice
…the Sun is directly overhead at noon on the Tropic of Cancer…
Arctic
Circle N N
Tropic of Cancer
Arctic
Circle
Equator
Sun’s
rays
Tropic of
Capricorn
Tropic
of Cancer
Equator
Antarctic
Circle
…and the Sun does not rise south of the Antarctic Circle.
Tropic of
Capricorn S S
Antarctic
Circle

14. Moon
Zodiac : band around Ecliptic where all planets, Sun, Moon are located.
There are 12 constellations in Zodiac

15. Change in the orientation of the rotation axis of a rotating object
Precession
Change in the orientation of the rotation axis of a rotating object

16. Precession Earth spin axis precesses with a period of 26,000 years.
Figure Precession
Because the Earth’s rotation axis is tilted, the gravitational pull of the Moon and the Sun on the Earth’s equatorial bulge together cause the Earth to precess.

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

18. Time, Meridian, and Transit
Korean sundial 600 years ago
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

19. Sun as a Timekeeper
Not a good one!

20. Mean Solar Time
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.

21. Sidereal Time To vernal equinox
Earth moves about 1° around its orbit in one day…
star as a timekeeper
 used by astronomers
Sun
…so Earth must make a complete rotation plus 1° to bring this location to local solar noon on March 22.
Local solar noon on March 21 is at this location on Earth. 1° 1°
Earth on
March 21
Earth on
March 22

22. Calendar : Leap Year system.
Length of a year ~ 365 ¼ days.
Sidereal year : time required for the Sun to return to the same position w.r.t. stars
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.
days
In leap years, we add one extra day in February (Feb 29).
Calendar gets complicated because of fractional days ( 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× days = days, but we added 1.0 day). The difference is days over 4 years or 3.12 days in 400 years.

23. Current Leap Year system = Gregorian Calendar
1. If year is dividable by 400
a leap year
2. If year is dividable by 100
 not a leap year
3. If year is dividable by 4
 leap year
Ex)
Year 2000 = leap year
2001 = no
2004 = yes
2100 = no
2400 = yes …
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 = 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.

24. Chapter/sections covered in this lecture : sections 2-5 through 2-8
In summary…
Important Concepts
Important Terms
Apparent solar time
Mean solar time
sidereal time
precession
Calendar
Ecliptic
precession
equinox
solstice
meridian
transit
tropical year
leap year, leap second
Chapter/sections covered in this lecture : sections 2-5 through 2-8