1. Precession of the Equinox
The Earth’s axis changes direction in a clockwise direction with a cycle of about 26,000 years

2. Precession of the Equinox 2
The precession is caused by the Earth’s equatorial bulge (the Earth is not a perfect sphere)
The longitude of the VE changes by 1°about every 72 years
It can be discovered by careful analysis of old star charts

3. Precession of the Equinox 3
Discovered by Hipparchus around 150 B.C.E.
Discovered independently in China around 320 C.E. by Yú Xǐ (虞喜)
These are the only known discoveries of precession
Don’t confuse it with “procession”

4. Precession of the Equinox 4
The equinox moves clockwise along the ecliptic
The NCP traces out a circle in the sky

5. Precession of the Equinox 5

6. Precession of the Equinox 6
Most of the time there will not be a good pole star
We are lucky to have Polaris in our age

7. Solar and Sidereal Day
Sidereal day is one rotation of the Earth or the day with the respect to the stars and is about 4 min shorter than the solar day
24 h/360 = 24*60 min/360 = 4 min
A year of about 365 solar days has about 366 sidereal days

8. Solar and Sidereal Day 2

9. Tropical and Sidereal Year
The tropical year of a bit less than days is (to a first approximation) the time from one vernal equinox to the next and corresponds to the seasons
Because of the precession of the equinox, this is NOT the same as the sidereal year of a bit more than days , which is one revolution in the orbit or the year with respect to the stars

10. Time Zones
The time at Greenwich is referred to as UTC (Coordinated Universal Time), but was earlier called GMT (Greenwich Mean Time)
Each 15° (360/24) east or west of Greenwich corresponds to one hour earlier or later

11. Time Zones 2

12. Time Zones 3
We are at longitude 104°, so we would expect to be in UTC+7, but in fact we are in UTC+8
We are on permanent Daylight Saving Time
Time zones tend to drift westward
Russia has 11 time zones!
China is the largest country with only one time zone

13. Time Zones 4
Before 1949, China had five time zones

14. Time Zones 5
China could not have enforced a “one China” policy on time zones if the capital had been in Chongqing or Xi’an
The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC +8) and Afghanistan (UTC+4:30)

15. Time Zones 6
There are many places where three time zones meet, like the tri-country border of Finland, Norway and Russia
Stations in Antarctica generally keep the time of their supply base

16. International Date Line 1
First discovered when Magellan's expedition sailed around the world in 1522
Made famous in “Round the World in 80 Days” by Jules Verne
The Philippines used to be a Spanish colony, and had its most important communication with Mexico, and was therefore on the east side of the date line
By 1844, trade interests had changed to China, so it switched to the west side

17. International Date Line 2
There are time all together 40 time zones ranging from UTC –12 to UTC +14
-12 are uninhabited islands belonging to the US
In 1995 Kiribati (Gilbert Islands) changed from −11 and −10 to +13 and +14

18. Daylight Saving Time 1 Introduced in Britain in 1916
Spring forward, fall back
More efficient use of early morning sunshine
More time for outdoor leisure activities in the afternoon and evening
Saves energy, lowers crime, improves traffic safety
Not used in the winter, because then Sun rises late anyway

19. Daylight Saving Time 2
In Europe DST is from the last Sunday in March to the last Sunday in October
In the US DST is from the second Sunday in March to the first Sunday in November
Countries in the southern hemisphere have DST in their summer
Britain used year-round DST from 1968 to 1971 but abandoned it because of its unpopularity in northern regions

20. Daylight Saving Time 3
Used in China from 1986 to 1991